The Cretaceous Austin Chalk of South Texas—A Petroleum Source Rock
Carbonate rocks commonly have been discounted as important source rocks because of their lower organic-carbon content and lower catalytic activity in comparison to shales. However, carbonate source rocks contain mostly sapropelic organic matter, which yields a higher percentage of oil earlier than the more humic organic matter of shales. Furthermore, carbonate source-reservoir sequences are at many places overlain by the perfect seal, evaporite, during the time of oil generation and accumulation. In contrast, many sandstone-shale sequences tend to leak petroleum during and after accumulation.
The richest source rocks in the world are the argillaceous and siliceous carbonates in formations such as the Green River of Utah, the La Luna of Venezuela, and the Nordegg of the Western Canada basin.
Carbonate rocks like the Cretaceous Austin Chalk of south Texas, which contains 60–90% CaC03, act as both source and reservoir rock. Light-hydrocarbon analyses and pyrolysis data both support the concept that most of the oil in the Austin Chalk is autochthonous.