--> Abstract: Carbonate Mud and Carbonate Source Rocks, by Paul M. (Mitch) Harris and Barry J. Katz; #90039 (2005)

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Carbonate Mud and Carbonate Source Rocks

Paul M. (Mitch) Harris1 and Barry J. Katz2
1 ChevronTexaco Energy Technology Company, San Ramon, CA
2 ChevronTexaco Energy Technology Company, Bellaire, TX

Studies of modern carbonate settings have documented varied origins for mud-size sediment, including disintegration of benthic and planktic organisms, precipitation from the water column, and mechanical breakdown of larger particles. Carbonate muds accumulate in protected “low-energy” settings on the platform top and are also exported to adjacent deeper-water settings. A limitation on source rock deposition is a result of the need for high primary productivity levels and/or elevated organic preservation. Carbonate petroleum source rocks tend to be mud/wackestones developed in two geological scenarios:

(1) Low latitude, embayments or sags where they are interbedded or underlie evaporates - Preservation can be promoted by salinity or temperature stratification which reduces diffusion of oxygen into bottom waters and near-surface pore waters. These sources are generally thin and discontinuous due to deposition in shallow fluctuating environments. Examples of this source type are the Jurassic Hanifa Formation of the Middle East, and the Lower Cretaceous Sunniland Limestone of South Florida.

(2) Shelves during periods of rising sea level – Formation of organic-rich sediment is promoted during major transgressions, which are times of high nutrient supply and poor oceanic ventilation. Carbonate source rocks of this type are often thick and widespread. Upper Devonian – Lower Mississippian examples of this type of source rock are the Duvernay Formation of Alberta, and the Domanik facies of the Timon-Pechora, Volga-Ural, and Caspian Basins. Cretaceous examples of these rocks include the Sulaiy Formation in the Middle East; La Luna and equivalents in Venezuela, Columbia, Ecuador, and Peru; and the Austin Chalk of Texas.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005