Sally J. Sutton¹, Frank G. Ethridge², William R. Almon³, William C. Dawson⁴

(1) Colorado State Univesity, Fort Collins, CO
(2) Colorado State University, Fort Collins, CO
(3) Texaco, Bellaire, TX
(4) Texaco, Inc, Bellaire, TX

ABSTRACT: Variables Controlling Sealing Capacity of Lower and Upper Cretaceous Shales, Denver Basin, Colorado

Considerable research has focused on the depositional processes, architecture, and sedimentological properties of reservoir sandstones, whereas relatively little is known about the interstratified seal lithologies. Consequently, seal competency is one of the greatest risk factors associated with petroleum exploration. The ultimate goal of this research is to develop predictive analog models for estimating seal capacity of shales and seal/flow barrier distribution for use in oil and gas exploration and risk analysis/mitigation. Initial research efforts have concentrated on the sealing capacity of Lower Cretaceous Skull Creek and Upper Cretaceous Graneros marine shales in the Denver basin, Colorado. The sealing capacity of these shales, as determined by mercury-injection capillary pressure analysis, varies with location (outcrop versus core), textural and compositional variables, and sequence stratigraphic position.

Core samples of Graneros shales have significantly higher mercury injection capillary pressure values than outcrop samples. Core samples also reveal generally higher correlations between PSIA and various textural and compositional variables. In general better seals with higher PSIA values have lower permeabilities, lower mean grain sizes, lower percentages of detrital silt grains, higher total organic carbon and higher hydrogen indexes. In contrast, most shales displaying high degrees of bioturbation are relatively poor seals. Shales from condensed sections and the upper portions of transgressive systems tracts are usually the best seals. However, occasionally shale from either one of these two stratigraphic intervals can be a relatively poor seal and shale from the lower portion of the highstand systems tract can be relatively good seal.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado