Concentration of Source Rock Organic Matter During Carbonate Diagenesis, Jurassic Smackover Formation

Roger Sassen, C. H. Moore, F. C. Meendsen

Although the Smackover is a regionally significant source rock for oil and gas, source potential is localized within certain carbonate facies, and is enhanced by the effects of carbonate diagenesis.

Oil-prone algal kerogen is preserved in the laminated carbonate mudstone facies of the lower Smackover. This facies was deposited in subtidal waters that were often anoxic and hypersaline. The mean organic carbon content of 189 conventional core samples of the lower Smackover from 14 wells, however, is a modest 0.46%. Very little source rock organic matter is preserved in upper Smackover carbonate grainstones and packstones that were deposited in shallow, oxygenated waters of normal salinity.

Pressure solution of Smackover carbonate has concentrated insoluble rock components in stylolites, resulting in exceptionally high contents of organic carbon. The mean organic carbon content of 48 stylolites isolated from conventional cores of 12 wells is 10.3%, with individual values as high as 63.3%. Much of the organic matter is algal-derived kerogen, but bitumen is also present.

The organic geochemistry of stylolites may help to explain why some carbonate source rocks with apparently modest organic carbon contents have given rise to large accumulations of oil and gas. Concentration of source rock kerogen during carbonate diagenesis could contribute to efficient expulsion from thermally mature carbonate source facies.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.