| The Austin Chalk is an impure "onshore" chalk that was deposited on a ramp marginal to the Gulf of Mexico during the Late Cretaceous. Basinal chalks are organic rich, commonly containing 0.5-5.0% amorphous, sapropelic kerogen derived from marine organic matter with only trace amounts of terrestial kerogen. Less organic matter was deposited and perserved in oxygenated shallow water, and fresh-water diagenesis oxidized the organic matter on outcrop. In each sample, the kerogen is concentrated in microstylolites, with organic fluids segregated in micropores in the chalk.;The Austin Chalk reservoirs petroleum sourced within the formation and petroleum that has migrated into the chalk from elsewhere in the chalk and the underlying Eagleford Formation. The hydrocarbons are reservoired in interparticulate pores and micro- and macrofractures. Migration results in an enrichment in saturated hydrocarbons and, to a lesser extent, in aromatic hydrocarbons and resins in porous, reservior chalks. Crude oil produced from fractured chalks tends to have a high API gravity and to be enriched in saturated hydrocarbons, probably due to migration. Biodegradation may lower the gravity and hydrocarbon content of crude oils reservoired in shallow chalks.;The bitumen from the Austin Chalk contains geochemical fossils characteristic of plankton and algae: steranes, triterpanes, isoprenoids (particularly pristane and phytane), and even-carbon numbered C(,15)-C(,22) n-alkanes. These geochemical fossils become less abundant with increasing burial depth, due to the generation of hydrocarbons during catagenesis. The kerogen darkens and becomes condensed and aromatic as the H/C atomic ratio decreases and hydrocarbons are liberated from the kerogen and from asphaltenes. Saturated hydrocarbons, particularly C(,12)-C(,26) n-alkanes and isoalkanes and single-ring cycloalkanes, increase more rapidly than aromatic hydrocarbons and resins. Kerogen alteration and hydrocarbon generation are unaffected by the mineral composition of the chalk, suggesting that the dominant controls on the rate of alteration and the composition of hydrocarbons generated are the type of organic matter and thermal history of the rocks. The principal zone of oil generation for the Austin Chalk occurs at the same depth as for shales of the same age. |
