Generation and Migration of Hydrocarbons in Upper Cretaceous Austin Chalk, South-Central Texas
George J. Grabowski, Jr., 1984. "Generation and Migration of Hydrocarbons in Upper Cretaceous Austin Chalk, South-Central Texas", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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The Austin Chalk of south-central Texas was deposited on a carbonate ramp marginal to the Gulf of Mexico during the Late Cretaceous Epoch. Dark-colored, laminated, and sparsely burrowed chalks containing > 1.5% total organic carbon (TOC) were deposited under disaerobic conditions and predominate in deep, basinal cores. Light-colored, thoroughly bioturbated chalks containing < 1.5% TOC formed in oxygenated shelfal environments and predominate in shallow cores.
The kerogen in the Austin Chalk is uniformly composed primarily of amorphous, type I or II material irrespective of carbonate content or lithology. The kerogen occurs disseminated in the dark-colored chalks and concentrated in microstylolites and stylolites. Extractable organic matter (EOM) is most abundant in porous chalk between zones of pressure solution.
EOM in shallow cores (< 5,000 ft or 1,525 m) contains 70% NSO (nitrogen, sulfur, and oxygen) compounds and 30% hydrocarbons. The saturated hydrocarbons are dominated by geochemical fossils in these shallow cores. With increasing depth of burial, the EOM becomes enriched in hydrocarbons, up to 60–80% in the deepest core (9,100 ft or 2,780 m). These hydrocarbons formed from alteration of kerogen and NSO compounds: the kerogen becomes progressively condensed and less aliphatic with increasing depth of burial. This alteration is independent of the carbonate content of the rocks. Peak oil generation occurs between 5,000 and 8,000 ft (1,525 and 2,440 m), with gaseous hydrocarbons becoming abundant below 8,000 ft (2,440 m).
EOM that was formed during catagenesis migrated in the Austin Chalk through micropores to zones of higher porosity. This migration caused enrichment of the EOM in both absolute amounts and relative proportions of hydrocarbons and resins in porous chalks. Larger molecules, such as asphaltenes, did not readily migrate in the chalk. Migration of EOM occurred at all depths in the Austin Chalk but was most prevalent in mature chalks below 5,000 ft (1,525 m). Changes in the composition of EOM owing to migration are independent of carbonate content or lithology and are dependent primarily on the maturity of the rock. The light (> 39° API) crude oils produced from deep wells in the Austin Chalk are similar to and may be formed from migrated EOM.