Regional Variation in Detrital Composition, Diagenesis, and Reservoir Quality of Deep Tuscaloosa and Woodbine Sandstones, Gulf of Mexico, USA

Dutton, Shirley P.; Ambrose, William A.; Loucks, Robert G.

Deep sandstone reservoirs of the Upper Cretaceous Tuscaloosa and Woodbine Formations occur along the Lower Cretaceous paleo-shelf edge in Louisiana and Texas. For many years, researchers have noted that porosity is preserved in some deep Tuscaloosa sandstones because thick, continuous chlorite coats formed around detrital grains as a result of dissolution of basic volcanic rock fragments (VRFs). We conducted petrographic analyses of 220 Tuscaloosa (4.3-6.5 km) and Woodbine (3.4-4.5 km) sandstones to document regional variations in detrital mineralogy, diagenesis, and reservoir quality. Tuscaloosa sandstones in this study were deposited in western, central, and eastern Louisiana; Woodbine sandstones were deposited in southeast Texas. Results provide insight into potential reservoir quality of deeply buried Tuscaloosa sandstones that are exploration targets beneath the Gulf of Mexico shelf and in the deep Gulf of Mexico.

Significant regional differences were observed in the distribution of VRFs and authigenic chlorite coats. Deep Tuscaloosa sandstones from central Louisiana have an average composition of Q86F1R13. VRFs compose an average of 5.4% of the whole rock volume, and chlorite coats have an average volume of 5.1%. Tuscaloosa sandstones in eastern and western Louisiana are more quartz rich and contain fewer detrital VRFs (0.2% and 0.1%, respectively); they also have less chlorite (2.8% and 0.2%). Deep Woodbine sandstones in southeast Texas have an average composition of Q95F1R4. VRFs compose an average of 0.4% of the whole rock volume, and chlorite has an average volume of 1.4%.

Chlorite coats are most abundant in Tuscaloosa sandstones in central Louisiana because the provenance of these sandstones included volcanic centers that shed VRFs. Chlorite-cement volume in central Louisiana sandstones varies little with depositional environment or systems tract. Deltaic sandstones in highstand systems tracts contain an average of 5% chlorite, fluvial sandstones in lowstand systems tracts contain 6%, and transgressive systems tract deposits contain 5%.

Deepwater Tuscaloosa sandstones that were deposited on the basin floor during the Late Cretaceous have been penetrated in deep Gulf of Mexico wells. Tuscaloosa basin-floor sandstones derived from the central Louisiana depocenter are more likely to contain abundant chlorite coats that preserve porosity than are sandstones derived from southeast Texas or eastern Louisiana because of provenance differences.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013