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Austin Chalk (!) Petroleum System Upper Cretaceous, Southeastern Texas: A Case Study
William C. Dawson (1), Barry Katz (1), Vaughn D. Robison (2)
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 the offshore Austin lithofacies display good to excellent source rock potential. Organic carbon content ranges upward to 20.0 wt.% with generation potentials exceeding 45 mg HC/g of rock being measured. Source potential increases basinward as the percentage of dark, organic-rich strata increases. The organic matter within the offshore chalk units display an affinity for the Type II reference curve. Hydrogen index values typically exceed 300 mg HC/g TOC.
Austin Chalk 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).
Several geochemical indices suggest that the oil-window is located at relatively shallow depths (2040 m 6693 ft). "Offshore" chalks charged "onshore" Austin reservoirs through lateral migration pathways (via fractures and styolites) which developed during the middle Tertiary. Hydrocarbons may also have been sourced from the underlying Eagle Ford Shale (Turonian).
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