ABSTRACT

Seeligson, Stratton, and Agua Dulce fields (South Texas) 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 five decades, each field has produced approximately 2 Tcf of gas from middle Frio reservoirs alone. Recent drilling, old-well workover results, and reservoir pressure data, however, point to the possibility of additional reserves within these fields.

The middle Frio (Oligocene) is composed of sand-rich channel-fill and splay deposits interstratified with floodplain mudstones, all forming part of the Gueydan fluvial system. Channel-fill deposits are 30 ft (9 m) thick and 2,500 ft (76 m) wide. Splay deposits are as much as 20 ft (6 m) thick proximal to channels and extend as much as 2 mi (3 km) from channels. Channel-fill and associated splay sandstones are reservoir facies (porosity = 20 percent; permeability = 10's to 100's md); floodplain mudstones and levee sandy mudstones impede or obstruct flow and separate individual reservoirs and compartments both vertically and laterally.

Deposition on an aggrading coastal plain resulted in a continuum of fluvial architectural styles that has important implications for reservoir compartmentalization. Relatively slow aggradation resulted in laterally stacked channel systems, whereas more rapid aggradation resulted in vertically stacked channel systems. In Seeligson field, laterally stacked architecture alternates with vertically stacked architecture through the 2,000-ft- (610-m-) thick middle Frio section. In Stratton and Agua Dulce fields, the same general alternation in architectural modes exists through the 2,500-ft- (763-m-) thick middle Frio section. In Stratton and Agua Dulce, however, the reservoir zones composed of laterally stacked architecture are not laterally continuous; instead, the laterally stacked architecture locally changes into a vertically stacked architecture. Because laterally stacked sandstone bodies lead to separate but potentially "leaky" reservoir compartments and vertically stacked sandstone bodies favor more "isolated" reservoir compartments, a high potential for reserve growth through the identification of untapped, poorly drained, and bypassed gas reservoir compartments exists in each of these fields. Differences in reservoir architecture also must be taken into account as part of exploitation strategies.