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Sedimentology and Petrophysical Character of Cretaceous Marine Shale Sequences in Foreland Basins—Potential Seismic Response Issues
W. R. Almon,1 Wm. C. Dawson,1 F. G. Ethridge,2 E. Rietsch,1 S. J. Sutton,2 B. Castelblanco-Torres3
1Chevron Texaco Inc., Houston, Texas, U.S.A.
2Colorado State University, Fort Collins, Colorado, U.S.A.
3Chevron Texaco Inc., Bakersfield, California, U.S.A.
The 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. Ward and A. Koenig made initial stratigraphic observations and collected outcrop samples. W. T. Lawrence prepared the thin sections. D. K. McCarty completed the XRD analyses, and B. J. Katz provided insight into the organic geochemical analyses. Poro-Technology (Houston, Texas) performed mercury injection capillary pressure analyses. The authors have benefited greatly from discussions concerning sequence stratigraphy with J. B. Sangree and L. M. Liro.
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 sedimentological, petrophysical, and geochemical analyses of Lewis Shale (lower Maastrichtian) samples from southeast Wyoming reveal considerable variability in petrophysically and 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 in the progradational Lewis package. Sealing capacity, as measured by mercury injection-capillary pressure (MICP) analysis, 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 3000 psia). Transgressive shales with enhanced sealing capacity are characterized by higher total organic carbon and hydrogen index values, lower permeability, and lower 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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