PSSedimentology and Petrophysical Character of Cretaceous Marine Shale Sequences, Foreland Basins: Seismic Response of Seals Horizons*
By
Wm. C. Dawson1, W.R. Almon1, E. Rietsch1, F.G. Ethridge2, S.J. Sutton2, and B. Castelblanco-Torres3
Search and Discovery Article #40089 (2003)
*Adapted from poster presentation at AAPG Annual Convention, Salt Lake City, May, 2003.
1ChevronTexaco, Bellaire, TX ([email protected])
2Colorado State University, Fort Collins, CO
3ChevronTexaco, Bakersfield, CA
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Development of predictive models to estimate the
distribution and petrophysical properties of potential mudstone flow barriers
can reduce risks inherent to exploration and exploitation programs. Such a
predictive model, founded in sequence
stratigraphy, requires calibration with
outcrop and subsurface analogs. Detailed sedimentologic, petrophysical, and
geochemical analyses of Lewis Shale (Lower Maastrichtian) samples from SE
Wyoming reveal considerable variability in seismically significant rock
properties. Lower Lewis strata represent late-stage transgressive deposits that
include a distinctive condensed interval. The overlying progradational Lewis
interval consists mostly of interstratified very silty shales and argillaceous
siltstones. High-frequency sheet and lenticular sandstone bodies occur within
the progradational Lewis package. Sealing capacity, as measured by mercury
injection capillary pressure
analysis
(MICP), varies with fabric, texture, and
compositional factors that are related to
sequence
stratigraphic position.
Samples from the Lewis Shale transgressive interval have significantly greater
MICP values (average 18,000 PSIA) and are markedly better seals relative to
samples from the overlying Lewis Shale progradational package (average 3,000
PSIA). Transgressive shales with enhanced sealing capacity are characterized by
higher total organic carbon and hydrogen index values, lower permeability, and
less detrital silt content. These transgressive shales are enriched in
iron-bearing clay minerals and authigenic pyrite. Greater shear wave velocities,
larger shear moduli, and higher bulk density also characterize transgressive
Lewis shales.
The most promising seal horizons are laterally extensive, silt-poor, pyritic shales occurring in the uppermost transgressive systems tract. Stacking patterns of slow and fast shale horizons can yield seismic responses comparable to those interpreted as hydrocarbon-bearing reservoirs.
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Figure Captions (1-2.1 - 1-2.3)
The ultimate goal of this research is to
develop
The Lewis Shale (Upper Cretaceous,
Maastrichtian), which crops out along the eastern margins of the Great
Divide and Washakie basins in south-central Wyoming, provides an
interesting analog for understanding stratigraphic architecture of
turbidite depositional systems (Figures 1-2.1,
1-2.2, and 1-2.3).
Previous outcrop and subsurface studies (e.g., Pyles and Slatt, 2000)
established a high-frequency The Lewis Shale is exposed intermittently along a 60-mile-long outcrop belt on the Rawlins-Sierra Madre uplift west of Cheyenne, Wyoming (1-2.1). Extensive subsurface data are provided by numerous producing fields west of the outcrop belt.
Stratigraphy (Figures 3.1-3.5) Figure Captions (3.1-3.5)
High-frequency
Subsurface DataChamplin 276 D-1, Section 13, T19N, R93W, Carbon County, Wyoming Figure Captions (4.1-4.2)
The Champlin 276 D-1 core (Figure 4.1)
represents the transgressive (TST) part of the Lewis Shale. These
samples have significantly higher MICP values (mean 18,000 psia)
relative to other Lewis Shale samples (Figure 4.1). Shales exhibiting
Total clay content varies from 54 to 64 percent (Figure 4.2) with a mean of 51 percent (std dev = 2.5 %). Quartz content ranges from 23 to 34 percent. The mean is 28 percent (std dev = 3.9 %). Detrital feldspars, pyrite, and carbonate are common accessory (18 to 26 percent; mean 20) minerals. The dominant clay type is the 2:1 aluminum family (Figure 4.2). Abundance ranges from 17 to 32 percent with a mean of 25 (std dev = 5.1 %). The 2:1 iron-bearing clays are also major components. Their abundance ranges from 15 to 27 percent with a mean of 21 percent (std dev = 4.6 %). Kaolinite (mean 4 %) and iron-bearing chlorite (mean = 1%) are minor components.
Section 25/25, T16N, R92W, Carbon County, Wyoming Figure Captions (5.1-5.3)
The Sierra Madre outcrop represents the highly progradational (3rd-order highstand) part of the Lewis Shale; the dominant lithofacies are silty shales (microfacies 2) and argillaceous siltstones (microfacies 5) (Figure 5.1). Several high-frequency (4th- or 5th-order) lowstand sandstone units are interstratified with this highstand systems tract (HST). Two major types of sandstone bodies (lenticular and tabular) are recognizable in this outcrop (Witton-Barnes, 2000) (Figure 5.2). Massive to weakly laminated shales and siltstones that compose the Lewis Shale HST are characterized by relatively high (mean 37 %) content of detrital silt, low TOC values, and the lowest sealing capacities (mean 1.150 psia) measured within the Lewis Shale (Figure 5.1). These relatively low sealing capacities are typical of shales from proximal parts of marine depositional systems (Dawson and Almon, 2002). Total clay content ranges from 35 to 71% (mean 52%) (Figure 5.3). Detrital silt (quartz + feldspars) abundance varies from 24 to 59% (mean 37%). Pyrite, siderite, Mg-calcite, and dolomite are accessory (1 to 4%) components. The normalized clay mineral composition is dominated (56 to 78%) by 2:1 aluminum clays (mean 67%) (Figure 5.3). Behind Outcrop CoreColorado School of Mines Stratigraphic Test 61Section 25, T16N, R92W, Carbon County, WyomingFigure Captions (6.1-6.2)
Samples from the Colorado School of Mines Strat Test 61 represent the lowstand Lewis Shale. These samples consist of very silty shales and siltstones that have relatively low MICP values (mean 2886 paia). Lower MICP values are typical of silt-rich shales wherein detrital sized grains are concentrated into high-frequency laminae (Figure 6.1). Total clay content varies from 35 to 69 percent (Figure 6.2) with a mean of 51 percent (std dev = 8.5 %). Quartz content ranges from 3 to 41 percent. The mean is 27 percent (std dev = 8.6 %). Detrital feldspars, pyrite and carbonate are common accessory (16 to 32 percent; mean 21) minerals. The dominant clay type is the 2:1 aluminum family. Abundance ranges from 0 to 46 percent with a mean of 31 (std dev = 5.1 %). The 2:1 iron-bearing clays are also major components (Figure 6.2). Their abundance ranges from 7 to 24 percent with a mean of 14 percent (std dev = 3.8 %). Kaolinite (mean 6 %) and iron-bearing chlorite (mean = 1%) are minor components.
Data SummaryFigure Captions (7.1-7.8)
Five microfacies have been recognized in the Lewis Shale in the study area; they are tabulated in Figure 7.1 and listed below:
Distal marine (TST) shales (microfacies 1 and 4) exhibit the “best” seal
character based on MICP Porosity
of TST shales is significantly lower than porosity in HST shales (Figure
7.5). MICP values are increased as porosity is reduced significantly in
the upper TST interval relative to all parts of the HST interval. The
reduced porosity in clay-rich TST shales is attributed to improved
organization of particles ( Additionally, there is a major difference in the permeability of TST and HST shales. Within the Lewis HST there is a weak trend of upward increasing permeability; this trend appears to correlate with a vertical increase in the content of detrital silt. There is a correlation between seal capacity and depositional systems, with an progressive increase in capacity from slumps/debris flows, HST, MFS, TST, condensed shales, to paleosols (Figure 7.6). A strong correlation between subsurface and outcrop samples, along with evidence of comparable burial history (Tmax data), suggests that other factors (e.g., diagenetic processes) are responsible for differences in seal character (Figure 7.7). Tmax values are essentially the same for all Lewis Shale samples; this implies that they have undergone comparable burial histories (Figure 7.8).
Seismic ModelFigure Captions (8.1-8.10)
Rock Properties Measurements reveal significant differences in the bulk density, Poisson’s ratio, and shear velocity of TST and HST shales (Figures 8.1, 8.2, and 8.3). The differences in rock properties across a shale-shale contact (i.e., low velocity and low density HST shale immediately above a high velocity and high density TST shale—Figures 8.1, 8.3, and 8.4) could generate a strong seismic reflection.
Shale Reflection Modeling Experiment
The basic modeling performed in this
experiment used measured elastic rock properties data and layer
thicknesses from
Each trace of this gather represents a
specific reflection angle and was convolved with an angle-specific
wavelet. These wavelets were derived from analog To assess the effect of the thin high-impedance shale (orange layer), it was removed, and the calculations repeated without it. The result (model 2--Figure 8.7) shows that the overall effect on the synthetic seismic is minimal. After removing both the orange and green layers (model 3--Figure 8.8), the effect is more noticeable but remains small (Figure 8.9).
The example of a seismic profile in
Figure
8.10 shows a shale horizon (strong seismic reflector) that could be
misinterpreted as hydrocarbon-saturated sandstone. Results from a
ConclusionsFigure Caption (9.1)
Lewis Shale strata consist of at least 5 argillaceous microfacies that exhibit distinctive sedimentological and petrophysical features along with significant variations in seal character. Uppermost transgressive and condensed shales (Lewis Shale microfacies 1 and 4) offer excellent to exceptional top seal potential. These shales occur preferentially in distal parts of marine depositional systems. The top seal capacity of highstand (Lewis Shale microfacies 2 and 5) and lowstand (Lewis Shale microfacies 3 and 5) intervals is reduced mainly because of elevated content (> 25%) of detrital silt and disrupted fabrics (extensive bioturbation). Significant stratigraphic separation (several hundred feet) can exist between a lowstand sandstone reservoir and its controlling top seal horizon (i.e., overlying transgressive shale) (Figure 9.1). Factors that tend to enhance sealing characteristics of marine shales include: low content (<25%) of detrital silt; relatively slow rates of accumulation; low oxygen levels and limited bioturbation (preservation of laminar fabrics); and increasing content of Fe- and Mg-enriched minerals.
Seismically significant parameters (e.g.,
density, shear velocity, Poisson’s ratio, and compressional velocity)
exhibit systematic variations that are consistent within the 3rd-order
Seismic modeling reveals a potential of some shales to exhibit an AVO response comparable to that exhibited by hydrocarbon-saturated sandstones.
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Spring, D.E., 1990, Schieber, J., 1999, Distribution of mudstone facies in the Upper Devonian Sonyea Group of New York: Journal Sedimentary Petrology, v. 69, p. 909-925. Showalter, T. T., 1979, Mechanics of secondary hydrocarbon migration: AAPG Bulletin, v. 63, p. 723-760. Winn, R. D., et al., 1987, Shallow-water and sub-storm-base deposition of Lewis Shale in Cretaceous Western Interior Seaway, south-central Wyoming: AAPG Bulletin, v. 71, p. 859-881. Winton-Barnes, E. M., et al., 2000, Outcrop and subsurface criteria for the differentiation of sheet and channel-fill strata: example from the Cretaceous Lewis Shale, Wyoming: GCSEPM Foundation 20th Annual Research Conference, p. 1087-1104.
AcknowledgmentsThe authors thank ChevronTexaco for permission to present these data and interpretations. We are especially grateful to R.M. Slatt, D.R. Pyles and S.M. Goolsby for sharing their knowledge concerning the Lewis Shale. C.W. Ward aided with the collection of samples. W.T. Lawrence prepared thin sections, and J.L. Jones contributed SEM images. D.K. McCarty completed XRD analyses, and Poro-Technology (Houston, TX) provided MICP analyses. Graphic design by L.K. Lovell (ChevronTexaco). |