Classification of Offshore Gulf of Mexico Oils and Gas Condensates (1)

Abstract

Five groups of petroleums can be recognized in the offshore Gulf of Mexico on the basis of carbon isotope and sulfur isotope ratios, sulfur content, and relative abundances of nickel and vanadium. Of 355 oils analyzed, 341 (96.1%) are evidently closely related, and cannot be subdivided on the basis of carbon or sulfur isotopes. We suggest that these oils represent a single broad superfamily. However, two subgroups within this set are distinguishable on the basis of sulfur content and relative abundances of nickel and vanadium: group 1A, (sigma){13}C = -26.5 +/- 0.4 parts per mil, (sigma){34}S = -5.6 +/- 2.4 parts per mil, %S = 0.25 +/- 0.15%, V/(Ni + V) = 0.27 +/- 0.10; and group 1B, (sigma){13}C = -26.5 +/- 0.3 parts per mil, (sigma){34}S = -5.9 +/- 1.2 parts per mil, %S = 0.73 +/- 0.43, V/(Ni + V) = 0.67 +/- 0.07. Group 1A oils are broadly distributed across the Louisiana and adjacent Texas continental shelf, whereas group 1B oils are found along the Louisiana shelf edge and continental slope. Group 1B oils have substantial genetic affinities to those from the Campeche Shelf, offshore Mexico, of Mesozoic origin. The remaining 14 analyzed petroleums include supermature condensates from offshore Texas, and oils from offshore southern Florida believed to be cogenetic with those from the Jurassic Smackover Formation. A three-dimensional appraisal of geohistory suggests that petroleum generation began in the Eocene in the region of the Rio Grande delta, progressed coastwise to the Mississippi delta area by the Miocene, and then occurred in regions of progres ively deeper water during the Pliocene-Pleistocene.