SUBSURFACE MAPPING OF A DOWNDIP VICKSBURG GROUP OIL AND GAS ASSESSMENT UNIT, TEXAS AND LOUISIANA GULF COAST ONSHORE AND STATE WATERS

The U.S. Geological Survey is assessing undiscovered oil and gas resources in the downdip portion of Paleogene strata in Texas (TX) and Louisiana (LA) onshore and State waters. This assessment addresses the estimated accumulations associated with potential Lower Oligocene Vicksburg Group reservoir lithofacies deposited basinward of the paleo-shelf margin. The Vicksburg Group consists of thick sandstones in southernmost TX, grading to predominantly interbedded shales and medium to thick sandstones along the middle to upper TX Gulf Coast and western LA, and interbedded carbonaceous shales and thin to medium sandstones in eastern LA. Currently, there are approximately 40 wells that produce from Vicksburg reservoirs downdip of the lowermost Vicksburg paleo-shelf margin. However, based on this limited reservoir data, undiscovered resources in the downdip Vicksburg are expected to be overpressured oil and gas accumulations found in slope turbidite, submarine canyon, and submarine fan sandstone deposits.

Vicksburg Group strata defined as ‘downdip’ along the TX Gulf Coast are delineated by the downdip extent of maximum normal fault expansion as observed in interpretation of three regional 2-D seismic surveys in southern TX. This coincides with the uppermost Vicksburg paleo-shelf edge, but does not incorporate some slope reservoirs in the lower Vicksburg landward of the downdip extent of faulting. At the time of Vicksburg deposition the major clastic sediment supply to the coast was from the Rio Grande Embayment in south TX. Lack of major sediment supply to LA during Vicksburg time abated the formation of extensive normal faulting at the shelf edge. Therefore, the ‘downdip’ Vicksburg Group in LA was defined by changes in stratal thickness and depositional character from well log interpretation, indicating transition from outer shelf to upper slope environments. The transition is also evident in structural and stratigraphic architecture observed in north-south 2-D seismic surveys for the LA Gulf Coast. Based on this subsurface mapping, the assessment unit encloses that volume of rock that contains Vicksburg Group sediments between the Upper Vicksburg paleo-shelf margin extending from the Mexican-USA border in south Texas to southeast Louisiana and seaward from that margin to the Federal-State waters boundary.