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U.S. Geological Survey Assessment of Undiscovered Hydrocarbons in the Deep Tertiary of the U.S. Gulf Coast Region

Abstract

In 2016, the U.S. Geological Survey (USGS) completed an assessment of undiscovered hydrocarbons in deep Tertiary strata of the U.S. Gulf Coast region. This assessment was motivated by recent discoveries at the offshore Davy Jones and Blackbeard prospects and shows at the onshore Highlander (Lomond) prospect, and it updated portions of the 2007 USGS Paleogene assessment. As part of this effort, eight conventional assessment units (AUs) located downdip of related expanded fault zones and three continuous AUs were identified in the region. The conventional AUs were quantitatively assessed, whereas the continuous AUs were not. The continuous AUs were not quantitatively assessed because there are no known wells or fields producing hydrocarbons from potential shale gas and oil reservoirs. All the AUs developed in this assessment are part of the Upper Jurassic-Cretaceous-Tertiary Composite Total Petroleum System defined by the USGS in 2007. These AUs, listed from oldest to youngest, are the Midway Group Continuous AU, Wilcox Group Continuous AU, Wilcox Group Slope Sandstones AU, Lower Claiborne Group Continuous AU, Lower Claiborne Group Slope Sandstones AU, Upper Claiborne Group Slope Sandstones AU, Jackson Group Slope Sandstones AU, Vicksburg Group Slope Sandstones AU, Frio Group Slope Sandstones AU, Hackberry Slope Sandstones AU, and Anahuac Formation Slope Sandstones AU. The downdip AU boundaries were defined by the State waters limit, whereas the updip boundaries of the conventional AUs are paleo-shelf margins defined by environments of deposition rather than structure, such as expanded fault zones. Although the expanded fault zone areas had borehole stratigraphy and production data detailed in commercial databases, the deeper downdip areas have a dearth of data, making these frontier exploration areas. Stratigraphic correlations were complicated by complex salt tectonics and normal faulting. Depositional environment of the deep, downdip intervals were interpreted to be paleo-slope, based on interpretation of paleontologic data, geophysical well logs from wildcat exploration wells, regional two-dimensional seismic lines, and source-to-sink paleogeographic reconstruction from recent sediment provenance studies. The continuous AUs were delineated based on revised interpretation of published data, new geochemical analysis of selected well samples, interpretations of well logs from legacy and recent drilling, and investigations of regional seismic profiles.