Paul R. Knox
Bureau of Economic Geology, The University of Texas at Austin

ABSTRACT

Risk assessment for petroleum prospects and aquifer contamination rely, in part, on successfully predicting either capillary characteristics or permeability of known shales that either form a potential reservoir seal or a confining aquitard. For these purposes, the ability of the shale to impede flow is frequently considered to be a consequence of its thickness. The distribution of hydrocarbons in Frio sandstone intervals of West Fulton Beach Field in Copano Bay, Texas, suggests that instead this ability is a function of the quality of the shale (fabric and ductility), which can be predicted by its relationship to a hierarchy of flooding surfaces and its position within a depositional cycle. More significant flooding-surface-related shales in the transgressive limb of a depositional cycle may be thinner because their distal setting results in less clastic input. However, they likely will have less silt content and higher organic content, resulting in a more aligned clay platelet fabric and increased ductility. This decreases pore throat size, leading to reduced vertical permeability (hydraulic conductivity) for wetting phases (usually water) and greater capillary entry pressures in nonwetting phases (usually hydrocarbons). This contrasts with higher-frequency flooding shales in progradational limbs, which may be thicker but siltier, with higher permeability (conductivity) and lower entry pressures.

If shale quality (permeability/capillarity) can be predicted using the same sequence stratigraphic techniques currently in use to evaluate sandstone reservoir quality, then more reliable values can easily be obtained for use in (1) models that assess contaminant risk for an aquifer, (2) subregional-scale fluid flow models used for groundwater availability modeling, and (3) assessments of hydrocarbon prospects. In hydrocarbon exploration, for example, the juxtaposition of high quality reservoir and high quality seal can impact the geographic focus of an exploration program. Yet to be assessed is the role that this hierarchy of shale quality plays in hydrocarbon migration and charge.