Considerations of oil origin, migration, and accumulation at Caillou Island and elsewhere in the Gulf Coast
1991; United States Department of the Interior; Linguagem: Inglês
10.3133/ofr91307
ISSN2332-4899
Autores Tópico(s)Global Energy and Sustainability Research
ResumoIn the oil fields of the bay Marchand-Caillou Island salt-ridge complex, offshore Louisiana, geometric considerations of oil location, extensive faulting associated vith this salt ridge, and the compartmentalized nature of each oil reservoir, all argue against an updip, lateral oil migration from a source to the south of the salt ridge, and suggest a local origin for the oils.ROCK EVAL analysis demonstrates that the Upper Tertiary elastics at Caillou Island have ultra-poor, source-rock characteristics, a fact universal for Pleistocene-Oligiocene shales throughout the Gulf Coast.An origin of the Caillou Island oils from the shales interbedded vith the oil s;inds is improbable because of: 1) this ultra-poor organic nature of the shales, 2) their limited volume from geometric considerations, 3) their low maturation rank, 4) primary migration considerations, and 5) compositional differences between shale bitumen and oil.Proponents of the argument that the large volumes of Tertiary elastics make up for their organic-poor nature (the "something from nothing" proposal), have disregarded four vast hydrocarbon (HC) sinks, viich cancel out favorable calculated ratios of generated EC's to in-place EC's.By process of elimination, an origin and migration of the Caillou Island (and other Gulf Coast oils) from depth appears to be the only viable alternative.With increase in both depth of burial and aje of rock in the Gulf Coast, organic richness increases in fine-grained rocks, although these increases are not constant vith respect to either geographic location ox geologic age.Three possible sources are proposed at depth for the Neogene (including the Caillou Island) oils: 1) thick sections of moderately organic-rich (total organic carbon (T.O.C.) = 2-5+2), hydrogen indices (Hi's) = 300-450+) Eocene to Upper Cretaceous (or older) shales vith hydrogen-rich (Types I and II organic matter (OM); 2) Lower Cretaceous to Jurassic black shales with high T.O.C. contents, high Hi's and Types I and II OM; and 3) Eocene-jPaleocene hydrogen-rich coals and carbonaceous shales.A call upon any one of these as a source of the Neogene oils will involve BC generation and migration at maturation ranks moderately to considerably above R =1.35, which is the hypothesized thermal deadline for C.,-+ HC's.However, the three principal lines of evidence which support the hypothesis of a thermal deadline for C-.C+ EC's at R « 1.35 can be explained by other causes and a large body of data contradicts it.Although this hypothesis is considered by many to be organic-geochemical law, I believe it to be in error.A substantial data base from both the laboratory and the natural system suggests that the different OM Types have far different reaction kinetics due to greatly different kerogen bond strengths.Typ i III OM undergoes: 1) the first detectable HC generation at R = 0.6, 2) both intense HC generation and primary migration with the first possibility of commercial oil accumulation) at R = 0.8, 3) maximum HC generation over R «= 0.8 to 1.6 or 2.0, and 4) loss of all HC generation potential (kerogen burnout) by R =2.0.Types I and II (hydrogen-rich) OM apparently undergo intense fiC generation and migration at more elevated (but yet undefined) maturation r^.nks, and suffer loss of all HC generation potential at R > 7.0.The three principal arguments against coals being possible oil-source rocks can be adequately replied to.Persuasive lines of logic and evidence exist which suggest that source rocks, and in other parts of the world, are.In light of these points, the preferred explanation here for the origin of the oils at Caillou Island, and for most other deeper HC generation from older, organic-rich rocks, and vertical migration of these oils up major fault zones, vith emplacement of the oils in shallower 1 hydrogen-rich coals can be oil it is taken for granted that they Neogene Gulf Coast oils, is sandstones.By this scenario, the fault zone becomes the most important element in Gulf Coast HC exploration, and specific exploration targets can be predicted from this model.
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