Structural and stratigraphic controls on reservoir architecture: A case study from the lower Oligocene Vicksburg Formation, Brooks County, Texas

A detailed structural and stratigraphic framework provides a precise understanding of the controls on the vertical and spatial distribution of reservoir units, as well as the identification of the most prospective hydrocarbon accumulation. In southern Brooks County, Texas, the Lower Oligocene Vicksburg Formation (LOVF), is being influenced by the Vicksburg Fault Zone (VFZ). This zone is characterized by listric-normal faults that have formed highly faulted rollover anticlines, which are valuable structural traps for hydrocarbon exploration. This study integrated 568 3D seismic lines, 39 well logs, and three velocity surveys to build a comprehensive conceptual model that explores how secondary synthetic (dipping east), antithetic (dipping west), and Texas Gulf Coast-perpendicular faults are influencing accumulation and spatial distribution of hydrocarbons within the LOVF (La Rucias Field). Results indicate that synthetic, antithetic, and coast-perpendicular faults affecting the V-102, V-17, and V-19 horizons provide conduits for hydrocarbon migration. Antithetic faults and coast-perpendicular faults within the rollover anticline are terminating beneath an overlying shale seal layer between the V-16 and V-17, influencing economic hydrocarbon accumulations through multiple reservoirs. While synthetic faults affect the overlying seal layer migrating hydrocarbon out of the V-102, V-17, and V-19, bidirectional faulting linking antithetic and coast-perpendicular faults provide additional pathways for hydrocarbon accumulation. Spatial distribution of hydrocarbons varies with the targeted horizon. V-102 reservoirs are situated on the western flank of the rollover anticline, whereas the V-17 and V-19 reservoirs are located on structural highs where overlying shale seal layer remains unaffected by the antithetic faults. V-17 and V-19 reservoirs are influenced by bidirectional faulting that terminates beneath the shale seal layer, enabling the accumulation within the rollover anticline. Petrophysical and lithological investigations of the seal layers within the V-102, V-17 and V-19 are highly recommended for better understanding of vertical compartmentalization of reservoir intervals and accumulation estimates within the La Rucias Field. Investigating the control of these fault systems enhances our understanding of subsurface fluid migration and accumulations (oil, gas, groundwater, and contaminants) in the expanded Vicksburg productivity trends. Furthermore, the insights gained from the examination of these fault systems not only contribute to our comprehension of subsurface fluid migrations within the Vicksburg trends but also serve as a valuable reference for similar geological systems and reservoirs worldwide, ultimately aiding in the development of more effective resource management.