STERANE AND TRITERPANE PATTERNS AS DIAGNOSTIC-TOOLS IN THE MAPPING OFOILS, 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 pattern s. These families are (1) carbonate-sourced oils reservoired in Jurass ic-Cretaceous reservoirs onshore and Plio-Pleistocene reservoirs offsh ore, (2) Austin Chalk reservoired oils, (3) Tuscaloosa oils, (4) Wilco x reservoired oils (La), (5) South Texas Palaeogene reservoired oils, (6) Mississippi delta oils, (7) post-mature oils of uncertain affiniti es, (8) migration contaminated oils (showing attributes of a mixing of immature and post-mature sources), and (9) oils and condensates lacki ng biomarkers. Each oil family is concentrated in specific age-defined producing trends, ranging in age from Jurassic to Pleistocene. For Te rtiary reservoirs, these producing trends shift geographically with ti me, in a manner analogous to the clockwise migrating Cenozoic depocent er 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 forma tions), (2) The Aptian (including Pine Island, Bexar and Sunniland for mations), (3) The Albian (Paluxy formation), (4) The Cenomanian-Turoni an (including Eagle Ford and Tuscaloosa formations), (5) The Coniacian Santonian (Eagle Ford and Austin Chalk), and (6) The Palaeocene (Wilc ox, Midway, Sparta formations). The Jurassic and Cretaceous source roc ks correspond to described oceanic anoxic events of probable global si gnificance. This paper will concentrate on describing the biomarker pa tterns of the oils and postulated source rocks, but due to reasons of space, isotopic, elemental and other data will only be briefly discuss ed.