Abstract:Diagenesis and Porosity/Permeability Prediction in Pennsylvanian Phylloid Algal Bioherms of the Paradox Basin, Southeastern Utah

GOURNAY, JONAS P., The University of Texas at Austin, Dept. of Geological Sciences, Austin, TX

Phylloid algal bioherms form significant hydrocarbon reservoirs (i.e. Aneth Field and the Horseshoe Atoll Trend). Problematic recovery results from both depositional processes and diagenesis. The goal of this project, is to identify the key fabrics and diagenetic events within these bioherms and to model their occurrence in order to develop a predictive model for porosity and permeability in related reservoirs.

Outcrop and core examination from the lower Ismay and Desert Creek zones of the Paradox Formation (Paradox Basin), as well as comparative work in Permian bioherms from the Laborcita and Hueco Canyon Formations (Orogrande Basin), reveal two end-member fabrics: a phylloid algal biomicrite with relatively high porosity, but poor permeability and a phylloid algal biosparite with high porosity and permeability. These fabrics tend to repeatedly stack, promoting reservoir heterogeneity.

Diagenesis is the other major control of heterogeneity. Numerous episodes of dissolution and precipitation with respect to calcite and dolomite, the implacement of anhydrite, compaction, and fracturing have greatly affected the fabrics. Open and cemented fractures have been identified which can either have a homogenizing effect, or further promote heterogeneity.

The final goal of this project is to model bioherm reservoir evolution in an attempt to identify those controls most significant to porosity and permeability formation and occlusion. This work will be performed in conjunction with the Texas Institute of Computational and Applied Mathematics Center for Subsurface Modeling group, using an in-house geochemical finite-difference model.

AAPG Search and Discovery Article #90931©1998 AAPG Foundation Grants-in-Aid