ABSTRACT

Carbon and sulphur stable isotope composition, metal and sulphur content, biomarker fingerprints and reservoir age; have been used to define the distribution of major oil types in the northern Gulf of Mexico. These groups include the oils with characteristics typical of carbonate sources (Smackover, Florida, Shelf Break, Campeche and a Cuban oil), the Austin Chalk oils, the Woodbine Trend oils, Tuscaloosa oils, Mississippi Delta oils, South Texas oils influenced by lignites, post-mature oils (condensates) and oils contaminated with bitumen during migration or pooling (see Figure 1). A given oil type is confined to reservoir rocks of specific age (see Figure 1). The spatial and chronological association of the oils suggest that multiple source rocks need to be invoked to explain the chemistry of Gulf of Mexico crude oils.

Oils exhibiting characteristics typical of carbonate influenced sources are probably of Mesozoic age, including the North Florida Smackover oils of Jurassic age and the South Florida oils of Lower Cretaceous age. The source of the Shelf Break (Flexure Trend) oils is unknown, however, little carbonate is present in the associated Tertiary section, suggesting that these Shelf Break oils are also of Mesozoic age (probably Lower Cretaceous). Other characteristics suggestive of a Mesozoic origin include relatively high sulphur contents and trace element compositions similar to onshore Mesozoic oils.

The three groups of oils produced from the Mississippi Delta have stable carbon and sulphur isotope ratios and trace element contents similar to the Flexure Trend oils. However, the oils lack biomarker fingerprints typical of carbonate influenced source rocks. The biomarker compositions of one minor group of Mississippi Delta oils are most similar to Tuscaloosa and Woodbine oil types (Upper Cretaceous). Further indication of a Mesozoic source is provided by the bio-marker content of the Pine Island formation (Lower Cretaceous) which has a signature similar to the largest group of the Mississippi Delta oils. However, many of the offshore Mississippi Delta oils (but not all) and Flexure Trend oils contain 18(H) oleanane and bisnorhopane. These biomarkers are also present in onshore Paleogene reservoired lignite influenced oils. One possible scenario is that much of the Gulf of Mexico shelf oils are a mixture of two oil types (Mesozoic and Paleogene). Thus, I8(H) oleanane and bisnorhopane are "tracers" of lignite sourced oils, since they are absent from Mesozoic oils reservoired onshore.

The offshore Mississippi area formed part of the "abyssal" carbonate rich source facies during the Late Jurassic-Early Cretaceous and it seems likely that these organic rich formations are the primary source of the Delta oils. A Mesozolc source would explain the huge volumetric quantity of oil and gas in the Mississippi Delta areas (and elsewhere in the northern Gulf of Mexico), but requires an explanation of how these oils were preserved and trapped in Neogene-Quaternary reservoirs. The presumed parent source rocks have "burnt out" due to deep burial and presently occur in the post-mature gas generation zone. A halokinetic model in which hydrocarbons and Jurassic Louann salt migrated vertically in response to Tertiary sediment loading, would postulate sequential movement of oil away (vertically) from the zone of thermal destruction into younger sediments. This reservoir-source decoupling model would predict the marked correlation between halokinetic phenomena and oil accumulation long noted in the Northern Gulf of Mexico. Thermal modelling of source rock data spuriously indicates that Neogene source rocks (presently mature and sourcing oil in some parts of offshore south Texas) are the major source of the northern Gulf of Mexico petroleum. However, Miocene sources would only be of local rather than regional significance due to their generally poor source qualities. The implications of this study are that only halokinetic phenomena, rather than source rock burial, ultimately control the distribution of oil and gas offshore Louisiana and that the Lower Cretaceous is the most likely source. The timing of generation and successive migration and remigration into shallower, younger reservoir rocks would determine whether the oil completely leaked out, was cracked to gas/or successfully reiterated to cooler reservoirs. This comply history and continuing generation of gas from deeply buried petroleum source rocks has lead to the surprisingly highly altered nature of many Gulf Coast crude oils.

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Figure 1. Sketch map of the northern Gulf of Mexico oil and reservoir ages. Due to the constraints imposed by shading techniques, areas of mixing between two or more oil types (offshore) are not shown.

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