Seismic Attribute Analysis of a Mississipian Chat, Osage County, Northeast
Oklahoma
Malleswar Yenugu and Kurt J Marfurt
Abstract
Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide
(SiO2). Chert occurs as nodules, concretionary masses and as layered deposits. Chert is an
unconventional reservoir rock that has been developed successfully in west Texas, Oklahoma, California
and Canada (Rogers and Longman, 2001). The deposition of chert reservoirs commonly occur in close
stratigraphic proximity to carbonate rocks, although chert reservoir origin, characterization, and
distribution may be quite different from adjacent carbonate rocks. Cherts which show high porosity and
low resistivity are sometimes called Chat. The Mississippian tripolitic chat is currently an exploration and
development objective through- out southern Kansas and northern Oklahoma, including our survey area
of Osage county.
Analysis of Mississippi Chat core from a well within our study area show porosity values ranging from
12% to 42%. Image (FMI) log of the Mississippi Chat shows complex internal architecture characterized
by vugs, nodules, fractures and little remnant of original depositional fabric. Through core and log
measurements, we are able to establish a good correlation between impedance and porosity. To this
end we map seismic acoustic impedance volumes to predict additional sweet spots. The chert is also
controlled by diagenetically altered fractures which we are able to seismically map using coherence and
most-negative principal curvature volumes. Combining these geometric attributes with impedance
estimates and well control, we generate 3D lithofacies maps with the goal of understanding the
distribution of fractures and the production from the wells.
Introduction
The geology of Oklahoma, while complex, is one of the best-interpreted geological provinces (Figure 1-a)
in the United States because of the amount of drilling and seismic exploration that was done in search of
hydrocarbons. Great thicknesses of sedimentary rock are preserved in a series of major depositional and
structural basins separated by the orogenic uplift that formed during the Pennsylvanian time. The
Figure 1-a: Map showing major geologic province of Oklahoma. The study area is shown in blue box.
sedimentary basins mentioned above contain ~20,000 to ~40,000 ft of sedimentary rock, mostly
Paleozoic and marine overlaying complex Precambrian basement igneous and some low-ranking meta
sedimentary rocks (Johnson et al, 1988). In the early Paleozoic, during Cambrian and Ordovician Period,
a tectonic event took place and three major depositional provinces formed: the Oklahoma basin, the
southern Oklahoma aulocogen, and the Ouachita trough (Figure 1-b).
Figure 1-b: Map of the southern USA showing boundary of Oklahoma basin and other major features that existed in the early
and middle Paleozoic time (Johnson, 1988)
The Oklahoma basin is a broad, shelf like area that includes a sequence of remarkably thick and
extensive shallow-marine carbonates interbedded with thinner marine shale and sandstone (Johnson
and others, 1988.) The southern Oklahoma aulacogen was the depocenter for the Oklahoma basin
(Figure 1-c) with a west-northwest-trending trough where sediments tend to be similar to those
elsewhere in the basin; however, they are two to three times thicker. The alocogen includes the
Anardako, Ardmore, and Marietta protobasin, along with the Arbuckle anticline and the Wichita
Mountain uplift (Kenneth, 2001). The Ouachita trough was where deep-water sedimentation took place
along a rift at the southern margin of the North America Craton. These sediments were then thrust ~50
mi where they reside as the “Ouachita Mountain uplift”. The Middle Paleozoic system, including the
Silurian, Devonian, and the Ordovician time, ranges from ~3,000 to ~6,000 feet thick in the aulocogen to
~1000 to 3000 feet thick in most other parts of the Oklahoma basin(Figure 1-c).The accumulation of a
thick column of sediment in the aulacogen was a result of rapid subsidence that took place around the
Cambrian and Ordovician time; however, sedimentation rapidly thinned in the Silurian and Devonian
Figure 1-c: Schematic cross sections showing restored thickness of Silurian, Devonian, and Mississippian strata in Oklahoma
(Carbott, 1992)
time, because of slow rate of subsidence. By the Mississippian time the aulocogen suffered more rapid
subsidence in the eastern part and, at the same time, as much as 10,000 ft of black shale, novaculite,
and flysch sediments was dumped into the Ouachita trough (Figure 1-c).
These three major
tectonic/depositional provinces persisted through the middle Paleozoic until Pennsylvanian time, when
sharply uplifted crustal blocks divided the Oklahoma basin and the Aulacogen into a series of welldefined marine basins. These marine basins created accommodation spaces for the deposition of what is
known to be the Osage Mississippian chat. Mississippian “chat,” MC is the representation of the
unconformity between the Pennsylvanian and Mississippian in north-central Oklahoma and southcentral Kansas, is a weathered and/or detrital interval of highly porous or hard, tight chert at or near the
top of the Osagean formation (Suzanne M. Rogers). Previous works, suggested that the MC spreads
throughout north-central Oklahoma as a compartmentalized reservoir and is a very heterogeneous type
of reservoir. Couple of modules have been proposed and concluded that the depositional environment
and diagenetic alteration of the MC was formed through a combination of uplift and either erosional
detritus or weathering-in-place of Osagean Mississippian cherty limestone (Figure 1-c). The diagenetic
processes included cherty limestone being replaced by silica followed by a change in conditions, such as
continued uplift resulting in infiltration of meteoric water, which resulted in flushing of the rock and
causing the dissolution of remaining calcite by low pH fluids. The deposition and the diagenetic
processes are very crucial in determining the characteristics (velocity, permeability, porosity, lithology)
of an unconventional reservoir like the Osage MC where rock properties are often affected by
diagenetic processes. For example, a study of logs and completion records of more than 6600 wells in
north-central Oklahoma shows that the chat appears on well logs as a low-resistivity zone having low
density and high porosity induced by fractures. Cherty reservoirs in northern Oklahoma are usually
characterized based on the reservoir quality; porosity, permeability and mineral compositon. Generally,
commercially productive zones have a minimum porosity of 25–30% and water saturations less than
80%. Whereas permeability results from indicate permeability as low as (0.1 to 50 md; average_ 20 md)
(Suzanne, 2001).
The study area (Figure 1-a) is located in the northern part of Osage county in northern Oklahoma. This
study is an integrated approach from Seismic attributes, Seismic inversion and petrophysical properties
to better characterize the Mississippian Chat, which is an exploration and development object in the
Osage county.
Seismic Attribute Analysis:
Different seismic attributes like RMS amplitude, fault lineaments from coherency volume, and curvature
attributes like most positive and most negative have been generated on the post stack seismic data. The
two way time structural map of MC is shown in figure-2. The RMS amplitude calculated along a window
of +-10ms along the Mississippian Chat with the fault lineaments have been shown in figure-3. It is
observed from the figure that the top of MC is compartmentalized with the fault lineaments. The wells 2
and 3 are falling in one compartment on the western side of the field whereas the well-1 is falling on
another compartment. The most negative and positive curvature attributes extracted along the
Mississippian chat are shown in the figure-4.
Figure-2: Time structure map on top of the Mississippian Chat
Figure-3: RMS amplitude map extracted along Miss Chat along a window of +-10ms, red ellipsoids show the compartments
formed by faults generated from coherency volume.
Figure-4: a) Most Negative b) Most Positive curvature slices along the Mississippian Chat
Post Stack Acoustic Impedance Inversion
Model based acoustic impedance inversion has been generated on the post stack data. The
petrophysical data from five wells in the field have been used. Petrophysical logs of well-1 are shown in
figure-6. High values of porosity upto 30%, low resistivity, low density and Vp are observed from the logs
of well-1. The petrophysical properties of the MC can be easily distinguishable from that of the
carbonates above and below it.
Figure-6: The petrophysical properties of well-1. Observe the high porous MC is showing low Vp and density.
The well-1 has the sonic Vp and Vs logs, Gardner’s reverse equation has been used to generate the Vp
and synthetic seismograms from density logs for all the rest of four wells. A combined wavelet of
constant phase (figure-7) has been extracted from all the five wells and used to remove the wavelet
effect from the seismic volume to get the reflectivity series of the subsurface.
Figure-7: a) Amplitude spectrum b) Frequency spectrum of the combined wavelet.
A model is created by using five horizons and the entire seismic volume is inverted to get the
impedance volume which shows the layer properties of the subsurface. The figure-8 shows the seismic
wiggles of a seismic line superimposed on the acoustic impedance response that is passing through the
well-1. The impedance values of MC are very low indicative of good lithofacies with hydrocarbon
saturation. A cross plot is drawn between the impedance log and neutron porosity of the well-1 (figure9), the MC is having the low impedance with high porosity than the rest of the formations that are
encountered in the well. An impedance slice 10ms below the MC has been extracted and shown in
figure-10 with the fault lineaments. The distribution of lithofacies are different from one compartment
to another compartment. The dark colors like blue and magenta indicative of poor facies with low
hydrocarbon saturation and the light colors like green and yellow are indicative of good lithofacies with
hydrocarbon saturation and the production of these wells infer the same. The three wells are producing
at different rates and the porosity distribution is also not uniform in the MC.
Figure-8: The seismic wiggle line superimposed on impedance response along the well-1.
Figure-9: Cross plot between acoustic impedance and Neutron porosity in the well-1
Figure-10: Impedance slice extracted 10ms below the Miss Chat.
Conclusions:
The petrophysical properties of Mississippian chat is different from other lithofacies in the area. The
RMS and impedance maps along the MC are indicating non-uniform distribution of the facies and the
reservoir is also compartmentalized leading to difference in distribution of reservoir properties like
porosity and permeability. The production rates from different wells drilled in different compartments
also supporting that the reservoir is compartmentalized.
References:
1.
James P. Rogers and Mark W. Longman, An Introduction to chert reservoirs of North America
2.
D. H. Mikkelson, The Origin and Age of the Mississippian Chat in North-Central Oklahoma
3.
Dongjun Fu, E. Charlotte Sullivan and Kurt J Marfurt, Rock-property and seismic-attribute analysis of a chert reservoir
in the Devonian Thirty-one Formation, west Texas, USA
Acknowledgements
The authors would like to say thanks to Spyglass for providing their data sets.