ABSTRACT

The Austin Chalk is an impure "onshore" chalk deposited marginally to the Gulf of Mexico during the Late Cretaceous. The chalk, as a reservoir, produces petroleum from the matrix and from fractures in the rock. In addition, the lower portion of the Austin Chalk contains 0.5 to 3.5 percent organic matter with some localized zones containing 20 percent organic matter.

The organic-rich chalks occur principally in deeper (greater than 5000 feet), basinward cores, whereas the organic-poor chalks occur in shallow cores on the San Marcos platform. The organic matter is similar in the chalk and in the underlying Eagleford Formation, although there is typically more organic matter in the shales of the Eagleford Formation. The kerogen is amorphous, sapropelic (Type II) kerogen that yields large amounts of saturated and aromatic hydrocarbons upon burial. Although hydrocarbon generation commences at about 2000 feet burial, the peak zone of petroleum formation is between 6000 and 8000 feet. At these depths, mature petroleum occurs in the matrix and in fractures in the chalk; whereas at greater depths gas forms.

The hydrocarbons in the chalk include those formed in place and those formed elsewhere (probably the Eagleford Formation), which have migrated into the chalk. Because of increasing generation and migration of hydrocarbons with depth, the petroleum becomes lighter and more enriched in saturated and total hydrocarbons. At less than 3000 feet, the petroleum is commonly heavy and depleted of saturated and total hydrocarbons, due to biodegradation or to the immaturity of the autochthonous oils.