Influence of Regional Tectonic Framework on Northern Gulf of Mexico Petroleum Maturation and Accumulation

Abstract

The geographic distribution of biomarker-determined genetic oil types and their spatial pattern of thermal alteration closely follow the regional strike, reflecting the influence of the northern Gulf of Mexico tectonic framework. Oils and source rocks are often in close geographic proximity. The regional distribution of oils reflects the following tectonic features: the Ouachita orogenic belt and thrust zone (onshore area) and the Louann Salt (salt tectonics) in controlling the offshore migrating depocenter. Additionally, great rivers such as the Rio Grande and Mississippi influenced basin formation by contributing large offshore sediment loads. Thermal isolines based on the C{sub 10+} fraction were used to connect many of the fields, but they often crosscut oil type. The pattern of thermal isolines was obtained by regional contouring of (1) methylphenanthrene indices; (2) 5{alpha}C{sub 29} 20R/S sterane ratio; (3) C{sub 20}/C{sub 20}-C{sub 27}, triaromatic steranes; and (4) moretane/hopane ratio. The derived isolines revealed, with differing sensitivity, the major tectonic elements. In particular, the Ouachita thrust zone appears to be a major control on the maturity of onshore oils. By contrast, the maturity of offshore oils is partly controlled by the subsidence history of the migrating depocenter. The Curtis multisource model is supported by the spatialmore » distribution of genetic oil types but complicated by the discovery of differing maturities of oil and gas in the same reservoir. Possibly larger tectonically induced late gas expulsion from mature source rocks may have enhanced oil migration out of immature Tertiary lignites and shales.« less