Lake-type controls on petroleum source rock potential in nonmarine

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Lake-type controls on
petroleum source rock potential
in nonmarine basins
Alan R. Carroll and Kevin M. Bohacs
ABSTRACT
Based on numerous empirical observations of lacustrine basin strata,
we propose a three-fold classification of lacustrine facies associations that accounts for the most important features of lacustrine
petroleum source rocks and provides a predictive framework for
exploration in nonmarine basins where lacustrine facies are incompletely delineated.
(1) The fluvial-lacustrine facies association is characterized by
freshwater lacustrine mudstones interbedded with fluvial-deltaic
deposits, commonly including coal. Shoreline progradation dominates basin fill, resulting in the stacking of indistinctly expressed
cycles up to 10 m thick. In map view, the deposits may be regionally
widespread but laterally discontinuous and contain strong facies
contrasts. Transported terrestrial organic matter contributes to
mixed type I–III kerogens that generate waxy oil (type I kerogen is
hydrogen rich and oil prone; type III kerogen is hydrogen poor and
mainly gas prone). The Luman Tongue of the Green River Formation (Wyoming) and the Honyanchi Formation (Junggar basin,
China) provide examples of this facies association, which is also
present in the Songliao basin of northeastern China, the Central
Sumatra basin, and the Cretaceous Doba/Doseo basins in westcentral Africa.
(2) The fluctuating profundal facies association represents a
combination of progradational and aggradational basin fill and includes some of the world’s richest source rocks. Deposits are regionally extensive in map view, having relatively homogenous
source facies containing oil-prone, type I kerogen. Examples include
the Laney Member of the Green River Formation (Wyoming), the
Lucaogou Formation (Junggar basin, China), the Bucomazi Formation (offshore west Africa), and the Lagoa Feia Formation (Campos basin, Brazil).
(3) The evaporative facies association represents dominantly
aggradational fill related to desiccation cycles in saline to hypersaline lakes and may include evaporite and eolianite deposits. Sublittoral organic-rich mudstone facies are relatively thin but may be
Copyright !2001. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received December 15, 1998; revised manuscript received June 28, 2000; final acceptance
August 31, 2000.
AAPG Bulletin, v. 85, no. 6 (June 2001), pp. 1033–1053
1033
AUTHORS
Alan R. Carroll ! Department of
Geological and Geophysical Sciences,
University of Wisconsin, 1215 W. Dayton St.,
Madison, Wisconsin, 53706;
[email protected]
Alan R. Carroll has been an assistant professor
at the University of Wisconsin, Madison, since
1996, specializing in sedimentary basins in
western China and the western United States.
He received geology degrees from Carleton
College (B.A. degree, 1980), the University of
Michigan, Ann Arbor (M.Sc. degree, 1983),
and Stanford University (Ph.D., 1991). He
worked as an exploration geologist for Sohio
(1983–1986) and a research geologist for
Exxon Production Research Company (1991–
1995). He is an associate editor of the AAPG
Bulletin.
Kevin M. Bohacs ! ExxonMobil Upstream
Research Co., 2189 Buffalo Speedway,
Houston, Texas, 77252;
[email protected]
Kevin M. Bohacs is a sedimentologist and
stratigrapher with the Petroleum Geochemistry
section of ExxonMobil Upstream Research
Company (URC) in Houston, Texas. He
received his B.Sc. (honors) degree in geology
from the University of Connecticut in 1976
and his Sc.D. degree in experimental
sedimentology from M.I.T. in 1981. At URC, he
leads the application of sequence stratigraphy
and sedimentology to organic-rich rocks from
deep sea to swamps and lakes in basins
around the world. As a research associate, his
primary focus is to keep the geo- in
geochemistry, integrating field work,
subsurface investigation, and laboratory
analyses. He has written numerous articles on
the stratigraphy and sedimentology of
mudrocks, hydrocarbon source rocks, and
lake systems. He was co-recipient of the AAPG
Jules Braunstein Memorial Award for best
poster session paper in 1995 for work on coal
sequence stratigraphy and the AAPG
International Paper Award in 1998 and was
an AAPG Distinguished Lecturer for 1999–
2000.
ACKNOWLEDGEMENTS
This article benefited from discussions with
many individuals, including S. C. Brassell, Y. Y.
Chen, R. Cunningham, D. J. Curry, G. Genik,
K. S. Glaser, G. J. Grabowski Jr., G. B. Hieshima, G. H. Isaksen, B. J. Katz, M. R. Mello, K.
Miskell-Gerhardt, J. E. Neal, P. Olsen, D. J.
Reynolds, C. Scholz, and K. O. Stanley. G. J.
Grabowski and H. B. Lo collected some of the
data used in this article. M. Wartes provided
useful comments on an early draft of the
manuscript. Field studies in the Junggar basin
in 1987, 1988, and 1992 were funded by the
Stanford-China Industrial Affiliates, a group of
companies that included AGIP, Amoco, Anschutz, BHP Petroleum, Chevron, Conoco, ElfAquitaine, Enterprise Oil, Exxon, Mobil, Occidental, Pecten, Phillips, Sun, Texaco,
Transworld Energy International, and Unocal.
Conoco provided additional support for this
article. The Donors of The Petroleum Research
Fund, administered by the American Chemical
Society, also provided support for this research. We thank Exxon Production Research
Co. for permission to publish this article. J. A.
Curiale, B. J. Katz, B. Wiggins, and an anonymous reviewer all provided very helpful
reviews.
1034
Lacustrine Petroleum Source Rocks
quite rich and widespread. The highest organic enrichment coincides with the deepest lake stages. Low input of land plant organic
matter results in minimal lateral contrasts in organic content. In
some cases a distinctive type I-S (sulfur-rich) kerogen may generate
oil at thermal maturities as low as 0.45% vitrinite reflectance equivalent. Examples include the Wilkins Peak Member of the Green
River Formation (Wyoming), the Jingjingzigou Formation (Junggar
basin, China), the Jianghan and Qaidam basins (China), and the
Blanca Lila Formation (Argentina).
INTRODUCTION
The deposits of nonmarine sedimentary basins account for a growing segment of current petroleum exploration and exploitation opportunities, especially in areas of rapid market growth such as
China, southeast Asia, and western Africa. The specific techniques
needed for locating, assessing, and developing hydrocarbon reserves
within lacustrine basins remain relatively undeveloped compared
to those for marine systems, and they constitute a significant source
of financial risk (cf. Sladen, 1994). This uncertainty stems not from
a lack of data on lacustrine systems, but instead from their great
sedimentologic complexity, as evident from numerous studies of
modern and ancient lakes. Furthermore, most lacustrine source rock
models have focused on inferred climatic controls (e.g., Eugster and
Kelts, 1983; Talbot, 1988), although paleoclimate modeling has
had only limited success in predicting the actual occurrence of
organic-rich lacustrine facies (e.g., Barron, 1990).
Fortunately, some of the complexity evident in modern lakes
resolves itself in ancient lacustrine deposits, resulting in the expression of three commonly recurring motifs in the lithology and
stratigraphy of lacustrine deposits. For example, Bradley (1925) was
able to generalize a trend within the heterogeneous Eocene Green
River Formation (Utah, Colorado, and Wyoming) from “relatively
shallow fresh-water lakes,” to “depositional basins that alternately
flooded and evaporated either partially or completely,” and finally
to a period when “the lakes became playa-like” and evaporites precipitated. A similar division, based on geography rather than on
geologic time, was proposed over 60 years later by Olsen (1990)
for the widespread lacustrine deposits of the Triassic–Jurassic Newark Supergroup (eastern United States). Olsen idealized lacustrine
facies associations as either Richmond-type, Newark-type, or
Fundy-type and proposed that these associations reflect differences
in climatically controlled basin hydrology. Mello et al. (1988) developed similar concepts involving organic facies, subdividing Brazilian source rocks into freshwater and brackish-saline lacustrine
facies based on relative concentrations of biological marker compounds that are sensitive to lake salinity and organic matter input.
Others have characterized biomarker distributions indicative of
hypersaline lacustrine source rocks (e.g., Jiang and Fowler, 1986).
Carroll and Bohacs (1995, 1999) proposed a classification of ancient
lake basins into three end-member types that correspond to commonly recurring lacustrine facies associations. The terms “overfilled,” “balanced-fill,” and “underfilled” lake basins refer to the balance between
tectonically controlled potential accommodation and
climatically controlled water plus sediment supply. A
unique advantage of this model is that it permits broad
characteristics of hydrocarbon source rock richness and
gas vs. oil generation to be predicted from limited geologic observations of nonsource sedimentary facies
(typically encountered on structure and along basin
margins). This information can also be incorporated
into basin thermal history models to estimate the timing of generation.
Several previous studies have presented overviews
of lacustrine petroleum source rocks (e.g., Powell,
1986; Katz, 1988, 1990; Kelts, 1988; Mello et al.,
1988; Mello and Maxwell, 1990; Williams et al.,
1995). This article is the first to classify petroleum
source rocks in accordance with the lake-type classification of Carroll and Bohacs (1995, 1999), and one of
very few to systematically integrate detailed observations of lacustrine sedimentary facies with geochemical
data on organic facies. We emphasize detailed, nonbiased sampling of carefully described stratigraphic intervals as the basis for interpreting the geologic controls
on organic enrichment. In contrast to previous work,
the model we present suggests specific, practical techniques for predicting source rock richness, distribution,
oil vs. gas potential, and generation timing based on
sparse geochemical or geologic data. Conversely, our
model permits broad aspects of the tectonic and climatic history of nonmarine basins to be predicted in
part from the character of petroleum and of source
rock extracts. Such data may be very useful in assessing
other exploration play elements, such as reservoir distribution and quality, migration pathways, fault timing, and basin thermal history.
Two important intervals of lacustrine strata are
used for illustrative purposes in this article: the
carbonate-rich Eocene Green River Formation of the
Washakie basin (Wyoming) and the predominantly silicic Upper Permian lacustrine formations of the Junggar basin (northwest China). Despite gross differences
in geologic age, lithology, and paleogeography, the
same three idealized facies associations are recognized
in both cases and in many other lake basins. The petroleum generative characteristics of mudrocks within
each of these facies associations differ significantly
from each other and must be considered for successful
exploration of nonmarine basins.
LACUSTRINE SEDIMENTARY FACIES
ASSOCIATIONS
Lacustrine deposits may be characterized according to
(1) their sedimentary facies, fauna, and flora, (2) their
internal stratigraphic relations (such as parasequence
stacking), and (3) the character of associated deposits.
In particular, evidence for open vs. closed basin hydrology and the presence and nature of depositional
cyclicity provide the fundamental bases for categorizing ancient lake systems according to types. Note that
these types represent end-member ideals and as such
need not be 100% representative of any one occurrence. Furthermore, the interpretation of depositional
cycles necessarily implies simultaneous observation of
several different subfacies, resulting in time averaging
of diachronous but evolutionary depositional environments. The concept of lake type as applied to ancient
deposits therefore may not be directly applicable to
modern lakes, for which only a synoptic or snapshot
view is available.
The Green River Formation in Wyoming consists
of approximately 2500 m of Eocene lacustrine and
associated alluvial strata deposited in several foreland
basins adjacent to Laramide-style uplifts (Sullivan,
1985; Roehler, 1992) (Figure 1). Late Permian basin
subsidence resulted in the preservation of more than
5000 m of nonmarine facies in the southern Junggar
basin, including more than 1000 m of organic-rich lacustrine mudstone (Liao et al., 1987; Carroll et al.,
1990, 1992, 1995; Wartes et al., 1998) (Figure 2).
Three lacustrine facies associations may be identified
within both deposits, as detailed subsequently: the
fluvial-lacustrine facies association, the fluctuating
profundal facies association, and the evaporative facies
association.
Fluvial-Lacustrine Facies Association
The Luman Tongue consists of freshwater to mildly
alkaline facies deposited in the Washakie basin as the
earliest lacustrine phase of the Green River Formation (Roehler, 1992; Horsfield et al., 1994) (Figure
1). As such, it resembles the Black Shale facies of the
Green River Formation in the Uinta basin, a source
of petroleum in Altamont-Bluebell and other fields
(Fouch, 1975; Ruble and Philp, 1998). The Luman
Tongue contains generally poorly expressed, shallowing-upward packages (parasequences) that were deposited by processes of shoreline progradation (Figure
3). Where evident, the base of these parasequences
Carroll and Bohacs
1035
Miles
0
Km
UT
ID
WY
0
100
ROCK
SPRINGS
UPLIFT
200 300 400
500
700
800
UT
42˚
ID
WY
Washakie Basin
WYOM
ING TH
RUST
East
Fresh
GREAT DIVIDE
BASIN
ROCK
SPRINGS
800
UPLIFT
900
1000 Figure 3C
1100
1200
WASHAKIE
BASIN
41˚
UINTA MOUNTAINS
Evap.
SAND WASH
BASIN
Wilkins Peak Member
WY
CO
Luman Tongue
(Figure 3A)
GREAT DIVIDE
BASIN
GREEN RIVER
BASIN
0
Saline lacustrine
Flood plain
Mudflat/saline lacustrine
Freshwater lacustrine
Evaporite/saline lacustrine
Figure 3A
UINTA MOUNTAINS
Coal
Luman Tongue
111˚
400
200 300
110˚
Figure 1. Location, sedimentary facies, and isopach maps of the Green River Formation in Wyoming, modified from Roehler (1992).
109˚
SAND WASH
BASIN
108˚
RA
ER RE
AD
M
Sandstone
100
SI
100
200
41˚
250 km
(156 miles)
ROCK
300
SPRINGS100
UPLIFT
WASHAKIE
200
300 BASIN
UT
CO
UT
42˚
ID
WY
Fresh
COAL
WYOM
ING TH
RUST
Alkal.
lake Water
Chemistry:
Lower
2500 m
(8200 feet)
Lwr. LaClede
Bed (Figure 3B)
300
RA
ER RE
AD
M
Alkal.
Wilkins Peak Member
(Figure 3C)
Eocene
500
600
700
400
100
WY
CO
SI
Middle
Laney Member
0
200
SAND WASH
BASIN
UT
CO
Rock Springs Uplift
Figure 3B
41˚
Laney Member
Green River Basin
WASHAKIE
BASIN
600
100
UINTA MOUNTAINS
West
0
RA
ER RE
AD
M
0
50
GREEN RIVER
BASIN
UT
CO
N
100
GREAT DIVIDE
BASIN
SI
Lacustrine Petroleum Source Rocks
USA
Map Area
42˚
WYOM
ING TH
RUST
1036
Isopach contour
interval = 100"
WY
CO
107˚
is typically a sharp surface overlain by organic-rich
calcareous mudstone and shale or organic-poor bioturbated mudstone and siltstone (Horsfield et al.,
1994; Bohacs, 1998). These facies grade upward into
littoral coquinas, small sandy deltas, and occasional
thin coals. Flood-plain and fluvial facies also are common. Cyclicity caused by lake-level fluctuation is
subtle or absent, consistent with relatively open lakebasin hydrology. The laterally equivalent and superjacent Niland Tongue of the Wasatch Formation contains predominantly lake-plain to alluvial deposits
(including coals) and is closely related to the Luman
Tongue (Figure 1).
The Hongyanchi Formation in the southern Junggar basin consists of interbedded profundal mudstone,
siltstone, limestone, and fluvial sandstone and conglomerate (Carroll et al., 1992, 1995) (Figures 2, 4). It
too lacks any strongly expressed depositional cyclicity,
although indistinct deepening and shoaling depositional trends on the scale of 10 m or more are evident.
Mudstone units are commonly slightly calcareous and
laminated in places but are commonly homogenous.
Amalgamated fluvial sandstone and conglomerate beds
may reach thicknesses up to tens of meters, recording
channeled flow within well-developed stream systems.
Desiccation cracks and other evidence of prolonged
subaerial exposure are absent. Limestone beds 20 to
50 cm thick occur in places and generally comprise
argillaceous micrite without visible fauna; they may be
algal in origin. Coals are not present in the Hongyanchi
Formation, but overlying fluvial units do contain fragments of petrified wood that have coaly residues (Carroll et al., 1992).
The aforementioned features are common to many
other hydrologically open lake basins, such as the Richmond and several other basins filled by elements of the
Newark Supergroup. Olsen (1990) characterized
Richmond-type basins as containing significant coals
and bioturbated shallow-water and fluvial facies and
thick intervals of laminated siltstone. Evidence for
subaerial exposure is absent, consistent with high
precipitation/evaporation ratios. Other examples of
this fluvial-lacustrine facies association include the Paleocene of the Fort Union Formation in Wyoming (Liro
and Pardus, 1990), some Cretaceous units in the Songliao basin of northeastern China (Yang et al., 1985;
Xue and Galloway, 1993; D. Li et al., 1995), some
Paleocene intervals in the Central Sumatra basin (Kelley et al., 1995), and the Cretaceous strata of the
Doba/Doseo basins in west-central Africa (Genik,
1993).
Fluctuating Profundal Facies Association
The lower LaClede Bed of the Laney Member of the
Green River Formation consists of alkaline lake deposits that are widely distributed over the greater Green
River basin (Roehler, 1992; Horsfield et al., 1994) (Figures 1, 3). It resembles similar facies in the Parachute
Creek Member of the Green River Formation in Utah
and Colorado, which, although immature with respect
to petroleum generation, constitutes the well-known
Green River oil shale deposits. In contrast to the Luman Tongue, the lower LaClede Bed contains welldefined parasequences that record major lake desiccation and flooding (Horsfield et al., 1994). The
parasequences typically begin with algal and oolitic facies, followed by deposition of finely laminated
organic-rich calcareous shale. Subsequent evaporative
contraction of the lake resulted in a gradual vertical
increase in dolomicrite, magadii-type chert, and efflorescent crusts of saline minerals, typically culminating in mud-crack horizons. During uncommonly humid phases, limited progradation of fluvial clastic facies
occurred at the lake margins, resulting in mixed
aggradational/progradational stratal geometries.
The Lucaogou Formation (Junggar basin, China)
consists primarily of profundal laminated mudstone
and occasional siltstone interbeds and includes the
most organic-rich facies in the southern Junggar basin
(Carroll et al., 1992; Carroll, 1998) (Figures 2, 4). The
Lucaogou Formation is predominantly siliciclastic,
with the exception of occasional nodular dolomite.
Distinct 1–4 m scale cycles occur near its base and are
punctuated by mud-cracked desiccation surfaces.
These cycles grade upward into the most organic-rich
interval of the Lucaogou Formation, where there are
no obvious signs of subaerial exposure. Cyclicity continues to be expressed, however, as variations in the
thickness and preservation of lamina. Flooding surfaces
have not been identified within this interval; therefore,
lamina thicknesses appear to record varying input of
clay and silt in an area distant from any shoreline (Carroll, 1998). These variations in detrital grainsize may
in turn be linked to fluctuating lake level. Occasional
centimeter-scale graded beds record fine-grained turbidite deposition, and minor soft-sediment slumping is
common.
Fluctuating profundal facies were also described by
Van Houten (1962) in the Newark and related basins.
Olsen (1990) attributed these cycles to lake level
changes driven by Milankovitch-scale climate changes
in basins where long-term water inflows were closely
Carroll and Bohacs
1037
LAKE MARGINAL FACIES
Hongyanchi Fm. measured section (Figure 4A)
1300
86
84
90
88
100
?
0
1200
46
JUNGGAR
BASIN
CHINA
50
0
FLUVIAL FACIES
HONGYANCHI FORMATION
1400
500
Karamay
1100
1
?
?
1000
?
1000
5
5
KELAMEILI
SHAN
50
0
45
JUNGGAR
900
1000
10
500
BASIN
?
46
10
00
800
150
0
LUCAOGOU FORMATION
DEEP LAKE FACIES
?
44
44
700
?
NORTH
600
N
2000
T
U
TIAN
50 KM
BOGDA
SHAN
?
SHAN
88
86
500
Upper Permian Sandst. and Congl. (approximate)
Permian Lake Deposits (approximate)
Pre-Permian Outcrops
T
TIANCHI SECTION
300
1 5 20
TOC (%)
0m
U
URUMQI SECTION
LAMINATED
ORGANIC-RICH
MUDSTONE
MUDSTONE
LIMESTONE
COVERED
INTERVAL
TOC < 0.5%
JINGJINGZIGOU FM.
SILTSTONE
TO SANDST.
DEEP LAKE FACIES
700
LUCAOGOU FORMATION
DEEP
Lucaogou Fm.
meas. section
(Figure 4B)
200
100
1038
U
Urumqi measured section
T
Tianchi measured section
800
600
500
400
LAKE MARGINAL FACIES
LAKE MARGINAL FACIES
400
300
200
100
0m
1 5 20
TOC (%)
Lacustrine Petroleum Source Rocks
Jingjingzigou Fm.
measured section
(Figure 4C)
Oil Fields
Thrust belts
00
10
5
Upper Permian
Thickness (m)
Mesozoic-Cenozoic
Thickness (km)
balanced by outflows. Van Houten cycles possess extreme lateral continuity, except where they intersect
basin-marginal alluvial fans. The ratio of organic-rich
to organic-poor units in the Newark basin, however, is
relatively low compared to the lower LaClede Bed or
Lucaogou Formation. The preservation of fine laminae
implies the lack of burrowing in fauna or resuspension
of bottom sediments by currents. Laminated intervals,
therefore, are interpreted to have been deposited in
water depths below storm wave base, and anoxic conditions prevail below the sediment-water interface
(Olsen, 1990). Similar profundal facies also occur in
the Bucomazi Formation offshore west Africa (Burwood et al., 1995) and in the Lagoa Feia Formation
offshore Brazil (Campos basin) (Trinidade et al.,
1995).
Evaporative Facies Association
The Wilkins Peak Member of the Green River Formation (Figures 1, 3) encompasses a wide variety of
facies, ranging from alluvial fan and sheetflood sandstone to laminated oil shale (e.g., Bradley and Eugster, 1969; Smoot, 1983). This member is best
known, however, for its bedded trona and halite,
which are interpreted to have been deposited in
evaporative playas (Eugster and Surdam, 1973; Eugster and Hardie, 1975). Basin-center deposits record
dominantly aggradational stacking of organic-rich
mudstone facies during lake flooding and carbonate
and evaporite facies during lake desiccation. The
basin-center organic-rich mudstone facies grade vertically and laterally into littoral and supralittoral deposits of dolomitic and siliciclastic mudstone and
grainstone facies that have desiccation cracks and
evaporitic minerals or their replacements (Bohacs,
1998). The lake-margin lithofacies intertongue with
carbonate and siliciclastic sandstone and mudstone
facies probably deposited in alluvial-fan and fluvial
systems. The basin-center evaporite beds lap marginward onto subaerially exposed lake and lake-margin
strata. Laterally equivalent eolian reworked grainstone and mudstone facies are uncommon. Figure 3C
illustrates the distribution of lithologies in a representative vertical section. Stratigraphic relationships
among littoral and lake-plain facies are complex be-
cause of interactions between sheetfloods and rising
lake levels (Smoot, 1983).
The Jingjingzigou Formation (Junggar basin,
China) (Figures 2, 4) consists of dolomitic gray and
gray-green mudstone, medium gray to dark gray mudstone, wave-rippled dolomitic siltstone, and finegrained sandstone (Carroll et al., 1992; Carroll, 1998).
Although evaporite minerals are absent from basinmarginal outcrop exposures, possible evaporite
pseudomorphs occur. Displacive dolomite nodules
ranging in size from 1 to 10 cm are common, particularly within the upper parts of siltstone beds. Shoalingupward lacustrine cycles 10 to 2 m thick occur
throughout the measured section. Tabular siltstone
and sandstone beds ranging from 20 to 50 cm in thickness appear at the base of many of the cycles. Minor
scour at the base of siltstone and sandstone beds is
common, as is soft-sediment loading into underlying
mudstone. The sandstone beds are interpreted to represent relatively unconfined, episodic flows across a
lake plain during onset of flooding and lacustrine transgression. Mudstone facies typically possess relatively
thick, discontinuous, wavy laminae (#1 mm) and commonly grade upward into poorly laminated or homogenous intervals. Thin intervals ($10 cm) of submillimeter scale laminae occur rarely. Abundant mud
cracks, commonly sand or silt-filled, occur near cycle
tops and attest to frequent subaerial exposure and
desiccation.
Although the composition of Wilkins Peak evaporites is unusual, evaporative lake facies occur in many
other lacustrine basins. For example, desiccation cycles
in parts of the Fundy and similar basins of the Newark
Supergroup include shallow lake and playa facies and
are associated with gypsum nodules, salt-collapse
structures, and eolian dunes (Hubert and Mertz, 1984;
Smoot and Olsen, 1988). Other examples include the
Blanca Lila Formation (Pleistocene of Argentina)
(Vandervoort, 1997) and deposits within several Tertiary basins in China (e.g., Fu et al., 1986).
LACUSTRINE ORGANIC FACIES
Each of the sedimentary facies associations described
previously correspond to distinctive organic facies as
Figure 2. Location of Junggar basin Permian lacustrine facies (modified from Carroll et al., 1992, and Carroll, 1998). Master sections
from Tianchi and Urumqi are aligned according to an approximate lithostratigraphic correlation between these localities. Permian
isopach contours represent the Jingjingzigou, Lucaogou, and Hongyanchi Formations but do not include the entire Upper Permian
interval.
Carroll and Bohacs
1039
A. Luman Tongue
B. Lower LaClede Bed,
Laney Member
C. Wilkins Peak Member
xxxxx
xxxxx
xxxxx
(36)
Ss
MSs
SMs
Ms
Sh
xxxxx
!15 m
30 0
0
%TOC
1000
HI
50 ft
10 m
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
Ss
MSs
SMs
Ms
Sh
0
0
30 0
%TOC
1000
HI
Scour
(42)
Ss
MSs
SMs
Ms
Sh
(48)
0
30 0
%TOC
1000
HI
0
LEGEND
Mudstone
Trough cross-beds
Intraclast
conglomerate
Current ripples
Evaporite
Wave ripples
Stylolites
Combined-flow ripples
Concretions
Climbing ripples
Oolite
Planar lamination
Stromatolite
Ss
Sandstone
Flaser or lenticular bedding
Plant fossils
MSs
Muddy sandstone
Convolute bedding
Ostracods
SMs
Sandy mudstone
Fining-upward bed
Molluscs
Ms
Mudstone
Mudcracks
Fish
Sh
Coal
xxxxx
Tuff
Laminated Mudstone
Sandstone
Limestone
Evaporite
Shale
Flooding surface
Figure 3. Green River Formation measured sections from (A) outcrop of Luman Tongue at Hiawatha locality (21-T12N-R100W), (B)
outcrop of lower LaClede Bed of the Laney Member at Trail Dugway locality (18-T14N-R99W), and (C) core of Wilkins Peak Member
from the UPR 41–43 well (23-T17N-R109W; see Figure 1 for locations). TOC " total organic carbon (%); HI " Rock-Eval hydrogen
index (mg/g).
Legend
M
30
submillimeter
laminated mudstone
nonlaminated
mudstone
limestone
mud cracks
(desiccation)
laminated
mudstone (>1mm)
siltstone
dolomitic
concretions
mud
intraclasts
FS = Flooding surface
M
13
M
12
12
11
11
10
10
20
9
9
FS
8
8
FS
7
FS
6
FS
FS
FS
FS
5
7
6
5
10
FS
4
4
FS
3
FS
Carroll and Bohacs
FS
2
3
2
1
1
FS
0
deep shallow
Water Depth
0
0
0
1
2
3
4
5
0
% TOC
A. Hongyanchi Formation
1000
HI (mg/g)
0
fine
coarse
Lamination
10
20
0
% TOC
B. Lucaogou Formation
1000
HI (mg/g)
deep shallow
Water Depth
0
1
2
3
% TOC
4
5
1000
0
HI (mg/g)
C. Jingjingzigou Formation
1041
Figure 4. Measured sections from (A) Hongyanchi Formation at Urumqi, (B) Lucaogou Formation at Tianchi, and (C) Jingjingzigou Formation at Tianchi (modified from Carroll,
1998; see Figure 2 for locations).
determined from such features as total organic carbon
(% TOC), Rock-Eval hydrogen indices (HIs), visual kerogen descriptions, pyrolysis products, and biomarker
geochemistry. Biomarkers, or biological marker compounds, are complex molecular fossils derived from
once-living organisms (Peters and Moldowan, 1993).
They have the capacity to provide detailed qualitative
information on organic matter input, water column oxygenation, and thermal maturity. A unique advantage
of biomarkers over other geochemical measurements is
that the same compounds are found in both rock extracts and expelled oils, making it possible to interpret
source rock depositional environments in cases where
the rocks themselves are inaccessible. Biomarkers are
also commonly used to assess the relative salinity of
source rock depositional environments, based on the
partial record they provide of the types of organisms
that were present. This information can be correlated
with physical characteristics that suggest a particular salinity range, such as abundant mud cracks or the presence of evaporites. It should be noted, however, that in
this context, terms such as “fresh”, “brackish”, “saline”,
or “hypersaline” do not normally imply specific solute
concentrations, but instead represent interpreted relative salinity levels. The following discussion summarizes the application of several biomarker parameters to
the interpretation of lacustrine depositional environments and organic matter sources; see Peters and Moldowan (1993) for more detailed documentation.
Table 1 summarizes bulk organic matter and biomarker parameters for Green River Formation and
Junggar basin samples, as well as data reported for other
lacustrine units. These occurrences are subdivided into
algal-terrestrial, algal, and algal-hypersaline organic facies. Note that for each facies, considerable variation
may occur in a given biomarker parameter, both within
one unit and among different units. This variation results from differing lake ecologies, differing levels of
thermal maturity, heterogeneous depositional environments, and possibly different analytical methods used
by different workers. Because only limited stratigraphic
data are available in most of the studies cited, some reported occurrences probably also include samples from
more than one organic facies. Despite these uncertainties, however, each organic facies exhibits a distinct pattern when all the parameters are considered together.
Algal-Terrestrial Organic Facies
The fluvial-lacustrine facies association corresponds to
algal-terrestrial organic facies in the Luman and Niland
1042
Lacustrine Petroleum Source Rocks
tongues and in the Hongyanchi Formation. The majority of TOC values in Luman Tongue and Niland
Tongue mudstone facies lie between 2 and 8% (Figures
3A, 5A), but some coaly samples exceed 50% TOC
(Figure 6). Rock-Eval hydrogen indices (HIs) for individual samples from a continuous section of the Luman
and Niland tongues are highly variable (Table 1). Their
average HI is 310, in sharp contrast to type I kerogen
HI values that range between 600 and 900 and can
exceed 1000 (e.g., Graham et al., 1990). Closer examination suggests the presence of two distinct organic
matter populations within these units; one population
averages 4.87% TOC and HI # 500, and the other
averages 9.34% TOC and HI $ 500 (Figure 6). Visual
kerogen descriptions indicate that organic macerals
likewise contain two populations; they are dominated
by alginite but also contain varying proportions of vitrinite (Horsfield et al., 1994). Luman Tongue and Niland Tongue source rocks, therefore, record mixed
aquatic and terrestrial organic matter input. Population
1 samples are tightly clustered around a linear regression line representing HI 805 (Figure 6), attesting to
the constancy of organic matter type within these samples. Variation in their TOC content likely resulted in
part from higher dilution of organic matter by silt and
clay. Population 2 samples are much more widely scattered around a line representing HI 324, suggesting a
more heterogenous mixture of aquatic and terrestrial
organic matter. Biomarker concentrations in extracts
from the Luman Tongue and Niland Tongue are indicative of relatively oxic freshwater environments that
have mixed input of aquatic and terrestrial organic
matter (Table 1). Analysis of kerogen structure via
pyrolysis–gas chromatography techniques indicates
that at higher thermal maturities, Luman Tongue
mudstone would generate waxy oil (Horsfield et al.,
1994).
The TOC values in the Hongyanchi Formation
are lower on average than the Luman and Niland
tongues (Figures 4A, 5B), although values up to 7.7%
have been reported (Carroll et al., 1992). The TOC
and HI values are also lower in part because of significantly greater thermal maturity of the Hongyanchi
Formation (Figure 5A, B). Overall organic matter
composition appears similar to the Luman and Niland tongues, although too few samples have been
collected for a full assessment. HI ranges from 58 to
444. The highest organic enrichments correspond to
more profundal intervals, whereas the lowest enrichments correspond to massive limestone and mudstone (Figure 4A). Vitrinite and inertinite combine
200
40
15
B. Hongyanchi Formation
35
S2 (mg/g)
n = 171
mean %TOC = 7.09
std. dev. = 8.63
Ro = 0.35-0.45%
100
n = 27
mean %TOC = 2.72
std. dev. = 1.85
Ro = 0.86-1.09%
30
25
9
y = 3.87x - 4.79
r 2 = 0.57
20
y = 3.10x - 9.23
r 2 = 0.78
12
6
15
Frequency
A. Luman and Niland tongues
>24%
10
3
5
0
0
0
200
15
15
C. Laney Member
D. Lucaogou Formation
10
9
y = 8.30x - 7.58
r 2 = 0.95
100
n = 69
mean %TOC = 4.41
std. dev. = 4.64
Ro = 0.77-0.88%
12
Frequency
S2 (mg/g)
n = 91
mean %TOC = 7.84
std. dev. = 3.71
Ro = 0.40%
y = 8.61x - 9.91
r 2 = 0.98
6
5
3
0
0
200
0
8
60
F. Jingjingzigou Formation
S2 (mg/g)
n = 201
mean %TOC = 4.09
std. dev. = 3.86
Ro = 0.20-0.45%
40
y = 7.90x - 2.87
r 2 = 0.97
100
7
n=8
mean %TOC = 1.44
std. dev. = 0.40
Ro = 0.88-0.91%
50
6
5
y = 5.60x - 2.55
r 2 = 0.82
30
4
3
Frequency
E. Wilkins Peak Member
20
2
10
1
0
0
0
0
5
10
15
20
25
% TOC
0
5
10
15
20
25
% TOC
Figure 5. Percent TOC (determined by LECO) vs. Rock-Eval S2 (points), and % TOC histograms (gray shading) for samples from six
lacustrine petroleum source rock units. The slope of the linear regression lines multiplied by 100 gives the average Rock-Eval HI for
each population, corrected for adsorption of pyrolysates on the rock matrix (cf. Langford and Blanc-Valleron, 1990). Ro " vitrinite
reflectance in oil for each population. Note that vitrinite reflectances for the Junggar basin samples (B, D, and F) have been affected
by outcrop oxidation and thus may be slightly higher than equivalent unweathered rocks (Carroll, 1998).
to average 48% of visible kerogen; the remainder is
mostly weakly fluorescent amorphous material. Biomarker distributions include features typical of fresh-
water, suboxic depositional environments that have
mixed terrestrial and aquatic organic matter input
(Table 1).
Carroll and Bohacs
1043
1044
Lacustrine Petroleum Source Rocks
Table 1. Selected Geochemical Characteristics of Lacustrine Source Rocks and Oils*
Basin Formation
% Ro
% TOC
HI
Pristane/Phytane**
b-carotane
Steranes
Tricyclic
Index**
Gamma.
Index
Hopane/Steranes
Algal-Terrestrial Organic Facies (Fluvial-Lacustrine Facies Association)
Wyoming, Luman Tongue
(rocks)†
Junggar basin (W. China),
Hongyanchi Fm. (rocks)††
Brazil basins
(rocks and oils)‡
Gabon Kissenda shale‡‡
Phisanulok basin
(Thailand; oils)§
C. Sumatra basin
(rocks and oils)§§
0.35–0.45
1.0–59.2
55–985
1.5–4.8
not detected
0.86–1.09
0–8
58–365
1.5–4.1
trace
C29 # C27 ! C28
abundant 4-methyl
C29 # C27 # C28
0.4–0.7
"6.5%
$779
#1.3
not detected
C29 ## C27
0.65–0.72
0.6–2.2
93–458
0.8–1.2
not detected
n.a.
n.a.
n.a.
2.7–4.0
not detected
0.4–0.8
0.7–9.2
110–946
2.2–3.2
(not reported) (negligible)
1.0–4.0
48–71
4–11
2.6–7.7
30–100
20–40
5–15
C27 # C29 " C28
abundant 4-methyl
C29 ## C28 # C27
(not reported)
7–35
4.2–17.5
(negligible)
(negligible)
28.1–47.5
not detected?
C29 # C28 % C27
abundant 4-methyl
12–27
4–6
3.8–7.7
0.1–0.5
present
(not reported)
(high)
0.4–0.8
79–85
19–36
2.5–5.2
100–200
20–70
5–15
(not reported)
13–150
1.5–7.7
Algal Organic Facies (Fluctuating Profundal Facies Association)
Wyoming, Laney Member
(rocks)†
Junggar basin (W. China),
Lucaogou Fm. (rocks)††
Brazil basins
(rocks and oils)‡
Bucomazi shales (Angola)
organic-rich zone (rocks)||
0.40
1.5–17.1
52–1003
0.77–0.88
0–23
481–766
1.0–1.6
present
C29 # C27 # C28
abundant 4-methyl
C29 ! C28 ## C27
0.4–0.8
"9
"900
#1.1
present
C29 # C27
0.56–0.81
2–24
#700
(not reported)
(not reported)
C27 # C29 ! C28
Hypersaline-Algal Organic Facies (Evaporative Facies Association)
Wyoming, Wilkins Peak
Member (rocks)|| ||
Junggar basin (W. China)
Jingjingzigou Fm. (rocks)††
Junggar basin (W. China)
Karamay trend (oils)#
Jianghan basin (E. China)
(rocks and oils)##
0.20–0.45
4.1–19.0
0.88–0.91
0.8–2.0
n.a.
0.45–0.55?
32–1001
0.1–1.1
abundant
C29 # C28 ## C27
6–21
8–82
0.2–2.0
129–477
0.8–1.1
n.a.
n.a.
0.9–2.3
"6.6
712–838
0.1–0.5
abundant to
dominant
abundant to
dominant
present
C29 # C28 # C27
79–245
15–34
1.7–7.1
C29 ! C28 # C27
260–580
31–56
0.7–2.2
C27 # C29 # C28
7–28
35–216
0.3–2.4
*% Ro " vitrinite reflectance in oil; % TOC " total organic carbon; HI " Rock-Eval Hydrogen Index; Tricyclic Index " 100(C23 tricyclic terpane/C30 17#(H), 21b(H)-hopane); Gamma. Index " gammacerane/C30 17#(H),
21b(H)-hopane; Hopane/Steranes " C30 17#(H), 21b(H)-hopane/C29 20S # 20R 14#(H), 17#(H)-steranes; n.a. " not applicable.
**Ratio strongly affected by thermal maturity.
†
Horsfield et al. (1994).
††
Carroll (1998).
‡
Mello et al. (1988).
‡‡
Kuo (1994).
§
Kulwadee and Philp (1991).
§§
Kelley et al. (1995).
||
Burwood et al. (1992, 1995).
|| ||
this study.
#
Clayton et al. (1997).
##
Fu et al. (1986), Philp and Fan (1987), Peters et al. (1996).
Carroll and Bohacs
1045
200
Luman + Niland Tongues
S2 (mg/g)
Population 1
n = 86
mean %TOC = 4.87
std. dev. = 1.54
Population 2
n = 85
mean %TOC = 9.34
std. dev. = 11.76
y = 8.05x - 8.08
r 2 = 0.94
100
y = 3.24x - 1.01
r 2 = 0.86
HI > 500
HI < 500
0
0
10
20
30
40
50
60
% TOC
Figure 6. Percent TOC vs. S2 plot for Luman Tongue and Niland Tongue samples. The samples have been divided into two
populations according to HI, and separate linear regression lines
have been calculated for each population.
Algal-terrestrial organic facies elsewhere typically
have moderately high TOC contents (!1–10%) and
mixed type I–type III kerogens (Table 1) and commonly occur in association with fluvial deposits and
coal. High molecular weight n-alkanes (waxes) are major constituents of both rock extracts and oils because
of input of protective tissues from higher land plants
(Tissot and Welte, 1984) and membrane lipids from
certain classes of freshwater algae (Goth et al., 1988;
Tegelaar et al., 1989). Selective enrichment in these
compounds may also result from selective bacterial
degradation of other components in oxic to suboxic
depositional environments (Powell, 1986). High
pristane/phytane ratios (Table 1) result both from
higher land plant input and oxic depositional conditions. High hopane/sterane ratios record bacterial degradation of relatively abundant terrestrial organic
matter.
Algal Organic Facies
The fluctuating profundal facies association typically
includes extremely organic-rich laminated mudstone
facies containing abundant algal-derived organic matter. The TOC values in the lower LaClede Bed of the
Laney Member of the Green River Formation are as
great as 20%, and the average HI is 830 (Figures 3B,
5C). Alginite is the principal organic maceral (Horsfield et al., 1994). Subsidiary vitrinite and intertinite,
1046
Lacustrine Petroleum Source Rocks
however, also occur in the highest TOC bluebeds, suggesting that these units were deposited during times of
enhanced terrestrial runoff. Ectogenic meromixis as a
result of increased inflow of fresh surface water has
been proposed as a mechanism for promoting salinity
stratification (Boyer, 1982), which in turn may have
helped preserve organic matter. Biomarker distributions are consistent with predominantly aquatic organic matter input in saline lakes with anoxic bottom
waters (Table 1).
Maximum Lucaogou Formation organic enrichment occurs in laminated mudstone, interpreted as anoxic profundal facies (Graham et al., 1990; Carroll et
al., 1992). Organic richness is highly variable within
the Lucaogou Formation, ranging from less than 0.5%
to greater than 20% TOC and is highest in submillimeter laminated intervals (Carroll, 1998) (Figures 4B,
5D). The lower average TOC values in the Lucaogou
Formation compared with the lower LaClede Bed may
in fact reflect more statistically representative sampling
of the Lucaogou Formation. Average Rock-Eval HI values over this interval are essentially identical with the
lower LaClede Bed (Figure 5C, D), and both are consistent with relatively homogenous type I kerogen.
Fluorescent amorphous material dominates Lucaogou
Formation kerogens, averaging 58% of visible organic
matter, and attests to probable algal input (Carroll,
1998). Very light d13C values in extracts of the richest
samples (Carroll, 1998) are inconsistent with the hypothesis that organic richness is controlled by high primary productivity, but correlations between productivity and organic richness have been reported in other
basins (e.g., Curiale and Gibling, 1994). The absence
of subaerial exposure surfaces, presence of fish fossils,
and modest elevation of b-carotane and gammacerane
in Lucagogou Formation extracts (Table 1) together
are consistent with deposition within a salinitystratified lake.
Algal organic facies in other basins represent some
of the world’s richest lacustrine source rocks and contain mostly type I kerogen that has HI values that typically reach maxima in the 600–900 range and may
exceed 1000 (Espitalié et al., 1977). Lower HI values
reflect temporal variations in primary productivity
(e.g., Curiale and Stout, 1993), degree of watercolumn anoxia (Demaison and Moore, 1980), and experimental artifacts arising from Rock-Eval pyrolysis
(Katz, 1983; Langford and Blanc-Valleron, 1990). Extracts and oils are commonly rich in n-alkanes derived
from membrane lipids of aquatic organisms. The distribution of n-alkanes may differ from freshwater fa-
5
S2 (mg/g)
4
y = 8.67x - 3.90
r 2 = 0.90
Frequency
6
100
3
2
1
0
0
10
Supralittoral
n = 25
mean %TOC = 3.23
std. dev. = 2.94
100
6
y = 7.58x - 2.67
r 2 = 0.96
4
Frequency
8
S2 (mg/g)
2
0
0
40
Littoral
35
n = 86
mean %TOC = 1.84
std. dev. = 1.11
30
y = 8.00x - 3.27
r 2 = 0.92
20
15
Frequency
25
100
10
5
0
0
Sublittoral
n = 76
mean %TOC = 7.33
std. dev. = 4.22
10
8
100
6
y = 7.83x - 1.82
r 2 = 0.96
4
Frequency
Oil shales in the Wilkins Peak Member of the Green
River Formation are thinner and slightly less areally extensive than those in the lower LaClede Bed but contain equally high maximum organic enrichments (up
to 18% TOC) (Bohacs, 1998) (Figure 5E). The TOC
contents in the Wilkins Peak Member vary widely but
are generally lowest in littoral and lake-plain mudstone
facies and highest in sublittoral mudstones and shales
(Figure 7). These relationships demonstrate that the
highest organic enrichments occur when the lakes were
deepest. Note that average HI values in the Wilkins
Peak Member (determined from S2 /TOC plots) are
essentially identical for each environment, indicating
very little lateral contrast in organic matter type.
Amorphous organic matter, derived from algal or bacterial sources, dominates visible organic matter, confirming very low input from land plants. Some of the
most organically enriched strata occur just above the
major flooding surface at the base of each sequence
(Figure 3C). Biomarker distributions are consistent
with deposition under hypersaline, anoxic conditions
(Table 1), although certain biomarker ratios (e.g.,
pristane/phytane ratios) may also reflect low thermal
maturity.
Organic enrichment in the Jingjingzigou Formation mudstone is relatively low (TOC less than 2%)
(Figures 4C, 5F). Total organic carbon is highest in medium and dark gray laminated mudstones and drops to
less than 1.0% in nonlaminated facies. Rock-Eval HI
values reach a maximum of 477, suggesting oxidation
of organic matter prior to burial. The Jingjingzigou
Formation contains high proportions of amorphous
7
n = 14
mean %TOC = 1.11
std. dev. = 0.43
S2 (mg/g)
Hypersaline Algal Organic Facies
8
Lake Plain/Alluvial
S2 (mg/g)
cies, however, in having lower odd-carbon-number
preference and less abundant n-C30# components.
Pristane/phytane ratios are typically close to or less
than 1.0, reflecting lower terrestrial input, anoxic depositional conditions, and possibly elevated salinity.
Other biomarker characteristics reflect the contributions of specific classes of aquatic organisms, as evidenced by the presence of moderate amounts of
b-carotane, elevated tricyclic terpanes, and gammacerane. Gammacerane is a commonly cited indicator of
salinity (e.g., Mello et al., 1988), although more recent
work with compound-specific isotopic analysis suggests that gammacerane may in fact record input from
bacterial communities living at or below the chemocline in stratified lakes (Schoell et al., 1994; Sinninghe
Damsté et al., 1995).
2
0
0
0
5
10
15
20
%TOC
Figure 7. Percent TOC histograms for core samples from the
Wilkins Peak Member, subdivided according to interpreted depositional environment.
Carroll and Bohacs
1047
kerogen, presumably of algal or bacterial origin (Carroll, 1998). Minor vitrinite, inertinite, and alginite
make up the remainder of the visible kerogen. Biomarker distributions are commonly dominated by bcarotane, which has been associated with shallow, hypersaline lakes in which certain highly specialized
organisms occur (Murphy et al., 1967; Hall and Douglas, 1983; Brassell et al., 1988; Duncan and Hamilton,
1988; ten Haven et al., 1988).
Hypersaline organic facies commonly feature
lower overall TOC values than do brackish-saline facies, although relatively thin intervals of high TOC occur during periods of maximum lake expansion.
Mixtures of type I and type III kerogens appear to result from varying degrees of preservation of aquatic organic matter and from the admixture of minor terrestrial material. Maximum HI values equal or exceed
those seen in algal organic facies (Table 1), but average
values tend to be lower. Biomarker distributions reflect
low-diversity assemblages of highly specialized organisms living under harsh environmental conditions.
Pristane/phytane ratios substantially below 1.0 may result either from anoxic depositional conditions in a
stratified, hypersaline lake, or from contributions from
halophilic bacteria (Goosens et al., 1984). b-carotane
is commonly an abundant to dominant constituent, as
are elevated concentrations of tricyclic terpanes and
gammacerane (Table 1). Hopane/sterane ratios are
typically low. In addition to the examples cited previously, hypersaline lacustrine organic facies are known
from several Tertiary basins in China (Shi et al., 1982;
Fu et al., 1986; Sheng et al., 1987; Brassell et al., 1988;
R. Li et al., 1988, 1992; Z. Chen et al., 1994; J. Chen
et al., 1996; Peters et al., 1996) and in Spain (e.g.,
Salvany and Ortı́, 1994; Sanz et al., 1994).
DISCUSSION: PETROLEUM GENERATION
FROM LACUSTRINE SOURCE ROCKS
Bradley (1925) noted that for certain members of the
Green River Formation, “large volumes of microscopic
plants and perhaps also animals accumulated.” Studies
of oil-shale facies of the Mahogany ledge of the Parachute Creek Member led to the recognition of type I
lacustrine kerogen, characterized by high atomic H/C
ratios and HI (van Krevelen, 1961; Espitalié et al.,
1977). Subsequent experience has shown, however,
that kerogens in many modern and ancient lake systems differ from type I, and that type I is not even
representative of all members of the Green River For1048
Lacustrine Petroleum Source Rocks
mation. For example, modern east African lakes preserve organic matter ranging from type I to type III
(Katz, 1988, 1990; Talbot, 1988). Recent studies have
demonstrated that these distinctions are critical not
only for estimating the relative quality and quantity of
oil and gas that may be generated, but also for determining the timing of hydrocarbon generation (e.g., Anders et al., 1992; Tegelaar and Noble, 1994; Peters et
al., 1996). The following discussion considers generative characteristics of mixed type I–III kerogen typical
of the algal-terrestrial organic facies (fluvial-lacustrine
facies association), type I kerogen typical of algal organic facies (fluctuating profundal facies association),
and type I-S kerogen that has been reported in association with some hypersaline-algal organic facies (evaporative facies association).
Oil and Gas Generation from Mixed Type I–III Kerogens
Tegelaar and Noble (1994) demonstrated through pyrolysis experiments that vitrinite-dominated (terrestrial) kerogen generally generates hydrocarbons over a
broader range of temperatures than do most algaldominated kerogens and that generation continues
well above the range of temperatures associated with
generation from algal-dominated kerogen. This result
suggests that given the same burial and thermal history,
mixtures of aquatic and terrestrial organic matter
should generate petroleum over a broader range of
thermal maturities than purely algal source facies. Data
from the Uinta basin in Utah are consistent with this
hypothesis. Oils in the Altamont-Bluebell fields were
sourced primarily from freshwater lake facies of the
lower Green River and underlying formations that resemble the Luman Tongue (Fouch, 1975; Ruble and
Philp, 1998). Based on Rock-Eval transformation ratios, light hydrocarbon yields, and atomic H/C ratios,
hydrocarbon generation from these facies occurred in
the deep Uinta basin over a broad range of thermal
maturities, ranging from approximately 0.70 to 1.35 %
Ro (Anders et al., 1992).
Uinta basin oils are solid at surface temperatures
because of their very high wax content. High wax content and pour point are also associated with freshwater
lacustrine source rocks in the Central Sumatra basin
(Kelley et al., 1995), in the Songliao basin of north
China (Yang et al., 1985), in central African rift basins
(Genik, 1993), and in some Brazilian rift basins (Mello
et al., 1988; Mello and Maxwell, 1990). Minor
amounts of natural gas are also common in oil fields
sourced by mixed type I–III mixed kerogens (e.g.,
Clem, 1985; Rice et al., 1992; Genik, 1993; Kelley et
al., 1995), but the sources of these gases have not been
clearly established.
Oil Generation from Type I Kerogen
Waples (1980) and Sweeney et al. (1987) showed that
type I kerogens generate oil over a very narrow range
of thermal maturities, reflecting little variation in
chemical bond type within these homogenous kerogens. Tegelaar and Noble (1994) found that the onset
of oil generation from Green River type I kerogen
should occur between 0.8 and 0.9% Ro, and peak generation is at 0.95–1.05% Ro. The overall oil window
(defined by 10–90% kerogen transformation ratios)
thus spans only about 0.3% Ro on average, or less than
30&C based on a 1&C/m.y. heating rate. They found
that the peak generation other type I kerogens also generate over a very narrow range of thermal maturities,
although the absolute thermal maturity at which generation begins could vary widely. They calculated that
the onset of oil generation could range between 0.54
and 0.96% Ro and concluded that bulk chemical
composition alone was not sufficient to predict generation timing. Using pyrolysis–gas chromatography
techniques to better discern kerogen chemical structures, they found that two other factors influence generation timing: (1) the types and mixtures of biomacromolecules preserved, and (2) organic sulfur content.
Whether absolute generation timing can be accurately
determined from bulk chemical measurements such as
Rock-Eval HIs is, therefore, currently unclear.
Type I kerogens generate paraffinic oils produced
from the Karamay and associated fields in the Junggar
basin (Clayton et al., 1997). Karamay oils are low in
sulfur, and except where biodegraded, contain abundant n-alkanes that have molecular weights less than
C30. A similar n-alkane distribution was reported for
oils sourced from type I kerogen in the Campos basin
of Brazil (Mello et al., 1988; Mello and Maxwell,
1990). The distribution of n-alkanes in oils generated
from type I kerogens may vary widely, however, and
are subject to modifications related to thermal maturity and biodegradation.
Early Generation from Type I-S Kerogen?
Several Tertiary lacustrine basins in China contain
oils that appear to have been generated at lower thermal maturities than are normally associated with the
breakdown of typical marine or lacustrine kerogens.
Such oils have been reported from onshore extensions of the Bohai Bay basin (Shengli oil field) (Shi
et al., 1982; Z. Chen et al., 1994), the Dongpu basin
(R. Li et al., 1988, 1992), the Jianghan basin (Fu et
al., 1986; Sheng et al., 1987; Brassell et al., 1988;
Peters et al., 1996), and the Qaidam basin (Huang
et al., 1991) and are commonly associated with saline
or hypersaline source rocks. Based on source rock–oil
correlations and biomarker maturity measurements,
significant generation appears to have occurred from
these rocks at vitrinite reflectance values as low as
0.45%. Several molecular maturity measurements, including the ratios of 20S/(20S # 20R) desmethyl
steranes and 22S/(22S # 22R) homohopanes, provide confirmation of the low thermal maturity of
these oils. Jianghan basin oils, for example, commonly have 20S sterane ratios as low as 0.17–0.24,
in contrast to the observation by (Mackenzie et al.,
1980) that petroleum generation from marine type II
kerogen begins at values of about 0.40. Likewise,
Jianghan oils have C32 22S hopane ratios as low as
0.44 (Fu et al., 1986), in contrast to values of approximately 0.50–0.55 that typically mark the onset
of petroleum generation from marine kerogens. If
these low maturities are representative of a significant
fraction of the total petroleum generation, then exploration models for basins containing such oils must
allow for earlier generation of oil relative to trap formation and other play elements.
Two explanations have been offered for these lowmaturity oils. The first is that they do not represent
thermal degradation of kerogens at all, but instead result from degradation and migration of original soluble
organic matter that was never incorporated into kerogen. For example, certain coals and carbonaceous
shales rich in higher land plant remains (particularly
from conifers) may contain abundant resins (soluble
organic matter), which may be expelled as liquids
(Snowdon and Powell, 1982; Stout, 1995). Huang et
al. (1991) invoked such an explanation for Qaidam basin light oils and condensates that have 20S sterane maturity ratios of 0.18–0.23. X. Li et al. (1998) proposed
that two distinct pulses of petroleum generation occurred in the East China Sea basin, the first occurring
at less than 0.55% Ro and the second between 0.55 and
1.30% Ro. Resinite derived oils reported from Canada
are naphthenic (high percentage of hydrocarbons having ring structures) rather than paraffinic, but different
resin types and varying proportions of resinite relative
to other macerals may partly account for more paraffinic oils in China.
Carroll and Bohacs
1049
Another important consideration is the potential
volumetric significance of oil generated from resins,
which typically comprise relatively low percentages of
the total organic matter (Tissot and Welte, 1984). Saline or hypersaline lacustrine facies would be expected
to contain relatively little resinous land plant organic
matter. For example, oils in the Shengli field are
thought to represent transformation of less than 10%
of the total organic matter present in the source rocks
(Cheng Keming, Research Institute of Petroleum Exploration and Development, Beijing, 1997, personal
communication). Further volumetric data on the Shengli oil field and other accumulations of low maturity
oils are needed to determine whether expulsion of biogenic bitumen is adequate to account for the known
oil reserves.
A second and possibly complementary explanation
is that low-maturity oils result from thermal degradation of a distinctive type I-S (sulfur-rich) kerogen. type
I-S kerogen is defined as having an HI above 600 and
a S/C atomic ratio greater than 0.04 (Sinninghe
Damsté et al., 1993; Peters et al., 1996). Jianghan basin
oils are generally high in sulfur (3.59–12.91%) (Sheng
et al., 1987), and aromatic organosulfur compounds
are abundant (Philp and Fan, 1987; Sheng et al., 1987).
Peters et al. (1996) presented experimental kinetic
data showing that the type I-S kerogen in Jianghan
hypersaline lacustrine source rocks reacts quickly to
thermal stress, in a manner similar to marine type II-S
kerogen in the Monterey Formation. Type II-S kerogens have been demonstrated to generate oil at lower
thermal exposures than other kerogens (Orr, 1986).
Hydrous pyrolysis experiments using Monterey kerogens provide evidence that this generation actually occurs via a two-step process (Baskin and Peters, 1992).
In the first step, cleavage of relatively weak S–C bonds
generates heavy bitumen that is retained on the kerogen matrix. Thermal degradation of this bitumen at
slightly higher temperatures results in the expulsion of
oil. Thus, this model appears to combine elements of
the “oil from resins” model discussed previously with
thermal degradation of an uncommonly reactive
sulfur-rich kerogen. Low maturity Monterey Formation oils, however, have low API gravities and are dominated by asphaltenes and resins, whereas Jianghan oils
are dominantly paraffinic (Philp and Fan, 1987). More
work is clearly needed to resolve the actual generation
mechanisms responsible for low-maturity oils in
China.
Type I-S kerogens appear to result from diagenetic sulfurization rather than from biological in1050
Lacustrine Petroleum Source Rocks
put of organosulfur compounds. Sheng et al. (1987)
identified long-chain normal alkyl-thiophenes and
alkyl-thiolanes, long-chain isoprenoid-thiophenes and
isoprenoid-thiolanes, and benzothiophenes in sulfurrich oils. The distribution of these compounds mirrors
that of the corresponding alkanes, leading Sheng et al.
(1987) to conclude that the sulfur compounds originated through diagenetic reactions between elemental
sulfur and sulfides with phytol, fatty acids, and alcohols. The apparent rarity of type I-S kerogen may
stem from the fact that most lake waters have low
sulfate concentrations relative to marine systems and
that in siliciclastic lacustrine systems there is commonly excess iron available to form pyrite or other
sulfides. These limitations, however, may be overcome if sulfide scavengers such as Fe and Mn are
scarce, especially if sulfate supply to the basin is unusually high. For example, Sinninghe Damsté et al.
(1993) documented type I-S kerogen in two Tertiary
Spanish oil shales, which they interpreted as having
formed as a result of weathering of Triassic evaporites.
Sulfate from the evaporites was microbially reduced
in a freshwater lake and incorporated into previously
deposited remains of Botryococcus braunii algae. A
more common setting for type I-S kerogen is probably
in evaporative lakes where siliciclastic sediment input
is low and evaporative concentration of sulfate occurs.
In the Chinese basins, the oil source rocks are closely
associated with gypsum and other evaporites and have
biomarker characteristics indicative of anoxic saline to
anoxic hypersaline environmental conditions (Shi et
al., 1982; Fu et al., 1986; Philp and Fan, 1987; Sheng
et al., 1987; Brassell et al., 1988; Huang et al., 1991;
Z. Chen et al., 1994).
CONCLUSIONS
1. Three end-member lacustrine facies associations
have been observed to recur in widely disparate
geographic settings and over a wide range of geologic time: fluvial lacustrine, fluctuating-profundal,
and evaporative. Significant differences exist among
each of the these associations in terms of oil vs. gas
generation and the timing of petroleum generation
relative to basin thermal history. These associations
can be used to make first-order predictions of petroleum generation type (oil vs. gas) and timing
from limited geologic data on nonsource facies.
Conversely, geochemical analysis of source rocks or
oils may aid in predicting lacustrine paleoenviron-
ments and the likely distribution and quality of reservoir and seal facies.
2. Lacustrine source rocks display a high degree of
geochemical heterogeneity relative to marine facies,
therefore their complete characterization requires
analysis of a relatively large number of samples selected at fixed, regular intervals. Selective sampling
of a few particularly rich beds may lead to erroneous
conclusions concerning overall generative potential.
3. No typical lacustrine source rock or oil exists. The
default-value kerogen properties and kinetics sometimes used in basin modeling are therefore of limited utility. For optimal precision, detailed kinetic
measurements should be made from immature
equivalents of the suspected source facies, if possible, and should be supported by molecular maturity
measurements from source rock extracts and generated oils.
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