Genetic Stratigraphy and Oil Recovery in an Upper Cretaceous Wave-Dominated Deltaic Reservoir, Big Wells (San Miguel) Field, South Texas

The Big Wells (San Miguel) reservoir in Dimmit and Zavala Counties, South Texas, produces from a broadly lenticular, wave-dominated deltaic sandstone encased in prodelta and shelf mudstones. An updip porosity pinch-out coincides with a gentle undulation on a uniformly gulfward-dipping monocline and forms a structurally modified stratigraphic trap. The reservoir is relatively tight and has average porosity of 21 percent and average permeability of 6 md; wells require fracturing to stimulate production. Ultimate recovery is projected to be 57 million barrels, or 29 percent of the 198-million-barrel field. The reservoir is subdivided into an upper nonproductive, transgressive shelf sandstone and a lower productive, intensely bioturbated, deltaic sandstone. The tight upper sandstone seals the reservoir, which consists of four major internal facies elements. A dip-oriented distributary system carried sediment into the basin, where it was transported along strike (dominantly to the southwest) by longshore drift. Here wave action reworked the sands into a beach-ridge plain. The resultant distributary/beach-ridge plain complex exhibits an asymmetric cuspate geometry elongated to the southwest. Landward of the delta system lay a muddy coastal plain. Prodelta and lower shoreface silts and shelf muds were deposited seaward of the deltaic/shorezone system. Biogenic activity destroyed most of the primary bedding structures. However, three associations of trace fossils together with subtle variation in lithology allow inference of depositional setting. The Zoophycos ichnofacies, a homogenized sandy and silty mudstone, grades upward into churned sandy siltstone of the Cruziana ichnofacies. Distinctive vertical Skolithos burrows in fine-grained sandstone characterize the Skolithos ichnofacies, which rests on the Cruziana ichnofacies and is transitional upward with the sole lithofacies, the weakly bioturbated, bedded sandstone facies. This vertical progression in ichnofacies represents a shoaling cycle from shelf through lower shoreface, upper shoreface, to foreshore environments. More important is the lateral distribution of ichnofacies, particularly in the oil column. Sand-rich foreshore deposits (composed of sparsely bioturbated, bedded sandstone) and the Skolithos ichnofacies are dominant in the southern half of the field; finer grained, less mature sediment of the Cruziana and Zoophycos ichnofacies characterize the northern half of the field. The northward transition from thicker and cleaner beach-ridge plain sandstones to argillaceous sandstones within and adjacent to the distributary system strongly affects oil recovery from the field. Critical parameters, reservoir permeability and induced-fracture half-lengths (calculated from pressure transient analysis), decrease dramatically to the north. Consequently, well performance peaks in the beach-plain sediments and decreases northward and updip and downdip into adjacent muddier sediments. Recovery efficiencies of the original oil in place average 50 percent in the southern half of the pool and drop to 20 to 30 percent in the north. However, recovery of movable oil is highly efficient. About 88 percent of the nonresidual oil in the pool will be produced. Wave dominated deltaic reservoirs are characterized by minimal well-to-well variability, excellent internal continuity, and consequently, maximum efficiency of mobile-oil recovery