Abstract: Austin Chalk (!) Petroleum System: Upper Cretaceous, Southeastern Texas

Wm. C. Dawson, B. J. Katz, V. D. Robison

The Austin Group (Coniacian-Santonian) is a sequence of interstratified chalk and marl deposited during a sea-level highstand as a transgressive unit. Austin Chalk deposition occurred on a southeastward-dipping carbonate ramp that exhibits distinctive onshore and offshore chalk lithofacies. Discrete intervals within offshore Austin lithofacies display good to excellent source rock potential. Organic carbon content ranges upward to 20.0 wt. percent with generation potentials exceeding 45 mg HC/g of rock measured. Source potential increases basinward where offshore chalk units exhibit increases in both organic richness and net thickness. These organically rich units display an affinity for the Type II reference curve. Hydrogen index values typically exceed 300 mg HC/g TOC. everal geochemical indices suggest that the oil-window is located at relatively shallow depths (6,700 ft). Offshore chalks sourced onshore Austin reservoirs through lateral migration (along fractures and stylolites) which occurred during the middle Tertiary. Hydrocarbons-may also have been sourced from the underlying Eagle Ford Shale (Turonian).

Austin Shale reservoirs are characterized by low porosity/low permeability dual pore systems consisting of microporous matrix and fractures. Diagenesis (mechanical compaction, stylolitization, and calcite cementation) has strongly modified Austin Chalk pore systems. Matrix porosity generally decreases with increasing depth because of progressive burial diagenesis. Austin reservoirs typically have two major sets of fractures; reservoir performance is related to fracture connectivity. Austin Chalk reservoirs are sealed by the overlying Taylor Marl (Campanian).

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana