Controls on Distribution of Chlorite Coats and Reservoir Quality in Cretaceous Tuscaloosa Sandstones, Gulf of Mexico

Some of the Upper Cretaceous Tuscaloosa sandstones from onshore central Louisiana, Gulf of Mexico basin, retain anomalously good reservoir quality (porosity >25% and permeability >100 md) at depths >6 km. High porosity and permeability occur in sandstones with thick, continuous chlorite coats around detrital grains that inhibited quartz cementation. Other Tuscaloosa sandstones at the same depth are tightly cemented by quartz or carbonate. We examined variable distribution of reservoir quality in Tuscaloosa sandstones in an area of uniform provenance and burial history. Sandstone composition was determined by point counts of 141 thin sections from 15 wells. Core descriptions and facies interpretations provided a sequence-stratigraphic and depositional-systems framework. Volcanic rock fragment content and grain size have weak but statistically significant correlations with chlorite volume. Higher permeability samples also contain fewer ductile grains and have higher intergranular volume. Chlorite is slightly more abundant in lowstand fluvial-channel deposits (5.9%) than in highstand deltaic deposits (4.7%) or in transgressive deposits (4.9%). Understanding the controls on porosity and permeability in onshore fluvial and deltaic Tuscaloosa sandstones will aid in predicting reservoir quality in deepwater Tuscaloosa sandstones deposited on the basin floor, which have been penetrated in deep Gulf of Mexico exploration wells.