Print this pageAbstract: Organic Diagenesis--Its Relation to Gulf Coast Oil
Sediments of the Barataria Bay area in coastal Louisiana provide a modern analogue for future petroleum source-reservoir lithologies. Beach and barrier-island sands have excellent reservoir potential. Sediments in areas of restricted circulation and high biogenic productivity have characteristics that suggest precursor hydrocarbon source rocks. Examination of the diagenesis of organic matter that occurs in these potential source rocks permits an assessment of basic parameters that influence the origin and distribution of Gulf Coast oil.
Petroleum hydrocarbons that have their origin in rock sequences similar to those of coastal Louisiana are the result of several processes. Biogenic hydrocarbons, mainly C15-C32 paraffins, are produced by metabolic activity of organisms indigenous to the region. These hydrocarbons are relatively unreactive and become concentrated with respect to other biochemical components. Secondly, biochemicals may be converted directly to hydrocarbons by degradational processes. These degradational processes have been examined qualitatively for specific characteristics, i.e., the chlorophyll to isoprenoid hydrocarbon degradation, but have not been assessed quantitatively. A third process that yields hydrocarbons concerns the thermally controlled reactions of kerogen (organic material in sediments that is not affected by organic solvents). Thermal maturation of kerogen generates a broad spectrum of products that begin with liquid oils, rich in heterocompounds, and progress to a point where methane is the dominant product.
Laboratory experiments indicate that biochemical-to-hydrocarbon degradational processes may be important reactions during early diagenesis. Conditions conducive to such reactions are present in several offshore Tertiary basins of the world. As basins continue to subside, temperature increases as a result of geothermal gradients become important, and kerogen-derived hydrocarbon products dominate. At higher maturity levels, preservation of hydrocarbons becomes a function of thermal stability, and concentrations of gaseous products are enhanced selectively.
AAPG Search and Discovery Article #90975©1976 GCAGS- GC Section SEPM Annual Meeting Shreveport, Louisiana