Sedimentology and Petrophysical Character of Cretaceous Marine Shale Sequences in Foreland Basins—Potential Seismic Response Issues
W. R. Almon, E. Rietsch, Wm. C. Dawson, S. J. Sutton, F. G. Ethridge, B. Castelblanco-Torres, 2005. "Sedimentology and Petrophysical Character of Cretaceous Marine Shale Sequences in Foreland Basins—Potential Seismic Response Issues", Evaluating Fault and Cap Rock Seals, Peter Boult, John Kaldi
Download citation file:
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.
Figures & Tables
This volume constitutes the proceedings of the AAPG Hedberg conference on seals held in Barossa Valley, South Australia, in 2002. The key driver for both the Hedberg conference and this publication was the recognition that knowledge of risk in the estimation of sealing capacity and fault-seal potential is important in making judgments at the exploration, appraisal, and development stages of the petroleum business. In addition, incorporating seal risk in the overall assessment of hydrocarbons in place can affect decisions to drill prospects and the location of appraisal and development wells, as well as reserve estimation. Improved methods to estimate seal capacity and fault integrity can lead to savings in well costs, improved recoveries through optimum placement of wells, and improved estimates of hydrocarbon in place. This volume contains 18 chapters that reflect the spectrum of presentations at the conference. The knowledge imparted by these chapters will be a window on the state of seal knowledge at this juncture of time and includes topics such as seal failure related to basin-scale processes, the role of geomechanics in seals, and the economic evaluation of prospects with a top seal risk.