CO2 INJECTION AND RESERVOIR
CHARACTERIZATION: AN INTEGRATED
PETROGRAPHIC AND GEOCHEMICAL STUDY OF
THE FRIO FORMATION, TEXAS.
A THESIS SUBMITTED TO THE GRADUATE
SCHOOL IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE MASTER OF
SCIENCE GEOLOGICAL SCIENCES
BY
KELLI A. MCGUIRE
(DR. JEFFRY D. GRIGSBY)
BALL STATE UNIVERSITY
MUNCIE, INDIANA
MAY 2009
3
CONTENTS
Page Number
ABSTRACT
LIST OF FIGURES
LIST OF TABLES
LIST OF APPENDICES
ACKNOWLEDGEMENTS
INTRODUCTION
GEOLOGIC BACKGROUND
STRUCTURAL GEOLOGY OF THE FRIO FORMATION
PREVIOUS PETROGRAPHIC ANALYSES OF THE FRIO FORMATION
Framework Mineralogy
Geographic Variation: Lower, Middle & Upper Frio Formation
Source
Frio Formation: Upper Stratigraphic Unit
Diagenesis & Authigenic Minerals
Quartz Overgrowths
Calcite
Clays
Feldspar Overgrowths & Dissolution
Pyrite
Porosity & Permeability
FRIO PILOT STUDY
Site Selection
Well Location & Installation
Pilot Experiment
Fluid Sampling & Geochemical Results
METHODS
Sample Collection & Preparation
Petrological Analysis
Thin Section Analysis
Scanning Electron Microscope
Geochemical Analysis
X-Ray Diffraction Analysis
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5
7
8
9
11
13
17
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RESULTS
39
Framework Mineralogy of the Upper Frio Formation
Diagenesis & Authigenic Mineralogy
Compaction
Authigenic Mineralogy
Clays
Pyrite
Distribution Frequency in Frio
29
34
4
Forms & Sizes
(a) Solitary Euhedral Crystals
(b) Framboidal
Unidentified covering
Quartz & Feldspar Overgrowths
Secondary Porosity
Minor Authigenic Phases
Porosity & Permeability
Geochemistry of Pyrite
DISCUSSION
78
Reservoir Characterization: Framework Mineralogy & Diagenesis
Pyrite
Occurrence of Pyrite
Geochemistry of Pyrite
Pyrite Alteration
CONCLUSIONS
95
REFERENCES
96
APPENDICES
104
5
LIST OF FIGURES
Figure
Page Number
Figure 1. Stratigraphic diagram of the Frio Formation
4
Figure 2. Stratigraphic column of “B” & “C” zones of injection
5
Figure 3. Ternary plot of Frio Formation sandstones
9
Figure 4. Map of the Texas Gulf Coast
11
Figure 5. Location map, Brazoria County, Texas
14
Figure 6. Location map, Liberty County, Texas & Pilot Experiment Site Map
21
Figure 7. Ternary plot of Frio “C” Formation sandstones
30
Figure 8. Photomicrograph of Frio “C” detrital grains
30
Figure 9. Photomicrograph of detrital quartz grains
31
Figure 10. Photomicrograph of embayed quartz grains
31
Figure 11. Photomicrograph of orthoclase feldspar grain
32
Figure 12. Photomicrograph displaying dominant detrital grains
32
Figure 13. Photomicrograph displaying detrital grains
33
Figure 14. Photomicrograph of volcanic rock fragments
34
Figure 15. X-ray diffraction graph of bulk sample material
34
Figure 16. Diagenesis chart
35
Figure 17. Photomicrographs displaying compaction features
36
Figure 18. Intergranular volume of upper Frio “C” sandstone
37
Figure 19. SEM image displaying I/S clay grain coats
39
Figure 20. SEM image and corresponding EDAX report of illite/smectite clay
40
Figure 21. SEM image and corresponding EDAX report of illite/smectite clay
41
Figure 22. XRD graph displaying the dominant clay phases
42
Figure 23. Photomicrograph displaying pyrite among detrital grains
43
Figure 24. Photomicrograph of pyrite in thin section
43
Figure 25. Photomicrograph of pyrite in-filling a benthic foraminifera
46
6
Figure 26. Photomicrographs of pyrite in-filling diatoms
47
Figure 27. SEM image of euhedral pyrite grains
48
Figure 28. SEM image of framboidal pyrite with unidentified covering
48
Figure 29. Framboidal pyrite
50
Figure 30. SEM image displaying a collection of octahedra pyrite grains
50
Figure 31. SEM images of framboidal pyrites attached to detrital grains
52
Figure 32. SEM images of euhedral pyrite with unidentified covering
53
Figure 33. SEM image of quartz overgrowth
54
Figure 34. SEM image of microcrystalline quartz on I/S clay coats
54
Figure 35. SEM image of feldspar dissolution along cleavage planes
56
Figure 36. SEM image of potassium-feldspar overgrowth & barite
56
Figure 37. Photomicrograph displaying dissolution of a feldspar grain
57
Figure 38. SEM image of barite
58
Figure 39. SEM image & EDAX report of halite
59
Figure 40. Porosity versus permeability for the upper Frio “C” Formation
60
Figure 41. SEM image of framboidal pyrite and corresponding EDAX report
62
Figure 42. SEM image & EDAX report of pyrite octahedra
63
Figure 43. SEM image of framboidal pyrite and corresponding EDAX report
64
Figure 44. Pyrite composition determined through electron microprobe analysis
67
Figure 45. Ternary diagram comparing upper “C” and middle Frio sandstone
70
7
TABLES
Table
Table 1. Pyrite composition determined by electron microprobe
Page Number
65
8
LIST OF APPENDICES
Appendix
Page Number
Appendix A. Point count data
93
Appendix B. Compositional analysis determined by point count
94
Appendix C. Electron microprobe data
96
Appendix D. Pyrite frequency chart
97
Appendix E. XRD Graphs
98
9
ACKNOWLEDGEMENTS
First and foremost I would like to thank Dr. Sue Hovorka and her collaborative team(s) at
the Gulf Coast Carbon Center and the Bureau of Economic Geology at the University of Texas at
Austin, for allowing me to join their carbon dioxide sequestration research efforts and providing
access to their data, materials, and reports. Without this group, I would not have been able to aid
in investigating a grass-roots effort of atmospheric CO2 reduction and sequestration, which will
serve me well in my future career, having been a part of something so ground-breaking and
environmentally relevant to these times. I would also like to express my sincere gratitude to the
American Association of Petroleum Geologists Grants-In-Aid program which provided funding
for this research project, through the Gustavus E. “Archie” Memorial Award. Without this grant,
much of my research would have been financially unlikely.
I would like to thank the University of Cincinnati‟s Dr. Warren Huff for his hospitality
and sincere assistance with the XRD system and interpretation and Mr. Doug Kohls for many
hours of assistance with the SEM+EDAX systems. I also want to thank Dr. Chusi Li of Indiana
University for allowing rental usage of the Department of Geological Science‟s electron
microprobe. I give many thanks to the faculty and staff of Ball State University‟s Department of
Geological Sciences as well, for their many forms of advice and support throughout my graduate
studies. Specifically, Dr. Kirsten Nicholson, Dr. Eva Weng, Dr. Rice-Snow, Mr. Chuck Betz and
Mike Kutis provided me with many words of advice, humor, and sincere encouragement. Dr.
Richard H. Fluegeman and Dr. Klaus Neumann also deserve many thanks, for both agreeing to
serve on my thesis committee, as well as their aid and assistance in approaching and reviewing
multiple aspects of my research. I would also like to thank Hanover College and Dr. Ken Bevis,
Dr. Heyo Van Iten, and Dr. Pete Worcester, for providing me with the base knowledge and
appreciation of the geological sciences in my undergraduate studies.
My sincerest gratitude goes to Dr. Jeffry D. Grigsby, my thesis advisor, for taking a
sincere interest in me as a student and support of my research as a scientist. I thank you, Dr.
Grigsby, for the many, many hours of instruction, travel, and review of my research as well as
your kind willingness to agree to work with me despite your new position as Associate Dean
within the college. You have provided invaluable guidance and assistance in my research, for
which I will be forever grateful. Thank you.
Finally, I would like to thank my family and friends for their support and love. To my
sister Shay and brother-in-law Jerry Hartley: thank you for welcoming me to Muncie and
providing unending support and love in all of my endeavors. As well as the many dinners and
relief from anything school-related; I will miss being so close (geographically) to you two. To
my best friend and fellow graduate student Dean Snidow: thank you for your immediate interest,
support, and advice upon my arrival to BSU. You provided me with many hours and days of
motivation, I hope you realize how much your love and support has encouraged me to excelthank you. To my parents, Jim and Kim McGuire, I express my sincerest gratitude and love for
all of the support (emotional and financial!) that you have provided me. I recognize that I owe all
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of my determination and desire to succeed to the values that you have instilled in me over the
years. Thank you for your endless phone calls, letters, and postcards of encouragement. Thank
you for stimulating my love of the Earth sciences and for keeping all of my boxes of rocks in
your attic! Mom and Dad- I love you both more than you know and I hope that I have made you
proud in my completed work; it is to you two that I dedicate this report.
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INTRODUCTION
A major hub for petroleum exploration and production, the Gulf Coast region of
the United States accounts for approximately 16 percent of the U.S. annual carbon
dioxide (CO2) emissions from fossil fuels (Gulf Coast Carbon Center, 2008). The
burning of fossil fuels has increased the amount of carbon dioxide released into the
Earth‟s atmosphere. Produced by the combustion of fossil fuels, CO2 is usually attributed
to refineries, factories, automobiles, and power plants. While the long-term effects of
CO2 released into the atmosphere are still unknown, it has been proposed by scientists
internationally that such pollution is resulting in global climate change. As a result of
this concern to our environment, major steps have been taken internationally by
governments, researchers, and academia to better understand the impact of CO2 on global
climate change, to reduce emissions, and develop alternative methods for CO2 storage.
The Gulf Coast Carbon Center (GCCC) has begun research to develop a suitable
plan for reducing atmospheric CO2 levels by reusing the depleted petroleum reservoirs of
the Gulf Coast subsurface. Geologic sequestration involves the capturing of CO2
emissions followed by injection into subsurface geologic formations such as saline
aquifers and hydrocarbon reservoirs. Separated from potable groundwater, these
reservoirs will contain the CO2 for long periods of geologic time. The GCCC is a
research organization, connected with the Bureau of Economic Geology (BEG) at the
Jackson School of Geosciences, University of Texas at Austin, and is partially
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government-funded through the Department of Energy (DOE). The BEG and
GCCC have access to the data, tools, and research necessary for an accurate assessment
of geologic formations in response to CO2 sequestration. Their mission is to aggressively
apply technical and educational resources to geologic storage of anthropogenic CO2 in a
region needing large-scale reduction of atmospheric CO2 releases. In 2004, the GCCC
conducted the “Frio Pilot Experiment” to test the feasibility of the Oligocene Frio
Formation, a bedrock unit that extends in the subsurface throughout the Gulf Coast
region, as a reservoir for CO2 storage (Hovorka et al., 2003). The GCCC‟s CO2
sequestration research may provide solutions for economically and environmentally
responsible storage of CO2. Complete analysis and investigation of the experiment,
including reactions of the subsurface geology to CO2 injection, will provide answers
critical to the prospect and long-term sustainability of geologic CO2 capture, in the
pursuit of alternative methods of disposal.
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GEOLOGIC BACKGROUND
Depositional Setting & Stratigraphy
Historically an oil and gas producer, the Frio Formation has been well-studied for
its characteristic reservoir properties including permeability and porosity. Examples of
these studies include analyses of variations in reservoir quality (Grigsby and Kerr, 1993),
formation water geochemistry and rock-water interaction (Morton and Land, 1987; Land,
1995; Lynch and Land, 1996), diagenesis and development of secondary porosity
(Lundegard et al., 1984; Moncure et al., 1984; Milliken et al., 1981, 1989, 1994), and
depositional and structural setting (Galloway et al., 1982; Kerr et al., 1992; Brown et al.,
2004). These studies provide the groundwork for the assessment of the Frio Formation as
a repository for anthropogenic CO2.
The Oligocene Frio Formation (from here on referred to as Frio) is a regionally
extensive sandstone deposited in the Tertiary, prograding basinward during the Cenozoic,
in the northwest margin of the Gulf of Mexico sedimentary basin (Lynch, 1996). A
progradational wedge, the Frio is underlain by the Oligocene Vicksburg Formation and
overlain by the Miocene-Oligocene Anahuac Shale, a 60 m (200 ft) – thick transgressive
marine shale wedge (Fig. 1) (Galloway et al., 1982). In the upper Texas coastal plain,
multiple fluvial sources collected downdip into the broad delta of the Houston
Embayment, creating a mixture of marine and non-marine layers (Galloway, 1984).
These interfingering sandstones and shales were deposited in an extracratonic basin
characterized by rapid subsidence in areas of sediment loading, creating the fluvial-
14
deltaic Frio Formation consisting of three main units, the lower, middle and upper
(Galloway et al., 1982). Generally, the upper Frio consists of upward-fining, finegrained, well-sorted sandstones locally separated by shales, indicative of their shore-zone
and shelf depositional history (Hovorka et al., 2006). The shore-zone facies include
lower, middle, and upper-shoreface and „back-barrier‟ sandstones, commonly associated
with environments such as cross-cutting channels, flood and ebb tidal deltas and their
associated deposits (Galloway, 1986). The shelf sandstone facies is comprised of stormderived sandstone beds forming upward-coarsening and upward-fining sandstone
packages, which decrease in thickness basinward (Galloway, 1986).
Figure 1. Stratigraphic associations of the Frio Formation for the (A) Houston and (B)
Rio Grande Embayments, Texas Gulf Coastal Plain (from Galloway et al., 1982).
15
Within the upper Frio, three sandstone layers, the “A”, “B”, and “C”, were
designated as target sequestration units for the experiment. The observation well was
installed in the “B” and “C” zones of the bedrock unit; the injection well was installed in
the “C” zone, as displayed in the stratigraphic column (Fig.2). The “C” zone of injection
is located at 1534-1542 m (5053-5073 ft), with the “B” overlying this unit at 1521.41527.5 m (5014- 5034 ft) (Hovorka et al., 2006). The two units are separated by a thin
shale that serves as the seal on the injection zone; both zones possess intermittent shale
layers. The injection zone of the “C” sandstone is a stratified fluvial unit approximately
24-m-thick (78 ft), while the “B” contains an approximately 4 m-thick (13 ft) re-worked,
fluvial sandstone layer near the top, increasing sand and siltstone layers with depth, and
an approximately 7 m-thick (22 ft) transgressive marine shale at the base (Kharaka et al.,
2006; Hovorka, 2003).
16
Figure 2. Stratigraphic column displaying the “B” and “C” zones of observation and
injection. Adapted from BEG written communications and Hovorka et al., 2006.
17
STRUCTURAL GEOLOGY OF THE FRIO FORMATION
As a part of a salt-diapir-defined subbasin, the area of study sits in the Houston
Embayment, a structural province of the Frio defined by salt diapirism, associated
faulting, and large salt-withdrawal subbasins (Galloway et al., 1982). The reworked
fluvial sandstone and siltstone beds and marine transgression shales of the Frio dip 16° to
the south and increase in depth prior to deposition of the overlying Anahuac Shale, which
is regionally extensive and characterized by shelf and prodeltaic mudstones (Galloway et
al., 1982, Kharaka et al., 2006 & Hovorka et al., 2006). Three-dimensional seismic work
defines the structural margins of the experiment site as northwest-dipping normal faults, a
part of the associated radial faults above the Liberty salt dome (Hovorka et al., 2006 &
Galloway et al., 1982). Only one normal fault occurs in the injection zone, bisecting the
storage reservoir (Hovorka et al., 2006). Structural compartmentalization, associated
with faulting along the flanks of the salt dome, is expected to limit lateral migration of
the CO2 plume (Hovorka and Knox, 2003).
18
PREVIOUS PETROGRAPHIC ANALYSES OF THE FRIO FORMATION
Framework Mineralogy
Geographic Variation: Lower, Middle, & Upper Frio Formation
Frio Formation sandstones have a wide range of mineral composition,
geographically separating the southern or lower, middle or central, and northern or upper
units. Loucks et al. (1979; 1984) and Lynch (1996) classify the southern Frio sandstones
as poorly-sorted, fine-grained, feldspathic litharenites, litharenites, sub-litharenites to
lithic arkoses with an abundance of volcanic rock fragments. The central Frio sandstones
are considered moderately to well-sorted, fine-grained, quartzose lithic arkoses with
abundant metamorphic rock fragments (Loucks et al., 1979; 1984; Grigsby & Kerr,
1991). The northern Frio, as classified by Loucks et al. (1979; 1984), is considered a
poorly-sorted, fine-grained, quartzose lithic arkose to subarkose with distinct populations
of rock fragments, mainly volcanic. In general, the Frio sandstones of the northern or
upper Texas Gulf Coast contain more quartz and less feldspar and volcanic rock
fragments than the Frio of the southern Texas Gulf Coast (Bebout et al., 1978). While all
three Frio geographic divisions possess rock fragments, the southern Frio contains the
greatest abundance, relative to its location near volcanic sources in west Texas and
Mexico (Loucks et al., 1979). This classification is displayed in the Folk diagram, Figure
3, created by Loucks et al., (1977).
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Figure 3. Sandstone composition of Frio Formation along the Texas Gulf Coast
according to Loucks et al., 1977. Sandstone classification after Folk, 1968.
Source
Deposited basinward of prior highstand systems tract (HST) shelf edges, the
Oligocene Frio Formation filled subbasins during third-order lowstand depositional
events (Brown et al., 2004). A drop in relative sea level during third-order sea-level
fluctuations resulted in sedimentation of the Frio as a prograding wedge (Brown et al.,
2004; Hammes et al., 2004). The formation can be arbitrarily divided, based on varying
material sources, into the south or lower, central or middle, and north or upper regions,
moving north along the Texas Gulf Coast (Fig. 4). The Frio Formation of south Texas
resulted from an influx of sediment from the Rio Grande, which carried material from
Cretaceous and Tertiary volcanoes of west Texas and northern Mexico (Lynch, 1996;
20
Galloway et al., 1982). Moving north along the coast lays a region of mixing between
the Rio Grande province and sediments derived from fluvial systems, known as the
central Frio Formation of the Texas Gulf Coast. The Frio Formation of the northern
Texas Gulf Coast, the study area for the Frio Pilot Experiment, was deposited as part of
the Houston Embayment, a broad fluvial-deltaic system responsible for most of the
terrigenous accumulation. Located in the up-dip portion of depositional sequences four,
five, and six of a stacked third-order sequence, the clastic prograding wedge of the upper
Frio is associated with a HST (Brown et al., 2004). The upper Frio deposition of the
northern Texas Gulf Coast is considered to be strongly influenced by the onset of the
Anahuac Shale unit (transgression), with many shale layers increasingly interfingered
with, and separated by, continuing delta progradation, also common to middle and lower
Frio deposition (Galloway et al., 1982). The varying sources of sediment supply to the
unit illustrate why the Frio shows the most regional variation in framework composition
of all Tertiary formations (Loucks et al., 1979).
21
Figure 4. Location map displaying divisions of Texas Gulf Coast. Area 3 is the focus of
this study. Circulars 75-1, 75-8, and 76-3 refer to a regional investigation of the
geothermal potential of the Frio Formation published by the Bureau of Economic
Geology, The University of Texas at Austin (Loucks et al., 1977; Bebout et al., 1978).
Frio Formation: Upper Stratigraphic Unit
Preliminary thin-section analysis completed by the BEG indicates that the
sandstones of the upper Frio (Fig. 2) are well-sorted, angular, “dominantly quartz with
about 20% fresh-to-leached orthoclase and plagioclase feldspars and altered rock
fragments, trace quantities of micas, calcite, organic materials, pyrite, and minor clays as
grain coats and local matrix” (Hovorka et al., 2006). Both “B” & “C” sandstones are
poorly cemented, moderately sorted, subarkose sandstones with minor amounts of illite
or smectite. According to a reservoir analysis of the Tertiary sandstone units of the Texas
22
Gulf Coast completed by Loucks et al. (1979), the Frio Formation displays the most
regional variation in mineral composition. Very little petrographic research of the upper
Frio of the northern Texas Gulf Coast has been published. The middle and lower Frio
have historically been the economically productive units of the Frio, rendering more
industry and research emphasis.
Diagenesis & Authigenic Mineralogy
Two main episodes of diagenesis have been identified in the Frio Formation,
syndepositional diagenesis and burial diagenesis, leading to multiple stages of
cementation and dissolution (Lynch, 1996; Lynch and Land, 1996). Lynch and Land
(1996) suggest that burial-diagenetic modification of Frio sandstones occurs at depths
shallower than 8,000 ft, fitting the depth range of the Frio sandstones studied in this
research. Cementation, associated with the presence of quartz overgrowths, calcite, and
clays (kaolinite) is considered to predate the formation of secondary porosity, usually
with quartz preceding all other diagenetic phases (Land & Fisher, 1987). Secondary
porosity is associated with the dissolution of detrital feldspars, rock fragments, and some
authigenic calcite, and is considered to be the last porosity-affecting diagenetic event to
occur in the Frio (Land and Fisher, 1987; Lynch and Land, 1996).
Quartz Overgrowths
Quartz overgrowths are commonly abundant in Frio sandstones according to
Lynch (1996), Milliken et al. (1981), and Loucks et al. (1984). Specifically, quartz
overgrowths may be more prevalent in Upper Texas Gulf Coast Frio sandstones, as they
tend to be more quartz-rich (Loucks et al., 1979). Isotopic research of Frio sandstones
23
(Brazoria County) by Milliken et al. (1981) indicates quartz cementation occurs at
shallow depths, possibly around 1500 m (4921.5 ft). This depth interval for quartz
overgrowth agrees with earlier work completed by Loucks et al. (1977, 1979), who state
that quartz cementation by overgrowth occurs at shallow (0-4,000 ft) to moderate (4,0008,000 ft) subsurface depths in the Gulf Coast, though occurrence increases with depth
(Land & Fisher, 1987). Additionally, quartz cement is thought to predate burialdiagenetic calcite precipitation in the Frio (Land and Fisher, 1987; Lynch and Land,
1996). Formation of quartz overgrowths can potentially fill pore space and reduce
porosity (Loucks et al., 1979).
Calcite
Both syndepositional and burial-diagenetic calcite is present in the Frio
Formation, relative to the depositional environment. Syndepositional calcite occurs in
shore-zone sandstones of the Frio and burial-diagenetic calcite postdates quartz
overgrowth in both shore-zone and shelf sandstones of the Frio (Lynch and Land, 1996).
It is suggested by Lynch (1996) and Lynch and Land (1996) that diagenetic calcite and
other minerals are spatially distributed relative to rock-water interaction and fluid flow,
both conditions specific to the depositional environment. Often, calcite is considered the
dominant carbonate in Frio sediments, occurring as cement infilling primary pore spaces
(Loucks et al., 1981; Milliken et al., 1981). Additionally, calcite is the most common
syndepositional-diagenetic mineral as well as the most volumetrically important burialdiagenetic mineral found in Frio sandstones of south Texas (Lynch, 1996; Lynch and
Land, 1996). In a study of northern Texas Gulf Coast Frio Formation sediments of
Brazoria County, Texas (Fig. 5), Milliken et al. (1981) identified that the presence of
24
carbonate cements in primary intergranular pore spaces increases with depth, solidifying
its presence around ~ 4000 ft- 13,500 ft, only intermittently present preceding this depth.
Liberty County, Frio
Pilot Experiment site
Figure 5. Location map, Brazoria County, Texas.
(http://commons.wikimedia.org/wiki/File:Map_of_Texas_highlighting_Brazoria_County.
svg, accessed 3/12/09).
Clays
Formation of clay coats as a surface-to-shallow-subsurface diagenetic process is
widespread in the Frio Formation, occurring as illite/smectite (I/S) (Loucks et al., 1979).
These clays precipitate from formation waters, forming a thin clay coat on grain surfaces,
but only occupy a small amount of pore space (Loucks et al., 1979). Lynch (1996),
however, states that I/S is the most abundant mineral in the Frio Formation, mostly
located in the shales, while Hovorka et al. (2006) observed only minor grain coating
clays, likely illite or smectite, at the site of the CO2 injection. Kaolinite in the Frio may
occur as a secondary pore-filling clay and as a feldspar replacement, reducing
permeability but not significantly reducing porosity (Milliken et al., 1981 and Loucks et
al., 1979).
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Feldspar Overgrowths & Dissolution
Diagenetic feldspar alteration may induce chemical changes in the Frio, as
suggested by Milliken et al. (1989). Loss of K-feldspar, Na-rich alkali feldspar, and Caplagioclase in the Frio resulting from dissolution and albitization may be depth-related,
with K-feldspar experiencing greater dissolution (Milliken, 1989; Milliken et al., 1994;
Land & Fisher, 1987; Land & Milliken, 1981). Enhanced Ca and Na values released into
the formation waters from plagioclase and feldspar dissolution have the potential to alter
the saline formation waters of the Frio and therefore, future diagenetic processes (Land,
1995). Dissolution may spatially and temporally vary with mineral and formation-water
composition, temperature, or fluid fluctuation (Milliken et al., 1989).
Feldspar cement precipitates, extending into spaces not occupied by detrital
grains. Such cement may occur in the form of an overgrowth of a detrital feldspar grain
or as a “free crystal” with no attachment to detrital feldspars (Milliken, 1989). Feldspar
overgrowths in the Frio may form less than 3 percent of the sample according to research
by Loucks et al. (1979). Both albite and K-feldspar can form intergranularly or as an
intragranular cement (Milliken, 1989).
Variation of local factors influencing diagenesis, such as formation fluid, provides
for a wide range of alteration of feldspars at any given depth in the Frio studied in south
Texas (Milliken, 1989; Milliken et al., 1989). However, Milliken et al. (1989) discovered
that the proximal, fluvial samples of the Frio Formation of south Texas displayed more
extensive feldspar alteration than the distal, deltaic and marine samples. Closer to the
Frio Pilot Experiment site of the northern Texas Gulf Coast, feldspar diagenesis studies
26
of the Frio Formation of Brazoria County (Fig. 5) show that both dissolution and
albitization (replacement) also occur locally in large quantities.
Pyrite
Commonly, only trace concentrations of pyrite have been reported in the Frio
Formation and are generally considered to be of minor importance (Hovorka et al., 2006,
Grigsby & Kerr, 1993, Kerr et al., 1991, Lindquist, 1977). Authigenic pyrite occurrence
in the Frio is considered a late diagenetic process created via precipitation, and has been
described in the lower to middle Frio of south Texas, sometimes as pyritization of
volcanic rock fragments (Levey & Grigsby, 1992; Grigsby & Kerr, 1993; Kerr et al.,
1991; Land & Fisher, 1987; Lindquist, 1977). Grigsby & Kerr (1991&1993) state that
diagenetic pyrite identified in the middle Frio postdates development of secondary
porosity and comprises up to 2 percent of the total rock volume. However, the
occurrence of pyrite filling both primary and secondary pores, may lead to locally
reduced porosity and permeability values. Land & Fisher (1987) discuss the lack of
quantitative assessments of pyrite in the Frio and its potential importance as sinks for iron
and magnesium in the formation. This suggests the need for further investigation into
pyrite (in the Frio) for its importance as a sink for not only iron, but other metals such as
magnesium, manganese, lead, zinc, and titanium. Similarly, no petrographic or
geochemical pyrite studies thoroughly investigate the occurrence and composition of
upper Frio pyrites; any mention of Frio pyrites focus on the middle and lower units and
do not discuss composition (Grigsby & Kerr, 1991 & 1993; Levey & Grigsby, 1992;
Lindquist, 1977).
27
Porosity & Permeability
Porosity and permeability values are associated with a formation‟s mineralogical
composition and diagenetic history, both of which control the formation‟s reservoir
quality (Loucks et al., 1979). High porosity and permeability values recorded in the Frio
combine to create an exceptional reservoir. The northern Texas Gulf Coast Frio
Formation has been described as possessing the best deep-reservoir quality of any Texas
Gulf Coast unit (Loucks et al., 1977, 1979; 1984). Reservoir quality of the Frio increases
from the southern to northern Texas Gulf Coast, corresponding with changing mineral
composition and cementation (Loucks et al., 1977; 1979; 1984). Frio sandstones of the
northern Texas Gulf Coast are rich in quartz and lower in volcanic and carbonate rock
fragments while also lacking cementation (Loucks et al., 1979; 1984). Because volcanic
and carbonate rock fragments are chemically unstable, they may alter and result in
precipitation of cements, reducing porosity. Additionally, the dissolution of volcanic and
carbonate rock fragments may enhance porosity. Quartz-rich mineral assemblages of the
northern Texas Gulf Coast are also thought to slow the formation of carbonate cements
(Loucks et al., 1977). Thus, the correlations in the northern Frio between high porosity
and quartz assemblages and low volcanic and carbonate rock fragment values signify
high reservoir quality (Loucks et al., 1977; 1979). Sharing a general relationship,
permeability increases as porosity increases in the Frio (Loucks, et al., 1984).
Additionally, porosity follows a regional trend in the Frio; mean porosity increases with
decreasing depth (Loucks, et al., 1979).
Lynch (1996) and Loucks et al. (1979) state that there is a large range in reservoir
quality in the Frio due to varying framework mineralogy and corresponding diagenetic
28
modifications including cementation, replacement, and dissolution. Stressing the
relationship between diagenesis and fluid flow of the Frio sandstones, Lynch (1996)
suggests that it is the modification of the rocks, or the reaction between detrital and
authigenic minerals and the formation water occurring in the pore space, that affects
reservoir quality. Diagenetic minerals (calcite, quartz overgrowths, and kaolinite or
illite/smectite clays) are commonly linked with reducing porosity and can greatly affect
Frio reservoir quality, if adequate physical and chemical conditions are present for
formation. Additionally, large quantities of feldspar dissolution, common to the Frio,
enhance secondary porosity values (Land and Milliken, 1981; Land and Fisher, 1987).
Diagenetic processes responsible for affecting primary porosity (loss or preservation of)
and the creation of secondary porosity determine the final reservoir quality of the unit
(Loucks et al., 1979).
29
FRIO PILOT STUDY
Site Selection
Using reservoir characterization, prior investigative studies, and numerical
modeling, the Frio Pilot Experiment was designed to test the feasibility of the Frio
Formation as a storage solution for anthropogenic CO2. The experiment took place in the
sandstone formation of eastern Texas to assess field methods and measuring techniques
to assist in understanding geochemical reactions and trapping capabilities of the Frio.
The experiment site was selected because of its well-characterized subsurface geology
including high porosity and permeability values, high storage capacity in numerous
sandstones, and its fault-bounded compartment (Hovorka & Knox, 2003). Additionally,
the Frio unit is regionally extensive and possesses many well-known shale seals
necessary for trapping brine and CO2, including the overlying seal: the Anahuac Shale
Formation. Finally, the geographic location of the Frio Formation, relative to abundant
anthropogenic CO2 sources and geotechnical expertise in the Gulf Coast, makes it an
ideal test site and geologic unit. Results of the experiment were intended to both aid and
develop monitoring methods of CO2 in reservoirs and migration containment of the
injected plume.
Well Location & Installation
The Pilot Experiment site is located in Liberty County, Texas, flanking the South
Liberty oil field and salt dome, 5.5 miles south of Liberty, Texas (Fig. 6). The BEG was
able to utilize existing infrastructure including an inactive oil well drilled in 1956, Sun-
30
Gulf-Humble Fee Tract 1, well #4, as the observation well for the experiment (Kharaka et
al., 2006; Sakurai et al., 2006). This provided a cement pad and observation well at a
depth of 1521.4-1527.5 m (5014- 5034 ft) in the “C” zone (Hovorka et al., 2006). Due to
the small volume of CO2 and short injection time, the existing well casings, tubing, and
other equipment of the observation well were not replaced by corrosion-resistant
equipment (Hovorka et al., 2003). The observation well is located up-dip from the
injection well, in the “C” zone, to identify any radial flow during the injection (Hovorka
et al., 2006). One-hundred feet downdip (~ 15°) from the observation well, an injection
well was drilled and perforated at depths of 1534-1542 m (5053- 5073 ft), also into the
“C” zone (Hovorka et al., 2006). Cored samples were collected in aluminum barrels,
frozen with dry ice, and slabbed before transportation to the BEG‟s Houston Core
Research Center where they remain frozen. Possessing only limited data for wells in the
study area, estimates of reservoir properties for the injection site were based on published
data from nearby Frio fields (Sakurai et al., 2006). Porosity and permeability values for
the injection zone were determined from core analyses of the new well (Sakurai et al.,
2006). To examine the mineralogy, cement/matrix, and pore geometries of the site,
31
conventional core analysis, XRD investigation, and thin section analyses were completed.
Figure 6. Location Map of Liberty County, Texas and site location
(http://commons.wikimedia.org/wiki/File:Map_of_Texas_highlighting_Liberty_County.s
vg & Hovorka et al., 2003).
Pilot Experiment
The experiment began by injecting 1600 metric tons of chilled, liquid
commercial-grade CO2 and perfluorocarbon tracers into the injection well at a rate of ~3
kg/s, over an 11-day interval (October 4-14) in 2004 (Kharaka et al., 2006; Sakurai et al.,
2006). The target injection zone, the “C” zone, sits at a depth of approximately 15341542 m (5053-5073 ft). The experiment was monitored by seismic profiles, gas and brine
sampling in the injection zone, as well as sampling in the sandstone unit above the
injection zone to monitor changes before and after injection (Hovorka et al., 2006;
Kharaka et al., 2006). Samples obtained for geochemical characterization were collected
before, during, and after CO2 injection to monitor the breakthrough of the CO2 plume
(Kharaka et al., 2006). Groundwater, soil, gas, and air were also monitored to identify
any changes in CO2 concentrations in the area that might present leakage concerns. The
32
specifics of the experiment including design, well construction, well logging, sampling
and modeling data may be found in Hovorka et al. (2006) and Kharaka et al. (2006).
Fluid Sampling & Geochemical Results
Fluid sampling, conducted during CO2 injection, provided visual observation of
discolored formation water (rusty-brown), suggesting the presence of Fe2+ in solution
within the formation (Hovorka, S.D., personal communication, 2008). Initial results of
the experiment, from hydrochemical analyses of formation waters, found a rapid decrease
in pH and increases in alkalinity and dissolved metals when CO2 concentrations
increased- marking the breakthrough of the CO2 plume on the third day of injection
(Kharaka et al. 2006). Specifically, there was an increase in Ca, Fe, Mn, Zn, Pb, Mo, and
HCO3 (Kharaka et al., 2006). The increase in ions and corresponding drop in pH raises
concerns that dissolution of the caprock may occur and allow for potential transport, or
upward migration, of these ions and the CO2 plume (into groundwater resources).
Questioning the source of this rise in ion concentrations, Knauss et al. (2005)
attempted to model the impact of CO2 on rock-water interaction, based on the initial
results of the experiment. However, these geochemical experiments were run without
exact parameters, usually provided by detailed petrographic examination of the unit.
Kharaka et al. (2006) suggests that the increase in Fe, Mn, and other metals is due to
dissolution of iron-oxyhydroxides and siderite, and the increases in HCO3- and Ca are
related to dissolution of calcite. The low pH values, in response to the formation‟s brine
coming into contact with injected CO2, are assumed to have caused the rapid dissolution
of minerals.
33
The purpose of this study is to provide petrographic and geochemical data,
interpreted from experiment-site core, to aid BEG evaluations which are critical to
determining the performance capability of the subsurface strata in trapping CO2.
Addressing concerns regarding rapid mineral dissolution in the Frio, in response to CO2
injection, will help to determine the effects of CO2 concentrations on rock-water
interaction in the formation. This research will attempt to identify the source of the ions
in solution, through petrographic, geochemical, and x-ray diffraction examination of the
Frio unit. Understanding the petrographic and geochemical reactions and characteristics
of the formation, in relation to CO2 sequestration, are essential to the development of
models to predict rock-water interactions and future sequestration experiments. This
research supports the GCCC‟s efforts in CO2 sequestration and their ultimate goal of
atmospheric CO2 reduction.
34
EXPERIMENTAL METHODS
Sample Collection & Preparation
Twenty-eight samples (plug/ chip) were obtained from the Bureau of Economic
Geology, ranging in depths of 4880 ft to 5466 ft (~1487- 1666 m), as well as sample
cuttings from a depth of 5752 ft (1753 m). Additional samples (36) were collected from
frozen cores ranging in depths from 4880-5469 ft (~1487- 1666 m), from the BEG‟s
Houston Core Research Center. Samples were collected for thin section petrography, Xray diffraction analysis, scanning electron microscopy (SEM) and geochemical analysis.
All samples were obtained from the injection well installed for the Frio Pilot Experiment
in Liberty County, Texas.
Petrological Analysis
Thin Section Analysis
Thin section analysis was performed using an Olympus BX-60 optical research
microscope. Point count analysis (300/slide) was completed on 32 thin sections,
representative of the formation at the injection site, to identify the framework mineralogy,
authigenic mineralogy and porosity trends. Thin sections were impregnated with bluedyed epoxy to indicate porosity. For more accurate point count interpretation and
35
feldspar identification, thin sections were also stained using amaranth and sodium
cobaltnitrite. The feldspar staining procedure required washing of the thin sections
followed by 40 seconds of hydrofluoric acid etching, 1.5 minutes soak-time in Na-Cobalt
nitrite, and a „swish‟ in tap water. This was followed by placing the thin section in BaCl2
solution for six seconds before dipping it into distilled H2O, then soaking in Amaranth
red for two minutes before finally being dipped into tap water and dried. Mean grain size
was determined by visual estimate. Thin sections used in electron microprobe
investigation were polished using a rotating wheel with polishing cloth and abrasives
(0.3µm and 1µm). Slides were then viewed under a reflected light microscope to verify
polishing quality.
Scanning Electron Microscope
In an attempt to identify microscopic elements and minerals, SEM investigation
was utilized at the University of Cincinnati. A Philips XL30 ESEM-FEG with an energydispersive x-ray system (EDAX) and a Hitachi S-4000 FESEM, also with an energy
dispersive x-ray system (EDS), allowed for both microscopic viewing as well as chemical
assessment of the material. SEM investigation of a variety of sample depths provided
petrographic and chemical insight as well as photographs of the sample and its
characteristics. Future references to SEM analyses will be indicated by the term SEM in
this publication and chemical assessment of samples will use the term EDAX.
Four samples at depths of 5061.65 ft, 5067.85 ft, 5075.5 ft, and ~5468-5469 ft
were used in SEM analyses. Three of the selected samples correspond with XRD
analyses as well (samples 5061.65 ft, 5067.85 ft, and 5075.5 ft). Samples chosen for
36
ESEM investigation were mounted on stubs with two-sided conducting tape. Samples
analyzed with the FESEM were also mounted on stubs with two-sided conducting tape
and were then coated in AuPd. Backscatter electron (BSE) imaging was used throughout
SEM investigation to aid in identifying variations in elements by detecting differences in
atomic number on the sample surface, providing clear identification of minerals and
elements. Gaseous secondary (GSE) imaging was used in SEM microscopy and usually
provided the best imaging conditions, displaying contrasting features.
Geochemical Analysis
Electron Microprobe Investigation
Pyrite chemistry was determined by WDS (wavelength-dispersive spectrometer)
analyses of 4 carbon-coated, polished thin sections using Indiana University‟s Cameca
SX-50 electron microprobe equipped with a backscatter electron detector (BSE) and a
beam spot size of 1µm. Sample depths of the four selected thin sections include 5050 ft,
5069.85 ft, 5076.6 ft, and 5469 ft. Prior to analysis, thin sections were polished using a
fine grit (1µm and 0.3µm) to even the surface of the grains to allow for better accuracy
with the electron microprobe (analysis). Elemental standards were run through the
microprobe for these selected elements: Fe, S, Cu, Ni, Co, Se, As, Zn, Mn, and Mg.
Standards included San Carlo olivine, synthetic indium arsenide compound, selenium,
chalcopyrite, zinc sulfide, pyrite, rhodonite, cobalt, and nickel.
37
X-Ray Diffraction Analysis
X-ray diffraction (XRD) was used to identify clays (coatings) as well as other
dominant primary and secondary minerals (> ~10% volume). Mineralogy of the selected
sample depths were determined using the University of Cincinnati‟s Siemens D-500
XRD system. XRD analyses were completed on samples from depths of: 4880 ft, 4892
ft, 4897 ft, 5053 ft, 5060 ft, 5066 ft, 5075 ft, 5460 ft, 5466 ft, and 5752 ft. Samples were
prepared in two initial forms for analysis: 11 bulk material samples and 20 mounted air
samples. Samples were then run through the XRD in four forms: bulk material, mounted
air, mounted heated, and mounted with added ethylene glycol. Following analysis of the
mounted air samples, the slides were then treated with ethylene glycol and re-run through
the XRD system. The duplicate set of slides only underwent heating for XRD analysis.
Sample preparation for air mounted XRD analysis included crushing the ten
sediment samples to a powder form, then mixing them in deionized water in an ultrasonic
bath for 10 minutes to allow the larger particles, such as quartz and feldspar, to settle out,
leaving the fine clay particles (minerals) to remain in suspension. Duplicate slides of
each depth were also prepared for the XRD analysis, for a total of 20 slides. All slides
were prepared by pipetting the sample onto the glass slide and then allowing them to air
dry, creating a thin layer of clay sample on the slide (Carroll, 1970). A set of duplicate
slides was treated with ethylene glycol prior to XRD analysis.
Each sample was analyzed at incident angles from 2° to 32° 2θ, 0.02 degree step
sizes, and at 1 second count times. The air mounted slides were run first, in the original
form and then again, after being treated with ethylene glycol. The duplicate slides were
38
heated at 149°C (300°F) to collapse any swelling minerals that might be present and to
aid in identifying any presence of kaolinite. They were then placed in the diffractometer
for further analysis. Following ethylene glycol treatment and heating, respectively, both
set of slides were run through the XRD system. Peak intensity plots were manifested
using Siemens D-500 operating software. The resulting graphs, shown as Figure 22 and
Appendix E., were then interpreted using the Table of Key Lines in X-ray Powder
Diffraction Patterns of Minerals in Clays and Associated Rocks (Chen, 1977). Dominant
X-ray peaks identified the major minerals and clays constituting the samples.
Eleven additional sample depths were chosen for XRD as bulk material, some at
depths corresponding with the mounted slides. The sample depths included: 4880.7 ft,
4897 ft, 5051.5 ft, 5056.4 ft, 5061.65 ft, 5067.85 ft, 5073.5 ft, 5075.5 ft, 5454 ft, 5460.35
ft, and 5466. Twenty milligrams of these samples were collected and then crushed into
fine material using a ceramic mortar and pestle. The bulk samples were analyzed to
determine non-clay mineral species, such as quartz, plagioclase, and potentially, ironbearing minerals (Fig. 15).
39
RESULTS
Framework Mineralogy of the Upper Frio Formation
Petrographic analysis identifies the upper Frio “C” sandstone as a very fine to
fine-grained, poorly cemented, subangular to subrounded subarkose with mean
composition of Q70F24L6 (Fig. 7). Detrital quartz is predominantly monocrystalline (31%
±10) with polycrystalline quartz less abundant 6% (±3) (Figures 8 & 9). Monocrystalline
quartz shows predominantly straight extinction with vacuoles and inclusions being rare.
Embayed quartz is also present in trace amounts (Fig. 10). Feldspar is dominated by
orthoclase 8% (±5) (Fig. 11) followed by plagioclase 4% (±2) (Fig. 12) and trace amounts
of microcline. Feldspar grains are mostly unaltered although a small percentage of grains
show signs of sericitization and dissolution. Compositional zoning is also present in a
number of plagioclase grains (Fig. 13). Lithic fragments are dominated by volcanic rock
fragments 2% (±2). These grains have a microlitic to lathwork texture with plagioclase
forming laths (Figures 13 & 14). Chert is present as 1% (±1) of the whole rock volume;
trace amounts of metamorphic rock fragments are also present. Other minerals present in
trace amounts include biotite and heavy minerals. Whole-rock X-ray diffraction (XRD)
analysis confirms the petrographic analysis (Fig. 15).
40
~200
µm
Figure 7. Ternary plot of Frio Formation samples, based off of Folk‟s Classification
system (1974). Photomicrograph image illustrating typical Frio “C” sandstone
composition.
Q
Q
F
Q
Q
VRF
~200 µm
Figure 8. Photomicrograph
displaying dominant detrital grains: quartz (Q), feldspar (F), and volcanic rock fragments
(VRF). Arrows indicate point-contacts.
41
Q
Q
Q
~200 µm
Figure 9. Photomicrograph of detrital quartz grains.
A.
B.
Figure 10. Photomicrograph of embayed quartz grains (A & B); photo width ~ 200 µm.
42
Figure 11. Photomicrograph displaying an orthoclase feldspar grain; photo width ~200
µm.
P
K-Spar
VRF
VRF
Q
Q
~200 µm
Figure 12. Photomicrograph displaying dominant detrital grains: quartz (Q), plagioclase
(P), K-feldspar (K-Spar), and volcanic rock fragments (VRF). Arrows indicate pointcontacts.
43
P
Q
Pyr
CC
VRF
~200 µm
Figure 13. Photomicrograph display detrital quartz grains (Q), plagioclase with twinning
(P), pyrite (Pyr), and a volcanic rock fragment (VRF). Notice clay coats (CC) forming
around grains, post-dating mechanical compaction.
44
Clay coat
~200 µm
A.
~200 µm
B.
Figure 14. Photomicrograph of volcanic rock fragments displaying microlitic to lathwork
texture (A & B); photo widths ~200 µm.
Figure 15. X-ray diffraction graph of bulk sample material at depth 5075 ft. D-spacing
values range from 0-61Å at a 2θ scale.
45
Diagenesis & Authigenic Mineralogy
Diagenetic processes important in modifying sandstone after burial include
mechanical and chemical compaction, cementation, and dissolution. Cementation in the
upper Frio “C” samples is minor, with authigenic phases dominated by clay (I/S) and
pyrite, followed by minor quartz and feldspar overgrowths and calcite occurring late in
diagenesis (Fig. 16). The following sections will describe the diagenesis and authigenic
mineralogy of the upper Frio “C” sandstone at Liberty field, Texas.
DIAGENESIS
Compaction
EARLY
LATE
Clay Coats
Quartz
Overgrowth
Pyrite
Secondary
Porosity
Figure 16. Diagenetic reactions occurring in the upper Frio Formation “C” sandstones at
Liberty oil field, Liberty County, Texas.
Compaction
According to Makowitz & Milliken (2003), sediment compaction is an
“irreversible process that decreases intergranular space through increased packing of the
solid grain volume.” In the upper Frio Formation of Liberty County, Texas, mechanical
compaction precedes other diagenetic processes (Fig. 16). Lundegard (1991) states that
46
compaction is likely the dominant mechanism of porosity loss in sandstones. Dissolution
of framework grains and cementation are minor in the upper Frio “C” sandstoneindicating that mechanical compaction may be the most significant, if only minor, burial
diagenetic process to have affected the sandstone. Thin section analysis finds that point
contacts, long contacts, and floating grains are common in the upper Frio (Fig. 17 A &
B).
~200 um
A.
B.
~200 um
Figure 17. Photomicrographs displaying compaction features; point-contacts and longcontacts are indicated by arrows.
47
An expression of the space between framework grains, intergranular volume is a
valuable indicator of mechanical and chemical compaction in framework-supported
sandstones (Paxton et al., 2002). Intergranular volume is the sum of intergranular pore
space, intergranular cement, and depositional matrix and is calculated to establish a
relationship between compaction and cementation (Paxton et al., 2002). Mean
intergranular volume is calculated to be 28% (±5) in the “C” sandstones. Mechanical
compaction is the dominant diagenetic process affecting intergranular volume in the
upper Frio “C” sandstone, illustrated in Figure 17. Figure 18 displays the role of both
compaction and cementation in modifying intergranular volume (after Houseknecht,
1987). The diagonal line separates samples that have experienced greater compaction
(lower left) than cementation (upper right).
Figure 17 (A & B) illustrates thin section images typical of the unit, displaying
virtually no cementation and only a minor reduction of intergranular volume. A lessened
degree of diagenetic processes in the formation, indicated by minor cements and
compaction, will enhance the intergranular porosity as well as the sandstone reservoir
quality (Houseknecht, 1987).
48
Cement (%)
0
10
20
30
40
Intergranular Volume (%)
40
30
20
10
0
Figure 18. Intergranular volume of upper Frio “C” sandstones.
Data used to plot the IGV graph can also be used to calculate the percent original
porosity destroyed by compaction (eq. 1) as well as the percent original porosity
destroyed by cementation (eq. 2) using the following equations (Houseknecht, 1987):
40- intergranular volume/40 x 100
(eq. 1)
Cement/40 x 100
(eq. 2)
According to Houseknecht (1987), earlier classifications by Beard and Weyl (1973)
suggest that well- sorted sand, such as the upper Frio, reports a mean porosity of 40%,
thus the value used for these equations to calculate the percentage of original porosity
destroyed by diagenetic processes. The upper Frio “C” sandstone is calculated to have
49
lost 29.8% original porosity due to compaction and 0.13 % original porosity loss due to
cementation.
Authigenic Mineralogy
Clays
Clay is the dominant authigenic phase in the upper Frio “C” sandstone at Liberty
oil field (Fig. 19). Illite/smectite (I/S) was identified by XRD and SEM as the principal
clay, occurring as thin grain-coats, with minor amounts of kaolinite present (Figures 2022). Illite/smectite formed following compaction of the unit (Figures 13 & 14).
Figure 19. SEM image displaying I/S clay grain coats (arrows).
Geochemical analyses of upper Frio clay coats by EDAX identified the clay
coatings as having a sodium- alumina-silicate composition, with additional amounts of
50
potassium, indicative of I/S and kaolinite composition (Figures 20 & 21). EDAX also
identified the illite/smectite (Na-Al-Silicates) clays as displaying minor amounts (~1%)
of Fe.
Figure 20. SEM image and corresponding EDAX report of illite/smectite clay.
51
Figure 21. SEM image and corresponding EDAX report of illite/smectite clay.
52
Figure 22. XRD graph displaying the dominant clay phases acquired from analysis of
sample material from 5075 ft on an air-mounted slide. D-spacing values range from 032Å at a 2θ scale.
Pyrite
Distribution Frequency in Frio
Authigenic pyrite occurs ubiquitously throughout the upper Frio “C” sandstone
(Fig. 23). It ranges from 0- 12% total volume and averages 1% with abundance
increasing with depth (see Appendix D). Pyrite was principally visible in thin section,
occurring as solitary grains or massive collections intermixed with matrix (Fig. 24 A-E)
or on top of other framework grains (Figures 13 and 23). Pyrite is generally intermixed
with the framework minerals, but also commonly occurs lining the edges of pore spaces
53
(Fig. 24 A, D, E).
~200 µm
Figure 23. Photomicrograph displaying pyrite among detrital grains.
~150 um
Figure 24 A. Photomicrographs of pyrite in thin section.
54
~150 um
B.
~150 um
C.
~150 um
D.
55
~150 um
E.
Through microscopic investigation, pyrite was also found in-filling diatoms and a
solitary benthic foraminifera. The benthic foraminifera‟s test has been replaced by pyrite,
leaving only its chambers visible (Fig. 25 A , B). Multiple diatom species were found,
some frustules in-filled by pyrite, partially or completely (Fig. 26 A, B).
~200 µm
A.
56
B.
~200 µm
Figure 25. Photomicrograph of pyrite in-filling a benthic foraminifera (5076.6 ft).
A.
~200 µm
57
B.
~200 µm
Figure 26. Photomicrographs of pyrite in-filling diatoms.
Forms & Sizes
Two forms of authigenic pyrite are present in these samples 1) euhedral crystals
and 2) framboids (Figures 27 & 28). The pyrite appears to have formed within or on the
surface of the clays, after the clay has grown into the pore spaces (Fig. 16).
58
Figure 27. SEM image of euhedral pyrite grains.
Figure 28. SEM image of framboidal pyrite with unidentified covering (arrow).
(a) Solitary Euhedral Crystals
59
Solitary pyrite grains occur as euhedral crystals embedded in the clay coats and
postdating quartz overgrowths (Figures 16 & 27). Such occurrences indicate that pyrite
formation is a late-occurring diagenetic phase (Fig. 16). Euhedral grains in these upper
Frio “C” samples range in size from ~500 nm to 2µm.
(b) Framboidal
Pyrite octahedra in the Frio “C” sandstone pack together to form spherical
framboids, ranging in size from ~10-15 µm. Framboidal pyrite may occur individually or
as collections or aggregates of microcrystalline crystals (octahedra), all uniform in size,
attached to clay coats covering detrital sandstone grains (Figure 28, 29, 30, & 31 A-D).
Pyrite was not observed in XRD because it occurred (per slide analyzed) in quantities less
than 10% of the total rock volume, a detection limit set by the XRD instrument.
Although pyrite comprises less than 10% of the total rock volume of the upper Frio “C”
sandstone, the fine-grained nature of the individual octahedra making up the framboids
increases the overall surface area of the pyrite (Evangelou, 1995).
60
Figure 29. Framboidal pyrite
Figure 30. SEM image displaying a collection of octahedra pyrite grains, forming a
framboid; Scale reads 8 µm.
A.
61
B.
C.
62
D.
Figure 31. SEM images of framboidal pyrites attached to detrital grains, indicated by
arrows (A,B, & C.)
Unidentified covering
An unidentified covering is visible on select pyrite grains when viewed using
backscatter imaging on the SEM (Fig. 32). The majority of pyrite grains however, are
not associated with this unknown covering. SEM imaging did not indicate a source or
display the extent of this covering. Previous studies have observed similar coverings,
found both surrounding framboids and in-between individual octahedra; some authors
identify the coating as organic and others as mineral in origin, although the role of this
unidentified covering is still undetermined (Love et al., 1984; Sweeney & Kaplan, 1973;
Wilkin & Barnes, 1997).
63
Figure 32. SEM images of euhedral pyrite, partially obscured by an unidentified
covering.
Quartz & Feldspar Overgrowths
Quartz and feldspar overgrowths were minor, occurring as less than 1% of the
total rock volume. However, one suspected quartz overgrowth was identified through
SEM, at a depth of 5067.85 ft, with chemistry confirmed through EDAX (Fig. 33).
Additionally, microcrystalline quartz can occur on the surface of I/S clay coats (Fig. 34).
64
Figure 33. SEM image of quartz overgrowth indicated by arrow.
Figure 34. SEM image of microcrystalline quartz on the surface of I/S clay coats.
65
Secondary Porosity
Secondary porosity in the upper Frio “C” sandstone is minor 1% (±1), occurring
as a late diagenetic phase, evident in thin section (Fig. 16). Dissolution of feldspar varies
and is a minor influence on the formation‟s porosity; however, feldspar dissolution is
more common than feldspar overgrowths (Fig. 36) in these sandstones, enhancing
secondary porosity of the unit (Figures 35 & 37). Dissolution along the cleavage plane of
a feldspathic grain can be observed in the SEM image shown in Figure 35. Additionally,
a moderate percentage of feldspar grains display secondary porosity in thin section (Fig.
37). Low secondary porosity indicates it has had little influence on total porosity in these
sandstones. Lynch (1996) and Lynch & Land (1996) suggest that the dissolution of
detrital feldspars, rock fragments, and some authigenic cements are the last porosityaffecting diagenetic event to occur in the Frio Formation (Fig. 16).
66
Figure 35. SEM image of feldspar dissolution along cleavage planes; scale reads 100
µm.
K-spar
B
Figure 36. SEM image of potassium-feldspar overgrowth (K-spar) plus barite (B).
67
Figure 37. Photomicrograph displaying dissolution of a feldspar grain, creating
secondary porosity; photo width ~ 200 µm.
Minor Authigenic Phases
Small amounts of an unknown titanium-bearing phase and barite was identified
through SEM+EDAX analyses, occurring as secondary grains on the surface of detrital
minerals and clay coats (Figures 36 & 38). Additionally, halite was identified through
XRD (bulk rock analysis) and in SEM analyses (Figures 15 & 30). It occurs as a
secondary phase on grain surfaces and likely precipitated during core collection and/or
storage.
68
Figure 38. SEM image of barite found in upper Frio sample, sitting atop clay coats;
corresponding EDAX report.
69
Figure 39. SEM image of halite precipitated on quartz grains of the upper Frio, likely
developing during core collection and/or storage (5,075 ft); corresponding EDAX report.
Porosity & Permeability
Porosity and permeability in the upper Frio Formation are controlled by the
framework mineralogy and diagenetic history of the formation. Calculations from coreplug analysis indicate that the “C” zone has a mean porosity of 32 percent (±3) and mean
70
permeability of approximately 2-3 Darcy (±933) (Figure 4) (Kharaka et al., 2006). These
porosity and permeability conditions make the formation an ideal candidate for
sequestration of CO2. Adversely, matrix-rich samples display lower porosity and
permeability values, as shown in Figure 40.
Figure 40. Porosity versus permeability for the upper Frio “C” Formation; bar scale is
~200 µm.
Geochemistry of Pyrite
Geochemical analysis of upper Frio “C” sandstone pyrite was completed using
SEM+EDAX systems and electron microprobe. EDAX systems confirmed chemical
71
concentrations of Fe and S in each pyrite mineral viewed with SEM. EDAX
investigation also identified a small amount of Mn present in the upper Frio “C” pyrite
samples. However, SEM+EDAX investigation of Mn was limited; it was often difficult
to determine the exact source of the Mn with this equipment. Nonetheless, subsequent
chemical analyses using SEM+EDAX identified steady concentrations of Mn (1-2%) in
the pyrite samples. Figures 41-43 display SEM images of framboidal pyrite analyzed
with EDAX and the corresponding chemical report, displaying Fe, S, and Mn. Au and Pd
result from the Au-Pd coating used in analysis.
72
Element Weight Atomic
%
%
CK
2.78
8.38
OK
11.08
25.11
Al K
0.49
0.66
Si K
1.88
2.42
SK
30.88
34.91
Mn K
1.46
0.96
Fe K
36.84
23.91
Pd L
6.22
2.12
Au M
8.36
1.54
Totals
100.00
Figure 41. SEM image of framboidal pyrite and corresponding EDAX report.
73
Figure 42. SEM image of individual pyrite octahedra (comprising framboid) and
corresponding EDAX report.
74
Figure 43. SEM image of framboidal pyrite and corresponding EDAX report.
75
Composition of upper Frio pyrites, determined through electron microprobe
analysis, identified the major elements as Fe, S, and Mn (Table 1). Additional elements
analyzed (Mg, As, Se, Cu, Zn, Co, and Ni) were not identified. If said elements are
present, they are below the microprobe‟s detection limit. Other methods, such as
synchrotron µ-XRF and ICP-MS, may yield more accurate results for identifying trace
elements in pyrite (Berner et al., 2006).
Table 1. Pyrite composition determined by electron microprobe.
Sample 5069.85Depth 2
(ft):
Fe
32.16
5069.85-11
5469-12
5469-7
5469-1a
5469-5
Detection
Limit
33.18
31.42
31.36
28.82
29.98
0.07
S
66.75
66.67
65.66
66.00
67.12
66.42
0.09
Mn
1.04
0.12
2.82
2.54
4.03
3.51
0.07
Manganese was identified as a minor constituent of upper Frio “C” pyrite
compositions, ranging from less than the detection limit (.07) to 4.03 percent (See
Appendix C). Due to the extremely small size of the majority of the pyrite grains
comprising these samples, only large clusters or formations could be analyzed, using a
1µm beam. Microprobe investigation identified two phases of pyrite: those that are Mnrich and those that are not.
76
Mn was found to occur in the chemical composition of some euhedral forms of
pyrite, confirmed by electron microprobe analyses, though this was not the case in all
euhedral pyrites investigated. However, all framboidal pyrite investigated was
discovered to possess Mn as part of its chemical composition, confirmed through both
microprobe and SEM investigations (Figures 41-43). Electron microprobe analyses of
framboidal pyrite displayed varying amounts of Mn in numerous samples; nonetheless,
Mn is a chemical component of these pyrite forms. Chemical analysis (EDAX) of an
individual octahedra pyrite crystal, one of many to comprise a framboid, shows Mn in the
chemical composition as well (Fig. 42). Additional EDAX analyses of whole framboids
also returned Mn concentrations (Figures 41 & 43).
Fe and Mn concentrations of pyrites were identified with electron microprobe,
displayed below in Figure 44. Fe concentrations of analyzed pyrites average 33% (±2)
total pyrite composition, S at 66% (±1) and Mn at 1% (±1) total pyrite composition. Mn
concentrations appear to collect into three groups in Figure 44, the lowest with Mn
concentrations ranging from ~0-1.5%, the second 2-3%, and the highest ranging from
3.5-4%.
77
Mn: Fe
4.5
4
3.5
Mn %
3
2.5
Series1
2
1.5
1
0.5
0
0
10
20
Fe %
30
40
Figure 44. Pyrite composition determined through electron microprobe analysis.
78
DISCUSSION
Formation waters sampled during CO2 injection exhibited a decrease in pH and an
increase in Ca, HCO3-, and dissolved metals: Fe, Zn, Mn, Pb, and Mo (Kharaka et al.,
2006). The decrease in pH poses potential concerns to the unit, as it can alter diagenesis,
enhance dissolution of minerals, and create pathways in rock seals allowing for CO2
leakage or mobilization of trace metals. Without a clear explanation for the changing
geochemistry of the formation waters, Kharaka et al. (2006) hypothesized that the
increase in ions was caused by the dissolution of carbonates and iron oxyhydroxides.
Specifically, Kharaka linked the increases in HCO3- and Ca to the dissolution of calcite,
the increase in Fe to the dissolution of siderite, and the additional increases in Mn, Zn,
Pb, and Mo to the dissolution of iron oxyhydroxides.
Initial Pilot Experiment computer modeling did not use representative
mineralogical parameters of the unit, to help explain the geochemical results found.
Lacking petrographic study of the upper Frio Formation (the injection unit) at the
sequestration site combined with unexpected geochemical results from the initial
injection has prompted a need for reservoir characterization of the Frio Formation at the
injection site, in Liberty, Texas. Reservoir characterization of the upper Frio “C”
sandstone is extremely important for two reasons: 1) it provides a detailed knowledge
about the upper Frio Formation at this location along the upper Texas Gulf Coast, 2) it
79
provides detailed information regarding the injection unit of the Pilot Experiment,
used to understand geochemical changes occurring in the Frio, as a result of the CO2
injection and rock-water interaction in the formation.
Reservoir Characterization: Framework Mineralogy & Diagenesis
Essential to reservoir characterization, petrographic analysis identifies the upper
Frio “C” sandstone, of Liberty County, Texas, along the northern Texas Gulf Coast, as
subarkose. Subarkose classification contrasts from the Frio sandstones of the central and
southern Texas Gulf Coast that are typically more enriched in rock fragments (volcanic)
and plagioclase (Fig. 45), indicating a change in the unit‟s composition from the lower or
southern Texas Gulf Coast to the upper, or northern Texas Gulf Coast (Bebout et al.,
1978; Loucks et al., 1979; Grigsby & Kerr, 1991,1993).
80
Figure 45. Ternary diagram displaying upper Frio “C” sandstones at Liberty County (Δ),
compared to Middle Frio sandstones (X) (Middle Frio data from Grigsby & Kerr, 1991).
A regional variation in the Frio Formation mineralogy from the southern Texas
Gulf Coast to the northern Texas Gulf Coast, observed in the mineralogy of the upper
Frio “C” sandstone, has resulted in a different diagenetic history for the formation at this
Pilot Experiment location compared to other Frio locations along the Texas Gulf Coast.
The diagenetic history of the upper Frio at the South Liberty oil field has produced
mineralogical and reservoir characteristics unlike the southern and central Frio units
along the Texas Gulf Coast. The same diagenetic history that makes the formation (at
this location) an ideal reservoir has also introduced secondary minerals which may alter
reservoir conditions. The upper Frio‟s high porosity and permeability values at the South
81
Liberty oil field are likely associated with the minor diagenetic features (clay coats,
quartz overgrowths, and cement fabrics) discussed below.
Quartz and feldspar overgrowths and subsequent cements are minor occurrences
in these Frio “C” sandstones, possibly related to the relatively shallow depths in which
these samples were buried. Unlike what has been reported in studies focusing on the
lower and middle Frio, these upper Frio “C” sandstones show very little to no calcite
(Lynch & Land, 1996). The lack of calcite in these upper Frio “C” sandstones enhances
the reservoir quality. According to Kaiser (1985), calcite serves as a major inhibitor of
porosity and permeability in the Frio Formation. Porosity and permeability values are
controlled by framework mineralogy and diagenetic history; the lack of calcite as a
cement increases the porosity in the upper Frio- helping to establish the formation as a
high porosity/permeability reservoir capable of trapping hydrocarbons and CO2. Further,
the formation‟s lack of calcite at the injection site reduces the likelihood that it serves as
the source for the increases in HCO3- and Ca found in the sampled formation waters, as
suggested by Kharaka et al. (2006). Although, the interlayering mudstones cannot be
overlooked as a potential source of calcite. The combined absence of calcite, siderite,
and iron oxyhydroxides in the formation as authigenic or detrital phases (at the injection
site), combined with unexplained hydrochemical results found in Kharaka et al. (2006),
initiated a review of the formation‟s other authigenic phases, specifically pyrite.
82
Pyrite
Occurrence of Pyrite
Pyrite occurs ubiquitously throughout the Frio “C” sandstones sampled in this
research. Prior studies (Grigsby and Kerr, 1991, 1993) suggest that the middle Frio
Formation pyrite accounts for less than 2% of total rock volume. However, the upper
Frio “C” sandstone contains up to 12% pyrite (1% on average) throughout the sample
depths, indicating that geochemical conditions were ideal for the formation of pyrite in
these sands. Further, the presence of pyrite as either euhedral or framboidal variety
influences the total exposed surface area of pyrites in the unit.
Models used to simulate the Frio Pilot Experiment did not account for finegrained pyrites with increased surface areas (framboids) (Knauss, K.G., personal
communication, 2009). Pyrite was considered to be only a minor accessory mineral
(0.5%), thus, the model simulations did not reflect the role that increased amounts of
pyrite surface area may play with changing pH conditions due to CO2 sequestration
(Knauss et al., 2005). Knauss emphasizes that more coupled simulations, with real data
(measurements of fluids and rocks from the reservoir) are necessary to mimic the
conditions present in the injection formation (Knauss et al., 2005). Therefore, the
modeling parameters for the initial experiment need to be re-evaluated to provide an
accurate estimation of the formation‟s response to increased amounts of CO2.
Upper Frio Formation pyrite is abundant in SEM and thin section analysis,
observed as large collections, framboids, and individual euhedral grains attached to other
framework minerals, intermixed with matrix material, as well as filling in pore spaces,
83
foraminifera, and diatoms. The multiple forms and sizes of pyrite present at all depths
sampled indicate widespread development, in varying geochemical environments, which
allow for variation in form and shape (see Figures 23-31).
Pyrite is known to form in one of four main chemical environments by 1) the
replacement of S-rich mineral phases such as mackinawite or greigite (Sweeney &
Kaplan, 1973), 2) the oxidation of aqueous Fe (II) monosulphides by H2S (Butler &
Rickard, 2000; Berner, 1970), 3) through biogenic processes involving bacteria or
fossilization (Berner, 1970; Raiswell, 1997) and 4) inorganically in areas of hydrothermal
activity (Shikazono, 1994). Chemical environments may vary based upon differences in
biogenic or bacterial sulphate reduction, or organic matter oxidation resulting in products
(Fe2+, Mn2+) which give rise to mineral precipitation and mineralogical replacement. Fe
and S saturation states play a distinct role in the formation of pyrite, and the degree of
supersaturation can determine the pyrite morphology (Ohfuji and Rickard, 2005; Wang
and Morse, 1996; Park et al., 2003). Specifically in the Frio, the abundance of pyrite
indicates that S production was likely greater than Fe production in the formation‟s
porewaters, therefore causing all of the Fe (II) present to form pyrite (Taylor, 1998).
The geochemical environment likely to have occurred in the upper Frio would
have included reducing conditions containing significant amounts of iron and sulfur in
diagenetic porewater. This would have induced diagenetic pyrite precipitation, at which
time the pyrite chemistry was able to uptake elements (for example, Mn) present in its
surrounding depositional environment (Taylor, 1998).
At shallow burial depths, diagenetic pyrite is thought to form by reduction of
sulphate, which is dissolved in porewaters, to H2S by S-reducing bacteria, followed by
84
the reaction between H2S and iron to form pyrite (Berner, 1969, 1970, 1984; McConville
et al., 2000; Huerta-Diaz & Morse, 1990). Thus, there are three factors controlling how
much pyrite can form in sediment: the amount of organic matter as well as reactive iron
minerals available in the sediment and the availability of dissolved sulphate (Berner,
1982,1984).
Alternatively, a study by Worden et al. (2003) suggests that H2S present in clastic
oil and gas fields may precipitate as a late-diagenetic pyrite in clean sandstones with
limited iron availability. Such growth may reduce H2S concentrations in reservoirs. This
finding has the potential to relate to the pyrites found in the Frio Formation of Liberty
County, Texas, as the reservoir has been a known oil and gas producer for decades.
Additional research is required for making any conclusive comparisons to this study.
Similarly, Schumacher & Abrams (1996) indicate that in a petroleum province, hydrogen
sulfide gas (H2S), anaerobic bacterial activity, or the oxidation of petroleum in the nearsurface can all serve as a major sources of sulfur. As these sulfur sources are combined
with an iron source and reducing conditions, pyrite can precipitate, creating pyrite
alteration zones in petroleum fields. The extent of hydrocarbon-induced alteration zones
related to previous oil and gas production of the area (Liberty County, Texas) is
unknown.
Pyrite found in the Frio “C” unit occurs in two forms, or textures, a result of
different modes of formation. The resultant texture of pyrite is a function of the Eh of the
initial reaction system, or environment of formation; solitary euhedral crystals dominate
at low Eh and framboids at high Eh, a function of crystal growth and nucleation (Butler
& Rickard, 2000; Sweeney & Kaplan, 1973; Wilkin & Barnes, 1997; Ohfuji & Rickard,
85
2005). At higher Eh, the reaction solution sits within the stability field of pyrite, allowing
pyrite to nucleate rapidly from a supersaturated environment (in respect to both pyrite
and iron sulphides), producing framboids. Lower Eh reduces pyrite supersaturation, or
supersaturation of the pore water with respect to iron sulphide, and nucleation slows,
allowing for coarse-crystalline euhedral pyrite precipitation (Butler & Rickard, 2000;
Taylor & Macquaker, 2000). Additionally, Butler & Rickard (2000) believe that
framboidal pyrite specifically favors low pH and high Eh conditions, similar nucleation
ideas also proposed by Sweeney & Kaplan (1973) and Wilkin & Barnes (1997). The
chemical and physical properties of both of these pyrite textures can provide details of
past environmental conditions, present at the time of (pyrite) precipitation.
Raiswell and Plant (1980), Sweeney and Kaplan (1973), Wilkin et al. (1996), and
Wilkin and Barnes (1997) suggest that framboidal pyrite forms as a replacement of S-rich
mineral phases such as mackinawite (FeS) and greigite (Fe3S4) likely at the oxic-anoxic
interface, the product of oxidation of FeS by H2S in aqueous solutions; whereas euhedral
pyrite can form without these metastable intermediates (See figure in R&P 1980). Wilkin
& Barnes (1997) later redefined the role of greigite, suggesting that it occurs as a
precursory framboid which is then pyritized. More recently, Butler & Rickard (2000)
declare that framboid formation can also occur without a metastable intermediate such as
greigite, but rather from the oxidation of aqueous iron (II) monosulphide (FeS(aq)) by H2S.
In such cases, FeS(aq) is an “electroactive, dissolved species and represents cluster
complexes of quantum-sized particles of FeS,” as described by Butler & Rickard (2000).
Further, these authors conclude that framboidal textures develop from rapid nucleation in
environments supersaturated with pyrite, whereas single, euhedral pyrite crystals develop
86
in environments of low pyrite supersaturation and limited nucleation (Butler and Rickard,
2000). The uniform-size nature of octahedra comprising framboids also indicates that the
octahedra of the framboid nucleated simultaneously and growth remains constant until
aggregation (Wilkin et al., 1996). Some researchers even suggest that some euhedral
pyrite may be a replacement of earlier framboids, with the initial framboid serving as a
nucleation site for euhedral pyrite crystals (Raiswell & Plant, 1980; Wilkin & Barnes,
1997; Love & Amstutz, 1966). However, Raiswell (1982) suggests that euhedral pyrite
may also form by reaction between mackinawite and elemental sulfur. Both massive and
euhedral pyrite may form by direct precipitation of Fe2+ (ferrous ions) by polysulfides
(Rickard, 1970).
It has been suggested that the two forms of pyrite, framboidal and euhedral, have
experienced different growth mechanisms, dependent upon their individual iron source,
or availability of iron within the sediment (Raiswell & Plant, 1980; Raiswell, 1982).
Growth of framboidal pyrite is possible as long as local iron sources remain available
within the sediments, a product of dissolved sulphides in the presence of abundant iron
oxides (Raiswell & Plant, 1980; Raiswell, 1982). As local iron sources are depleted,
framboid formation ceases and the iron source is dependent on host sediment via pore
waters, forming euhedral pyrites. Rather, euhedral pyrite precipitates directly from
porewaters as FeS2 saturation levels are reached (Taylor & Macquaker, 2000).
Decreasing saturation levels of iron sulfides have a kinetic influence on pyrite texture,
changing from framboidal to euhedral pyrite crystallization (Raiswell, 1982). Thus,
framboidal pyrites tend to be restricted to early stages of diagenesis and fine-grained in
nature, resulting from rapid mineral growth whereas euhedral pyrites are more crystalline
87
and result from slower growth rates (Raiswell & Plant, 1980; Wilkin et al., 1996; Butler
& Rickard, 2000). This suggests that the pyrites of the upper Frio “C” sands have
precipitated in multiple phases, based on different textures and growth conditions.
Geochemistry of Pyrite
Manganese was the dominant trace element identified in pyrite samples through
electron microprobe analyses. SEM+EDAX analyses also identified Mn to be associated
with pyrite (see Figures 41-43). Chemical confirmation provided by electron microprobe
has lead to the theory that pyrite of the upper Frio “C” Formation (Liberty County, TX)
served as a sink for Mn. Based upon the idea that iron sulphides are known to act as
sinks for metals within marine sediments, incorporating metals during mineral
precipitation, the pyrite present in the Frio is likely the source of the Mn discovered in the
formation waters from the Frio Pilot Experiment (Taylor & Boult, 2007; Huerta-Diaz &
Morse, 1990). Electron microprobe analysis confirms that pyrite serves as a source for
the Fe and Mn sampled in the Frio Pilot Experiment formation waters.
Raiswell & Plant (1980) further suggest that the different pyrite morphologies and
modes of formation likely result in different trace element compositions, even in local
collections. Multiple studies of pyrite composition, from a variety of depositional
environments, have identified/associated manganese as a trace element of the mineral
(Mitchell, 1967; Dellwig et al., 2002; Mercer et al., 1993; Cambel & Jarkovsky, 1967;
Raiswell & Plant, 1980; Shikazono et al., 1994; Huerta-Diaz & Morse, 1990).
Depositional environments studied include shales, peats, Ordovician sediments, Jurassic
mudstones, hydrothermal areas and convergent plate boundaries, metamorphic, volcanic,
88
marine and even modern freshwater sediments. It is generally agreed upon that during
pyrite formation, the mineral will incorporate any available elements associated with its
(depositional) environment. As authigenic pyrite forms in a reducing environment, iron
is drawn into solution and other elements present in the surrounding sediments may be
incorporated into the growing sulphide (Mitchell, 1967; Cambel & Jarkovsky, 1967).
Raiswell & Plant (1980), however, believe that the fine-grained nature of framboidal
pyrite, formed by rapid growth, favors the uptake of large concentrations of Mn, Ni, Co,
and Zn from local iron hydroxide sources by adsorption, occlusion and solid solution.
Although, iron hydroxides were not observed in the upper Frio “C” sands, it is likely they
were present at the time of deposition and provided metals during framboidal pyrite
precipitation. Whereas euhedral pyrites, coarse-crystalline in nature due to slower
growth rates, are more likely controlled by Fe supply from external sources, carried
through the formation sediment by porewaters. However, the overall behavior of trace
elements (and the up-take of trace elements) during diagenesis is poorly understood and
requires additional research to confirm the differences in trace element assemblages of
framboidal and euhedral pyrite (Raiswell & Plant, 1980).
Mitchell (1967) stated that the trace element composition of any environment is
highly variable, thus, the trace element content of authigenic pyrite is controlled by the
trace element content of surrounding sediments of the environment of formation
(Raiswell & Plant, 1980). Additionally, the composition of the sediment at depth
controls the porewater distribution of elements such as Fe and Mn (Taylor & Boult,
2007). Therefore, pyrite composition is dependent upon the environment and sediment in
89
which it is formed, and may provide accurate indications of the environment at the time
of formation.
Additional studies suggest that Mn in pyrite may occur either as a substitute for
Fe or S or may be associated with minerals (vivianite, siderite) which pyrite has replaced
(Mitchell, 1967; Roberts, 1982; Taylor & Boult, 2007; Taylor, 1998). In a trace element
study by Roberts (1982), it was suggested that pyrites with higher Mn, Ti, and Ni
contents reflect diagenetic or metamorphic pyrite, aligned with earlier theories suggesting
that pyrite trace element chemistry may distinguish types of mineral occurrences,
specifically base metal deposits, in specific environments. This idea links the mode of
pyrite formation and occurrence to its chemistry. Roberts (1982) also suggests that the
occurrence of Co, Mn, Ni and Ti in pyrites reflects substitution of Fe by these elements,
as both Mn and Ni were found to have effectively replaced Fe in pyrite crystals of a
diagenetic and metamorphic origin. Such reasoning combined with the lack of siderite
and vivianite (minerals commonly replaced by pyrite) in the upper Frio provides support
that the Mn identified in electron microprobe occurs as a substitute for Fe in the pyrite, as
the upper Frio pyrite is diagenetic in origin. Further, the additional elements tested for
with the electron microprobe, including Co and Ni, could be present in these pyrites
however, their concentrations are below the detection limit of the electron microprobe.
Further, Huerta-Diaz & Morse (1990) conclude that most trace metals are generally too
low in concentration to be quantitatively determined through SEM+EDAX systems or
microprobe analysis.
90
In a study of Holocene coastal peats, Dellwig et al. (2002) state that in freshwater
systems, high amounts of Fe and Mn are supplied by the detrital material of the
environment. In bottom mud-waters, a reducing anoxic environment occurs,
synonymous with pyrite and peat formation. Trace metals carried into the environment
are commonly incorporated into authigenic minerals like pyrite; pyrite-rich layers of
sediment (peat) are often associated with layers of tidal-channel clastic sediments of
marine influence. Further, Mn has the potential to be incorporated into pyrite, with no
clear distribution, and may be affected by diagenetic processes occurring in the
depositional environment (Dellwig et al., 2002).
A study of trace metal concentrations in sedimentary pyrite from the Gulf of
Mexico was able to establish a relationship between the transfer of Fe and trace metals to
the pyrite phase by comparing the degree of pyritization (DOP) with degree of trace
metal pyritization (DTMP), a method developed by the authors, Huerta-Diaz & Morse
(1990). These authors suggest that trace metals such as metal oxides and organic matter
may co-precipitate with iron sulfide minerals in reducing marine environments. Citing
previous studies of sedimentary framboids, Huerta-Diaz & Morse (1990) agree with the
idea that Mn may precipitate as a solid solution with iron sulfides based on their analysis
of DTMP of sedimentary pyrites. Huerta-Diaz & Morse (1990) propose that the chemical
behaviors of trace metals are strongly influenced by the authigenic iron sulfide minerals
and hydrogen sulfide concentrations in such environments. Mn and Ni trace metals in
these sedimentary pyrites showed a linear increase in DTMP with increasing DOP.
Huerta-Diaz & Morse (1990) believe that the linear increase can be attributed to
91
hydrogen sulfide, stating that increased hydrogen sulfide concentrations will increase the
transfer of transition metals from a reactive state to the pyrite phase precipitating.
Therefore, the hydrogen sulfide concentration can allow for the co-precipitation of trace
metals with iron or manganese oxides or re-precipitate them as solid sulfides (into the
pyrite phase). Further, it was observed that pyrite containing Mn greatly increased in
concentration with depth.
In summary, it is suggested that the depositional environment‟s mineralogy may
control the porewater (chemistry) during early diagenesis, and therefore, the overall
geochemistry of the formation. Thus, if sediment and subsequent porewater provide a
source for ions (Fe, Mn), minerals precipitating from this geochemical environment will
reflect the conditions of the environment at the time of formation. In the upper Frio, at
some point during diagenesis, a sedimentary source and/or the porewater, likely supplied
the S and Fe needed for iron sulphide precipitation, which included uptake of Mn (Fe
substitution) during pyrite formation.
Pyrite Alteration
Microprobe analysis confirmed that pyrite is serving as a sink/source for Mn.
This holds significant value for the Frio Pilot experiment as well as general petrographic
study and knowledge of the upper Frio Formation. However, no clear explanation exists
for how the Mn present in the pyrite is being released into the formation porewater
sampled at the time of CO2 sequestration. Abiotic oxidation, one idea for the breakdown
of pyrite within the Frio, is likely occurring, although no study has been completed
regarding pyrite or pyrite oxidation within the upper Frio Formation.
92
In oxic conditions, at low pH (<3.5), biotic pyrite oxidation occurs via ironoxidizing bacterium, T. ferrooxidans, which accelerates the oxidation of pyrite by
oxidizing Fe2+ to Fe3+, demonstrated below (Evangelou, 1995):
FeS2 + 14Fe3+ + 8H2O→ 15Fe2+ + 2SO42-+ 16H+
O2
Iron-oxidizing bacteria
This bacterium is considered to be primarily responsible for the rapid oxidation of pyrite
in acid mine waste at low pH, accompanied by atmospheric O2, which oxidizes Fe2+ to
Fe3+ to create a continuous oxidation cycle (Evangelou, 1995). However, the pH
recorded during the Pilot Experiment did not drop to values lower than 5.7 and the
anoxic, methane-saturated environment (Knauss, K.G., personal communication, 2009)
of the Frio Formation makes the oxidation of pyrite by this type of bacteria an unlikely
cause.
Research by Evangelou et al. (1998) suggests that the addition of CO2 to pyrite
will enhance pyrite surface complexes, converting Fe2+ to an Fe(II) HCO3+ complex,
which increases the oxidation rate of the pyrite (in the presence of H2O2). Thus,
nonmicrobial pyrite oxidation is promoted by accelerating Fe(II) oxidation, enhanced by
the Fe(II) HCO3+ surface complex (Evangelou, 1995). Evangelou et al. (1998) conclude
that the influence of solution HCO3- enhances pyrite oxidation through oxidation of Fe2+
and rapid regeneration of Fe3+. Precipitation of Fe3+ (rapidly oxidized from Fe2+ in the
presence of CO2) serves as a pyrite oxidizing agent (Evangelou, 1995). Hydrochemical
93
results from the Frio Pilot Experiment indicate that CO2 injection resulted in an increase
in HCO3- concentrations (Kharaka et al., 2006) by the reaction:
CO2 (g)+ H2O ↔ CO2 (aq)+ H2O ↔ HCO3-+ H+
(Hovorka et al., 2003).
It was also observed at the sampling site at the time of the Frio Pilot Experiment
that waters sampled were rusty-brown (Hovorka, S.D., personal communication, 2008),
indicating the presence of Fe2+ in solution within the formation, becoming oxidized with
exposure at the surface. This leads to the hypothesis that Fe2+ (and other metals) is being
released into the “C” sandstone of the upper Frio Formation during CO2 injection through
the development of Fe(II) HCO3+ surface complexes. Whether these complexes play a
significant role in a reducing, methane saturated environment is for future study.
In any case, the presence of pyrite, particularly framboidal pyrite with its known
high surface area and compositional complexity (Evangelou, 1995; Mercer et al., 1993),
provides a source for the reported increase in Fe and Mn (and other metals) found during
the Frio Pilot Experiment (Kharaka et al., 2006). The drop in pH and release of metals
into the porewaters is a concern, as this could alter the formation‟s geochemical
environment, perhaps inducing or preventing additional diagenetic activities.
Potential dissolution of the overlying caprock- the Anahuac Shale, carbonates,
and other minerals, caused by changing geochemistry (dropping pH) could ultimately
create pathways in the rock seals, allowing CO2 and brine leakage (Kharaka et al., 2006).
Mineral dissolution mobilizing toxic trace metals or toxic organic compounds where
residual oil or suitable organics are present would migrate with the CO2 plume, posing
94
geologic and environmental concerns (Kharaka et al., 2006). Accurate reservoir
characterization combined with geochemical modeling is essential to understanding the
impact of CO2 injection and the successful sequestration of CO2 in subsurface reservoirs.
95
CONCLUSIONS
1. The upper Frio “C” sandstone is a very fine to fine-grained, poorly cemented,
subangular to subrounded subarkose with mean composition of Q70F24L6.
2. During burial diagenesis, mechanical compaction processes dominated over
cementation, however, only minor compaction and cementation occurred. Primary
porosity, through point count analysis, averages 23% (±11). Intergranular volume
averages ~28%.
3. Calculations from core-plug analysis indicate that the “C” zone has a mean
porosity of 32 % and mean permeability of approximately 2-3 Darcy (Kharaka et al.,
2006). The Frio Formation‟s high porosity and permeability values display a linear trend
and make the formation an ideal reservoir unit for sequestration of CO2.
4. Euhedral and framboidal pyrite occurs ubiquitously in the upper Frio
Formation “C” sandstones and serves as the source for Mn.
5. Injection of CO2 into the formation creates an increase in HCO3- and
potentially the formation of Fe(II)HCO3- surface complexes, resulting in the release of
Fe2+ and other metals such as Mn, into solution. This resulted in the increase in Fe and
Mn during the Frio Pilot Experiment.
6. The addition of CO2 alters the geochemical environment of the upper Frio,
causing mineralogical and geochemical changes within the unit that need to be predicted
and understood before large scale sequestration occurs.
96
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104
Appendix A. Point-count Data
105
Appendix B. Compositional analysis determined by point count
Sample
5051.5
Quartz
K-Feldspar
Rock
Fragment
Plagioclase
Other
Pyrite
5071.75
34
11
4
5
2
0
5052.90
5054.7
5054.70
5056.4
5061.65
40
0
34
4
40
6
46
3
49
0
48
1
45
13
2
2
0
0
2
8
0
0
2
8
0
0
5
5
0
0
3
5
1
0
7
4
0
0
5
3
0
0
5061.65#2 5062.4
42
11
4
1
0
0
5052.9
44
12
3
6
0
0
5062.40
5065.85
5067.85
5069.85
5069.85#2
45
19
5
3
1
0
47
18
4
1
0
0
38
14
2
4
0
0
41
12
2
3
0
3
40
9
3
9
0
0
5071.75#2 5073.5
34
8
3
5
2
1
38
9
3
5
3
1
5075.5
35
11
5
5
3.25
1.5
5050
5056
3
6
0
1
0
12
5057
42
11
4
5
0
0
38
9
1
7
0
0
106
5066
5076.60
41
13
3
6
0
1
0
0
0
0
0
7
5469
26
10
2
9
2
5
107
Appendix C. Electron Microprobe Data
Fe
Mn
S
Total
Depth
5050-3
5050-4
5050-5
5069.85-1
5069.85-2
5069.85-3
5069.85-4
5069.85-5
5069.85-6
5069.85-7
5069.85-8
5069.85-9
5069.85-10
5069.85-11
5069.85-12
5469-1
5469-1a
5469-2
5469-3
5469-4
5469-5
5469-6
5469-7
5469-8
5469-9
5469-10
5469-11
5469-12
5469-13
5469-14
5469-15
5469-16
5469-17
5469-18
Avg Totals:
33.8951
0.0414
65.98
34.0839
0.0013
65.7115
41.9432
0.4801
52.5158
33.468
0.1545
66.332
32.1558
1.0367
66.7492
33.5438
0.1133
66.306
33.7846
0.5468
65.566
33.6685
0.2552
65.9489
32.9045
0.7998
66.21
32.1947
1.5057
66.1959
78.7558
0
3.1728
33.3243
0.4348
66.2393
33.4617
0.957
65.3121
33.1809
0.116
66.6703
33.282
0.0478
66.5729
36.0069
2.139
43.7902
28.8168
4.0255
67.124
31.2176
2.3848
66.2498
31.6029
2.1994
66.1176
31.5967
2.3223
66.0242
29.9758
3.5099
66.4243
30.9594
2.2797
66.6912
31.3581
2.5405
66.0037
37.5055
0.8349
61.6376
30.7929
3.8152
65.2811
31.5205
2.0061
66.4628
32.358
1.5799
66.0275
31.4184
2.8218
65.6582
31.9074
2.2477
65.798
34.5825
0.3605
64.9311
33.9099
1.3067
64.6198
33.019
0.61
66.333
32.3297
1.3177
66.2873
32.3358
1.3046
66.2483
34.875 1.56883611 60.1032278
98.5086
97.3352
10.2393
99.7139
100.592
99.0788
98.5919
99.196
91.7998
97.0318
0.5295
101.6474
94.9358
100.932
100.4828
1.6556
101.2807
99.894
100.7043
100.446
101.1178
101.441
96.674
56.9386
97.1661
101.3806
98.0132
98.8788
93.7104
92.1509
92.7528
101. 1510
100.5833
100.74
83.8794286
108
Appendix D. Pyrite frequency in the upper Frio “C” Formation
Pyrite Frequency in upper Frio Formation,
Liberty County, Texas
25
Frequency
20
15
10
Pyrite
5
0
5050
5055
5060
5065
Sample Depth (ft)
5070
5075
5080
109
Appendix E. XRD graphs
110
111
112
113
114
115