Abstract

Long the subject of speculation, the origin, distribution, and quality of Mesozoic source beds in the deepwater Gulf of Mexico (GOM) are now open to analytical study and hypothesis. We have developed new maps and concepts for organic richness and lithofacies patterns of the primary Upper Jurassic oil-prone source rock interval spanning the Kimmeridgian to Lower Berriasian in the northern GOM. This interval, previously referred to as the Tithonian-centered source, includes the Haynesville and Bossier shales, which lie within supersequences representing second-order transgressive and high-stand systems tracts, respectively. A newly developed gulf-wide Cotton Valley-Bossier paleogeographic map based on a novel paleotectonic model for the Mesozoic provides the framework for this source mapping study. Organic richness averages up to 4.7% and 6.5% total organic carbon for the Kimmeridgian and Tithonian-Lower Berriasian supersequences, respectively, based on the log overlay ΔlogR technique and increases toward the basin center. Lithofacies-sensitive geochemical parameters from reservoired oils and oil seeps tied to the Tithonian-centered oil family demonstrate several potential entry locations for siliciclastic sediments into the Tithonian distal calcareous environment. The region from Garden Banks to Alaminos Canyon appears to be the locus of siliciclastic mixing with carbonates in the deepwater and link with updip evidence of a wide progradational clastic apron sourced by the paleo-Mississippi river. This siliciclastic input may contribute to the potential for improved oil quality by lowering the content of sulfur incorporated into kerogen moving southwest from Ewing Bank-Mississippi Canyon to Alaminos Canyon. Ultimately, enhanced Upper Jurassic source deposition ended with a ventilation event whereby more oxygenated, deeper water entered the northern GOM. Declines in calculated total organic carbon occurring later to the west during the earliest Cretaceous (Berriasian) indicate the opening of the gateway between the GOM and the central Atlantic possibly related to the termination of seafloor spreading in the eastern GOM.

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