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The distribution and percentages of oil and gas in Cenozoic delta systems have often been attributed to the variations in the distribution and amount of "oil-prone" and "gas-prone" organic matter in the presumed source rock. This genetic relationship has been greatly exaggerated. Data have become available in the past 10 yr indicating that the distribution and amount of oil versus gas in reservoirs primarily reflects the relative ease of migration of the 2 hydrocarbon phases. For example, pressure (active compaction) seals are the preferred cap for gas in the McKenzie delta, fault conduits were not available hydrocarbon migration until the primary source rock was in the gas stage of thermal maturation in the eastern portion of the ancestral Orinoco delta, mature source ro ks in the Mahakam delta yield oil to the reservoirs if they are normally pressured but only gas when they are overpressured, the amount and distribution of oil and gas in the Niger delta are primarily controlled by fault displacement and leakage, and in the Mississippi delta the primary control is overall structural complexity.
Although specific source rock units have been tentatively identified in several of the Cenozoic deltas, it is possible that the more than 10,000 ft of overmature marine shales that underlie all of these delta systems contain the most important driving force for the migration of both oil and gas. Although the TOC's are usually modest, the total amount of methane generated is very large. As the methane moves upward through the section, it sequentially dissolves, moves, and precipitates the liquid hydrocarbons. Faults are very important in the focusing of the migration and the accumulation of the hydrocarbons.
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