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The chemical and isotopic composition of natural gases from 55 fields in the offshore Gulf of Mexico province has been analyzed. The gases display a trend of more positive ^dgr13C1 values (-70 to -35 per mil) with increasing depth and age of producing reservoir. The mechanisms responsible for this fractionation are biogenic enrichment of 12C1, thermal cracking, and mixing. Separate trends are present in Texas and Louisiana which suggest a higher geothermal gradient or different type of organic matter in offshore Texas. There is considerable scatter along the general trend because gases generated from deeper, thermally mature source rocks have commonly migrated to shallower immature reservoirs.
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