--> Gulf of Mexico Petroleum: A Ring of Black Gold

AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis

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Gulf of Mexico Petroleum: A Ring of Black Gold


The span of 30 years has mostly passed since the interpretation of the "Oil Correlation Study" at Texas A & M university at the Geochemical & Environmental Research Group under James Brooks & Mahlon C. Kennicutt. The study consisted of a huge project consisting of nearly 1000 oils from all over the Gulf of Mexico & included both onshore & offshore representatives. The purpose of the project was to determine the origin of the GOM oils using GCMS “fingerprinting”, carbon isotopes, metals etc. In 1989 4 models were proposed for explaining the origin of the oils of the Gulf of Mexico: 1) Quaternary sources. Wally Dow proposed Miocene deep water sub ‐ basins associated with anoxic high density brines from salt dome emergence at the sea floor are the principal petroleum source. Similar GOM sub‐basins sediments are high in TOC today. 2) Tertiary sources ‐ Roger Sassen, followed by Mahlon C Kennicutt, noting that many oils of the Gulf of Mexico, both onshore & offshore, contain 18a(H)oleanane, proposed that the majority of oils, both onshore & offshore are sourced from the Paleocene ‐ Eocene Wilcox formation. Very extensive lateral migration from the deep, offshore, Tertiary & Quaternary depocenters was invoked by Sassen to explain filling of many onshore reservoirs, other than the Jurassic, though Kennicutt incorporated halokinesis from largely Tertiary sources to his model. 3) Cretaceous sources: Keith Thompson noted that many of the onshore oils derive from Cretaceous sources. Paul Comet noted that the oils of the GOM could be subdivided & classified according to the scheme of Oceanic Anoxic events described by Schlanger & Jenkyns. In particular, the Eagleford Cenomanian/Turonian source was highlighted. Paul Comet also proposed that the oils could be mapped "like rocks", similar to magma, & could be correlated by classical geological principles. The "mapping" idea was first emphasized by Keith Thompson & this idea encouraged the idea of a huge ring of carbonate ‐ sourced oils around the entire Gulf of Mexico. 4) Jurassic sources. The Smackover oils are well described by many authors as marine carbonate sourced. Some hints of pre ‐ salt lacustrine oils have been noted as possibly deriving from the Eagle Mills formation. Low sulfur, high nitrogen oils, are to be found in Northern Louisiana. The major problem with proposing, in 1990, that Cretaceous & Jurassic sources are major in the GOM, is that in many places these rocks are buried to 10 kilometers & would be post mature. This problem was solved by invoking halokinesis (oil moves with salt away from the zone of thermal destruction; the famous “migrations & remigrations”) proposed by Audrey Bissada & Barry Katz at Texaco, providing a major impetus to investigate the subsalt play. Though all these ideas, at the time, were highly controversial, they are widely accepted now; as the main focus for “fracking” on the Gulf Coast & elsewhere, are those same OAE, particularly 2 & 3 within the Eagleford play, which has sourced the majority of the offshore oils.