ABSTRACT: Fluvial Architecture and Reservoir Compartmentalization in the Oligocene Middle Frio Formation of South Texas

D. R. Kerr, L. A. Jirik

Seeligson, Stratton, and Agua Dulce fields are being studied as part of a Gas Research Institute/Department of Energy/State of Texas cosponsored program designed to develop and test methodologies and technologies for gas reserve growth in conventional reservoirs in mature gas fields. Over the last four decades, each field has produced approximately 2 tcf of gas from middle Frio reservoirs alone. Recent drilling and workover results and reservoir pressure data, however, point to the possibility of additional reserves. Stratigraphic and sedimentologic studies based on well logs and cores indicate that middle Frio reservoirs are architecturally complex.

Deposition on an aggrading coastal plain resulted in a continuum of architectural styles that has important implications for reservoir compartmentalization. The middle Frio is composed of sand-rich channel-fill and splay deposits interstratified with floodplain mudstones, all forming part of the Gueydan fluvial system. Relatively slow aggradation resulted in laterally stacked channel systems; whereas more rapid aggradation resulted in vertically stacked channel systems. Laterally stacked sandstone bodies predominate at Seeligson field, leading to separate but potentially "leaky" reservoir compartments. By contrast, vertically stacked sandstone bodies predominate at Stratton and Agua Dulce fields, favoring more "isolated" reservoir compartments. Thus, a high potential for reserve growt through the identification of untapped compartments, poorly drained acreage, and bypassed zones exists for each of these fields, but differences in reservoir architecture must be taken into account as part of exploitation strategies.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990