Abstract: The Tuscaloosa Formation Revisited

Paula L. Hansley

A petrologic study of the Upper Cretaceous lower Tuscaloosa Formation in the Gulf Coast from depths of 2,700 to 6,000 m indicates that anomalously high porosity (>20 percent) in deep gas and condensate-bearing sandstones (5,000 to 6,000 m) is approximately evenly divided between primary and secondary porosity. Primary porosity was preserved by early, iron-rich grain-rimming chlorite and quartz overgrowths. Most secondary porosity resulted from dissolution of carbonate cements. Moldic pores outlined by chlorite were created by dissolution of unstable feldspars and rock fragments. Interparticle clay microporosity is significant in sandstones containing authigenic kaolinite and (or) chlorite. Pores were filled in the deepest sandstones by quartz overgrowths and a late mag esium-rich chlorite that is commonly obscured by fibrous illite.

Voids were created in the early Tertiary(?) by acidic meteoric waters and during deep burial by brines carrying organic and inorganic acids that were released during hydrocarbon maturation in neighboring shales. Oil fills dissolution voids in ankerite cement and albitized plagioclase and coats most authigenic minerals. Two-phase primary fluid inclusions in quartz overgrowths which also contain oil-bearing inclusions have homogenization temperatures between 125°C and 134°C. These temperatures combined with a burial history reconstruction indicate that hydrocarbons migrated into Tuscaloosa sandstones during the Miocene. Overpressuring began in the middle Tertiary along with gas generation in the Tuscaloosa. These events coincided with the end of deep meteoric flow through the ulf section and the beginning of a compactional hydrologic regime. Precipitation of quartz overgrowths and hydrocarbons at this time locally created effective pressure seals.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas