INVESTIGATING EFFECTS OF FAULT GEOMERTY ON GAS RESERVIORS: A CASE STUDY FROM LA RUCIA FIELD, BROOKS COUNTY, TEXAS

The Lower Oligocene Vicksburg Formation (fluvial, nearshore marine, and deltaic sandstones; Rupelian stage approximately 35-33 million years ago) in southern Brooks County Texas, is being affected by the Vicksburg Fault Zone (VFZ). The VFZ is comprised of listric normal faults oriented parallel to the Texas Gulf Coast and forming rollover anticline structures. Explanations of how these faults are affecting accumulations and spatial distributions of gas have been investigated in this study. We analyzed post-stack 3D seismic with high spatial coverage and good resolution (289 inlines, 279 crosslines; area: 49.1 km²; 80,631 common depth points and 24.38m spacing) integrated with velocity, well logs, and production data to investigate the proposed objectives. The V-17 and V-19 sand members within the Vicksburg Formation represent prolific gas reservoirs. Faults affecting these horizons have been interpreted, tied, and mapped to explain how fault geometry is either acting as a conduit or as a barrier for gas accumulations. Results from detailed interpretation indicate that (1) western dipping faults created a favorable condition for gas accumulation beneath a top-lapping unconformity and thick shale sealing layer (between the V-17 and V-16 members), (2) eastern dipping faults are affecting the overlying sealing shale and enhanced gas migration, (3) intersecting fault systems (parallel and perpendicular to regional SW-NE Gulf Coast) act as conduits rather than traps and provide additional pathways for gas migration into the V-17 and V-19 sand reservoirs. Investigating the control of these fault systems enhances our understanding on subsurface fluid migrations and accumulations (oil, gas, groundwater, and contaminants) in the expanded Vicksburg productivity trends.