Environments of Deposition and Petroleum Geology of Tuscaloosa Group (Upper Cretaceous), South Carlton and Pollard Fields, Southwestern Alabama

Abstract

In southwestern Alabama, the lower Tuscaloosa Group (Upper Cretaceous) consists of two informally defined units, the Massive and Pilot sand intervals. The Massive sand interval accumulated principally as sands in a wave-dominated, high-destructive delta system. These sandstones are structureless, well sorted, micaceous, locally fossiliferous, calcareous, glauconitic, fine grained, and quartz rich, containing angular to subangular quartz grains. The Massive sand interval unconformably overlies fluvial-deltaic sediments of Lower Cretaceous strata. The Pilot sand interval, which overlies the Massive sand interval, accumulated as shelf sands and clays during a marine transgression. The sandstones are well sorted, micaceous, fossiliferous, calcareous, glauconitic, very fine to fine grained, and quartz rich, containing sub-angular to subrounded quartz grains. The sandstones appear massive but may be structureless as a result of extensive bioturbation. Marine bivalves, such as inoceramids, are present in the sandstones and claystones. The Pilot sand interval is overlain by a marine claystone (Marine shale) containing a diverse faunal assemblage of macroinvertebrates, including ammonites, inoceramids and other bivalves, and a rich microfossil assemblage of planktonic foraminifera and calcareous nannofossils. The Marine shale accumulated in an open-marine shelf environment. Petroleum traps in the Tuscaloosa are structural traps involving salt anticlines (South Carlton field) and extensional fault traps associated with salt movement (Pollard field). Reservoir-grade porosity occurs in the Massive and Pilot sandstone units as primary intergranular porosity. Although Tuscaloosa marine claystones contain significant amounts of organic carbon, these rocks are thermally too immature to be the petroleum source rocks for the Tuscaloosa crude oils in South Carlton and Pollard fields.