Predicting Reservoir Quality and Geometry in Rudist Buildups Rimming an Intrashelf Basin; Albian (Cretaceous), Maverick Basin, Pecos River Canyon, Southwest Texas

By

KERANS, CHARLIE

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

The Pecos River Canyon exposures of upper Albian (Cretaceous) limestones provide a spectrum of styles of rudist reef development. These rudist buildups vary in style from isolated mounds having up to 15 ft of local relief to hydrodynamic accumulations within tidal inlet fills and along the foreshore-upper shoreface transition. These buildups also vary widely in terms of faunal content and rock-fabric and porosity-permeability characteristics. This study analyzes these buildups and presents a means for predicting style and reservoir quality of buildups by placing them into a sequence stratigraphic framework.

Considering an ideal model for a single high-frequency sequence, we show that the TST is dominated by mud-rich facies that contain low-relief, pancake-shaped buildups with a core colonizing community of chondrodontid clams and a capping facies of radiolitid rudist rudestone/bafflestone. In contrast, HST deposits contain large accumulations of caprinid rudists that are clustered as hydrodynamic piles within tidal inlets in tide-dominated settings and as lags along the upper shoreface of wave-dominated sequences. The high-energy highstand setting of the caprinid buildups makes the reservoir quality of these facies far superior to their chondrodont-radiolitid counterparts in the mud-dominated TST. Comparison of highstand rudist accumulations from early transgressive sequence set (TSS) sequences with those of later highstand sequence set (HSS) sequences within the upper Albian composite sequence illustrates a different trend. High-frequency sequences in the TSS have highstand caprinid rudist complexes with greater dip dimension, coarser grain size, and higher interparticle porosity than those of their wave-dominated HSS counterparts. Knowledge of setting within the sequence/systems tract framework thus allows prediction of rudist buildup type, gross external geometry, and rock-fabric/reservoir quality.