Geochemical analyses of well cuttings from offshore Louisiana reveal trends inorganic matter characteristics that mimic thermal alteration effects, but are in fact related to depositional environments. Sediments deposited on the continental slope contain organic matter that is considerably higher in hydrogen content than organic matter from adjacent environments. These chemical variations are reflected in headspace composition of canned cuttings. Slope sediments, at or near the top of the oil window over a widespread area, are commonly encountered near the base of the penetrated section, leaving the greater part of the principal zone of oil generation unsampled. Active petroleum generation must occur deeper than 8,000-15,000 ft, depending on local geothermal gradient. Mod l calculations of thermal maturity, used in conjunction with burial history analysis, are consistent with our empirical observations of the maturation of organic matter based on vitrinite reflectance and geothermal gradient.

In the absence of direct evidence from the source beds of Gulf of Mexico oils, detailed biomarker analysis can provide information on the nature and thermal history of the organic matter from which the oils were generated. Thermal maturity of the oils increases from west to east, indicating that source beds of eastern oils are older than those of western oils. Oils produced from the eastern offshore region were probably generated from early to middle Miocene sediments. The depocenter was located to the northwest, and deposition of oil-generating sediments enriched in marine organic matter occurred in deep water. Oils produced from the western offshore region were probably generated from Pliocene sediments. The depocenter was located close to where the oils are found today; hence, the ils are enriched in components derived from land plants deposited in oxygenated sediments. Oils produced from two locations close to the present-day shelf edge are significantly different from the other offshore oils. These were probably generated from middle to late Miocene sediments deposited in anoxic basins with high bacterial input.

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