On the Origin of the Gulf Coast Neogene Oils

Abstract

ABSTRACT In the oil fields of the Bay Marchand-Caillou Island salt-ridge complex, Louisiana Gulf Coast, geometric considerations of oil location, extensive faulting associated with this ridge, and the compartmentalization of each oil reservoir all argue against updip lateral oil migration from a source south of the salt ridge, and suggest a local origin for the oils. However, an origin of the Caillou Island oils from shales interbedded with the oil-bearing sands is improbable because of: 1) the organ ic-poor nature of the shales, a characteristic of all Pleistocene through Oligocene shales in the Gulf Coast); 2) limited shale volume; 3) the low shale maturation rank; 4) primary migration considerations; and 5) compositional differences between shale bitumens and oils. The vertically-stacked oils at Caillou Island, and adjacent oil fields, comprise one genetically-related family, which is true for other vertically-stacked Neogene Gulf Coast oil fields. This genetic relationship suggests that the oils originated from a source, or sources, at depth. Three possible deep sources are proposed for the Neogene oils: 1) thick sections of moderately organic-rich (total organic carbon (TOC) = 2-5+%, hydrogen indices (HIs) = 300-450+) Eocene to Upper Cretaceous (or older) shales with hydrogen-rich (Type II) organic matter (OM); 2) Lower Cretaceous to Jurassic, organic-rich, black shales with high TOC and HIs, and Types I and II OM; and 3) Eocene to Paleocene hydrogen-rich coals and (or) carbonaceous shales. For any of these to be a source of the Neogene oils, hydrocarbon (HC) generation and migration must have occurred at maturation ranks at or above Ro = 1.35, the hypothesized thermal deadline for C15+ HCs. However, the three principal lines of evidence that support this deadline can be explained by other causes, and a large body of data contradicts this deadline. Consequently, this deadline is rejected here. A substantial data base from both nature and the laboratory suggests that different OM types may have different reaction kinetics due to different kerogen bond strengths, with Types I and II (hydrogen-rich) OM requiring higher maturation ranks for intense HC generation and migration than Types II-S and III OM. Persuasive lines of logic and evidence suggest that hydrogen-rich coals can be oil source rocks, and in other parts of the world it is taken for granted that they are. Thus, it is proposed here that the Gulf Coast Neogene oils may have originated from deeper, older, organic-rich rocks, from which the oils migrated vertically up major fault zones to be emplaced in shallower sandstones. Thus, faults are important elements in Gulf Coast HC exploration, and specific exploration targets can be predicted from this model.