Comparing Chlorite-Coat Coverage and Reservoir Quality in Deep Tuscaloosa Sandstones, Louisiana Gulf Coast, USA

Abstract

Continuous chlorite coats on detrital grains in deep Upper Cretaceous Tuscaloosa sandstones preserve porosity by inhibiting quartz cementation. Where chlorite-coats are discontinuous and incomplete, Tuscaloosa sandstones are extensively cemented by quartz. However, it is not known what percentage of the exposed grain surfaces must be coated by chlorite in order to reduce quartz cementation and preserve porosity in hot (>135°C), deeply buried (> 5 km) Tuscaloosa sandstones. Previous work has quantified the whole-rock volume of chlorite cement in Tuscaloosa sandstones by thin-section point counts but has not measured the 2D proportion of chlorite-coated grain surfaces. In this study, we quantified the percentage of exposed quartz-grain surfaces covered by chlorite cement in deep Tuscaloosa sandstones and compared it to reservoir quality. Petrography and routine core analysis of 141 samples from central Louisiana documented composition, diagenesis, and reservoir quality in fluvial and deltaic Tuscaloosa sandstones. Porosity, permeability, and chlorite-cement volume all have bimodal distributions. Twelve samples representing a wide range of porosity and permeability were selected for quantifying chlorite grain-coat coverage on 50 detrital quartz grains per sample. Average chlorite-coat coverage in these samples ranges from 29% to 95% and has a bimodal distribution. Statistically significant correlations exist between chlorite-coat coverage and volume of chlorite cement, quartz cement, porosity, and permeability. Samples having average chlorite-coat coverage of >60% have preserved high porosity (15–28%) and permeability (4–1249 md) at temperatures of 160°C to 195°C by inhibiting quartz cementation. On the basis of point-count data of chlorite volume from the larger petrographic database, we estimate that less than half the Tuscaloosa sandstones in central Louisiana contain an average of >60% chlorite-coat coverage. The occurrence of continuous chlorite coats has an important influence on reservoir quality in Tuscaloosa sandstones, but local variations in coat coverage are difficult to predict. Although one cannot predict chlorite-coat coverage and reservoir quality foot by foot, a risk estimate can be assigned for encountering higher reservoir quality.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015