Sterane and triterpane patterns as diagnostic tools in the mapping of oils, condensates and source rocks of the Gulf of Mexico region

Four hundred and eighty-one Gulf of Mexico oils and condensates can be subdivided into nine major families based on their sterane ( m/ z 217, 218) and triterpane ( m/ z 191, 177) biomarker mass chromatogram patterns. These families are (1) carbonate-sourced oils reservoired in Jurassic-Cretaceous reservoirs onshore and Plio-Pleistocene reservoirs offshore, (2) Austin Chalk reservoired oils, (3) Tuscaloosa oils, (4) Wilcox reservoired oils (La), (5) South Texas Palaeogene reservoired oils, (6) Mississippi delta oils, (7) post-mature oils of uncertain affinities, (8) migration contaminated oils (showing attributes of a mixing of immature and post-mature sources), and (9) oils and condensates lacking biomarkers. Each oil family is concentrated in specific age-defined producing trends, ranging in age from Jurassic to Pleistocene. For Tertiary reservoirs, these producing trends shift geographically with time, in a manner analogous to the clockwise migrating Cenozoic depocenter model for the northern Gulf of Mexico. At least six ages of marine source rock deposition may be implicated in the genesis of these oils (1) The Kimmeridgian-Oxfordian (including Smackover and Norphlet formations), (2) The Aptian (including Pine Island, Bexar and Sunniland formations), (3) The Albian (Paluxy formation), (4) The Cenomanian-Turonian (including Eagle Ford and Tuscaloosa formations), (5) The Coniacian-Santonian (Eagle Ford and Austin Chalk), and (6) The Palaeocene (Wilcox, Midway, Sparta formations). The Jurassic and Cretaceous source rocks correspond to described oceanic anoxic events of probable global significance. This paper will concentrate on describing the biomarker patterns of the oils and postulated source rocks, but due to reasons of space, isotopic, elemental and other data will only be briefly discussed.