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 sequence stratigraphic-based models for predicting seal occurrence and estimating top seal capacity for application in hydrocarbon exploration and risk analysis. Few systematic studies of seal character and shale sedimentology are available. Consequently, seals remain the least understood element of petroleum systems. 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 sequence stratigraphic framework for the Lewis Shale. Winton-Barnes et al. (2000) characterized sandstone lithotypes within the Lewis Shale, and Costeblanco-Torres (2003) completed a detailed study of shale lithotypes from Lewis Shale outcrops and cores. Almon et al. (2002) documented considerable variability in petrophysical properties of shales within the Lewis Shale. 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 sequence stratigraphic cross-section reveals that the Lewis Shale consists of at least twenty (probable 4th-order) depositional sequences (Figure 3.3). Beneath the “Asquith Marker” Lewis Shale deposition was basically aggradational (Figure 3.1). The overlying progradational unit consists dominantly of silty shales (3rd-order highstand [HST]) with interstratified 4th-order “lowstand” (LST) sandstones (Figure 3.2). These sandstones record below storm wave base deposition from storm-induced gravity flows. Relatively weak seals (HST shales) are interstratified with the sandstones (potential reservoirs).
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 well-developed laminar fabrics and enrichment in iron-bearing clay minerals, TOC, and authigenic pyrite have excellent to exceptional seal. 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 analysis (Figure 7.2). Discriminant function analysis of Lewis Shale microfacies yielded two functions that account for nearly 99% of the total variance (Figure 7.3). TST shales are enriched in iron-bearing clay minerals and pyrite and have strongly elevated MICP values relative to HST shales (Figure 7.4). 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 (well-developed laminar fabrics) as well as the precipitation of Fe-carbonate cements during early submarine diagenesis. 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).
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