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The Austin Chalk is an impure "onshore" chalk deposited marginal to the Gulf of Mexico during the Late Cretaceous. The chalk is a reservoir, producing petroleum from the matrix and from fractures in the rock. In addition, the lower part of the Austin Chalk contains 0.5 to 3.5% organic matter, with some localized zones containing 20% organic matter.
The organic-rich chalks occur principally in deeper (greater than 5,000 ft or 1,524 m), 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 Eagle Ford Formation, although there is typically more organic matter in the shales of the Eagle Ford 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 2,000 ft (610 m) burial, the peak zone of petroleum formation is between 6,000 and 8,000 ft (1,828 and 2,438 m). At these depths, mature petroleum occurs in the matrix and in fractures in the chalk, whereas at greater depths gas is forming.
The hydrocarbons in the chalk include those formed in place and those formed elsewhere (probably the Eagle Ford Formation) which have migrated into the chalk. Due to increasing generation and migration of hydrocarbons with depth, the petroleum becomes lighter and enriched in saturated and total hydrocarbons with depth. At less than 3,000 ft (914 m), the petroleum is commonly heavy and depleted in saturated and total hydrocarbons, due to biodegradation or to the immaturity of the autochthonous oils.
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