Increased Oil Recovery Potential from Barrier/Strandplain Reservoirs, Jackson-Yegua Trend, by Geologically Targeted Infill Drilling: Examples from Seventy-Six West and Colmena-Cedro Hill Fields, South Texas

The Jackson-Yegua Barrier/Strandplain Sandstone play is a prolific oil-producing trend in South Texas that has high average porosity (31 percent) and permeability (600 md). Nevertheless, it is characterized by low recovery efficiencies because of low average oil gravity, weak solution-gas drive, complex stratigraphy, and structural complications. In this study, the potential for substantial reserve growth was ascertained, and the remaining mobile oil resource contained within the internally heterogeneous and structurally disrupted sandstone reservoirs was targeted.Two fields, Seventy-Six West and Colmena-Cedro Hill, were chosen as typical of the Jackson-Yegua Barrier/Strandplain Sandstone play. Detailed reservoir characterization of these fields indicated that the highly complex reservoir architecture consists of a mosaic of strike-oriented barrier-core, shoreface, and inner-shelf-shoal sandstones and mudstone-rich back-barrier-lagoonal deposits. The principal strike-oriented reservoir trend is crosscut by dip-oriented tidal-inlet fills and fluvial channels and has been modified by erosion and deposition of washover sandstones during storm events. Small-scale (10 to 100 ft) normal faults and dip reversal of units in the Jackson Group, forming in response to reactivation of older growth faults along the deeper Wilcox fault trend, have further disrupted reservoir continuity. The oil-trapping mechanism is primarily an updip permeability pinch-out where barrier sandstones thin against the muddy backbarrier-lagoonal facies. However, faulting and dip reversal played an important role in providing secondary structural traps.At Seventy-Six West field, production trends are controlled by a complex geometry of reservoir sandstones and faults: eight discrete compartments were identified from the reservoir architecture, structure, and trends in oil and water production. At Colmena-Cedro Hill field, in contrast, structural control dominates production trends, whereas reservoir architecture is less important. Because Colmena-Cedro Hill field has an extensive gas cap and strong aquifer drive, subtle structural movement determines whether the reservoir sandstones lie within the gas-, oil-, or water-saturated parts of the field. Internal reservoir heterogeneity at Seventy-Six West and Colmena-Cedro Hill fields has created a strong fluid-flow anisotropy and, thus, the potential for bypassed mobile oil in untapped compartments and partly swept zones in waterflood areas targets for infill drilling and optimum waterflood design.Four prospects were identified at Seventy-Six West field. Two have been drilled and successfully completed as oil producers. A third well was drilled primarily as a location for water injection in the geologically optimized waterflood currently being designed. Three prospects were identified at Colmena-Cedro Hill field, and to date, two have been drilled and successfully completed as oil producers, although one of the wells is considered only a marginal financial success. This case study of Seventy-Six West and Colmena-Cedro Hill fields can apply directly to all fields in the Jackson-Yegua Barrier/Strandplain Sandstone play, ultimately helping to increase oil recovery.