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Chemical and isotopic data obtained on gases from High Island 511A indicate that the reservoirs contain hydrocarbons from multiple sources. A significant portion, 17 to 34 percent, of the produced methane is biogenic. Thermogenic gases were generated from a single source lying at depths greater than 16,000 feet, considerably deeper than the Pleistocene reservoir sands. Condensates coproduced with natural gas were also studied. Chromatographic analysis revealed that the condensates possess very unusual compound distributions, atypical of light petroleum fluids derived from phaseseparated crude oils or thermal cracking. Normal and branched alkanes are noticeably depleted, and cyclic hydrocarbons are enriched. Aromatic compounds are in exceptionally high concentrations. In the C9-C18 range, isoprenoid hydrocarbons are the dominant components. No one process appears to totally explain the atypical compositions of the High Island 511A condensates. A model was developed which is consistent with the compositional data and production history. Thermogenic natural gas-condensate from a single source charged the 511A Pleistocene reservoirs where biodegradation occurred to varying degrees. Hydrocarbons generated from terrestrial Type III kerogen in local immature sediments were extracted by the biogenic methane. As the biogenic methane accumulated in the sands, the extracted, low maturity thermal hydrocarbons were also added to the reservoir in roughly equal proportions as the high maturity thermogenic condensates. This resulted in condensates with a light hydrocarbon distribution characteristic of both immature and mature generation. Biodegradation and water washing may have further per-turbed the light hydrocarbon abundances. The geological situation which gave rise to the 511A immature condensates is common throughout the offshore Gulf of Mexico and may be responsible for many of the unusual light hydrocarbon distributions found in petroleums from this area.

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