The origins of petroleum in the northern Gulf of Mexico

Abstract

The distribution and chemistry of oils in the northern Gulf of Mexico and the adjacent onshore can be explained by multiple sources, fades variations, maturation, and post-generation alteration. Genetic families include Jurassic Smackover, Flexure Trend, Upper Cretaceous, and Paleogene oils. Smackover oils have high sulfur contents, pristane/phytane ratios < 1.0, CPI < 1.0, abundant extended hopanes, C 35 C 34 hopane ratios ≥1.0, and C 30 C 29 hopane ratios usuall ≤ 1.0. Flexure Trend oils are similar and contain abundant extended hopanes, high sulfur contents, and V (Ni + V) > 0.5 ; these oils are inferred to be sourced in Upper Jurassic/Lower Cretaceous strata. Upper Cretaceous oils contain a distinctive suite of tricyclic and nonhopanoid triterpanes; oleanane is low or absent. Paleogene oils can be recognized by the presence of 18α(H) oleanane and a lack of extended hopanes. These oils occur from south Texas to southern and offshore Louisiana. Two subfamilies can be recognized based on the relative abundance of triterpanes and steranes. These oils have a presumed Paleogene source. Mixing of oil types is quite prevalent at the geographic boundaries of oil types. The initial control on the distribution of oil and gas in the northern Gulf of Mexico is provided by the areal extent of source rocks. Secondary control is due to Cenozoic deposition, which provides the thermal stress to generate, as well as destroy, oil. Salt tectonics provide conduits for migrating fluids to escape the zone of thermal destruction. Mesozoic source strata again become important in the deep Gulf of Mexico, where Cenozoic sediments thin.