ABSTRACT

The Miocene reservoirs of Pabst Field on Main Pass Block 259 contain thermogenic gas dominated by methane (mean = 95.6%). The isotopic properties of methane, ethane, and propane are indicators of thermal cracking because the molecules become isotopically heavier with increasing exposure to time and temperature. Methane at Pabst Field has a relatively heavy carbon isotopic composition, but carbon and hydrogen isotopes suggest a mixture with some bacterial methane. Ethane and propane are also isotopically heavy. The gas at Pabst Field is proposed to be the result of thermal cracking of crude oil at depth before migrating to reservoirs. Minor thermal condensate shows differences in carbon isotopic properties between reservoir compartments that may be related to variable thermal cracking prior to migration. Gas and condensate charge entered reservoirs in discrete pulses by vertical migration. Pabst Field is markedly different than nearby Petronius Field in Viosca Knoll 786, which contains oil and less mature gas in Miocene reservoirs. As yet undiscovered gas and condensate are likely to be found in older and deeper reservoir rocks in the Pabst-Petronius area. Gas from Pleistocene reservoirs and gas vents of the deepwater Gulf slope is isotopically lighter, indicating a simpler and more recent migration history.