Reservoir Heterogeneity in the Middle Frio Formation: Case Studies in Stratton and Agua Dulce Fields, Nueces County, Texas: ABSTRACT

Abstract

ABSTRACT Selected middle Frio (Oligocene) reservoirs of Stratton field and the contiguous Agua Dulce field are being studied as part of a Gas Research Institute/Department of Energy/State of Texas cosponsored program designed to improve reserve growth in mature gas fields. During the past four decades, Stratton has produced 1.8 Tcf of gas from 110 middle Frio reservoirs, and Agua Dulce has produced 1.6 Tcf from 120 reservoirs. Recent drilling and workover activities, however, suggest the presence of additional untapped or bypassed middle Frio reservoirs. 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. 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 = tens to hundreds of millidarcys); floodplain mudstones and levee sandy mudstones impede or obstruct flow and separate individual reservoirs and compartments both vertically and laterally. The E31/6100 and E25/6050 reservoirs are examples of vertically stacked channel systems reflecting higher rates of coastal plain aggradation. Pressure tests strongly suggest that bypassed compartments have only recently been contacted or are still present for exploitation. Vertically stacked architectures are considered to be more favorable for isolated compartments and therefore are better candidates for infield reserve growth.