Chemical and Isotopic Evidence of the Origins of Natural Gases in Offshore Gulf of Mexico

ABSTRACT 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 δ13C1 values (−70 to −35 ‰ (permil)) 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. The province is primarily a gas-producing region for three reasons. Many gas fields, particularly those of Pleistocene age, are of apparent biogenic origin. This gas is characterized by enrichment of the light isotope 12C in methane (δ13C1 more negative than −55 ‰) and by large amounts of methane (C1/C1-5 > 0.99). Miocene gas accumulations in the western part of the province are either the result of early stages of thermal cracking of liquid hydrocarbons because of a higher geothermal gradient, or a different type of organic matter from which the hydrocarbons were derived. This gas is wetter than biogenic gas (C1/C1-5 > 0.94) and isotopically heavy (δ13C1 more positive than −40 ‰). Many gas fields in reservoirs of all ages are the result of “separation-migration” in which the gas phase was physically segregated from a petroleum accumulation. These gases are similar in composition to gas associated with petroleum.