The Upper Cretaceous 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 an average porosity of 21% and average permeability of 6 md; wells require fracturing to stimulate production. Ultimate recovery, based on current production trends and technology, is projected to be 57 million bbl, or 29% of the 198 million bbl field.

The reservoir is subdivided into an upper, nonproductive, transgressive shelf sandstone and a lower, productive, but intensely bioturbated, deltaic sandstone. The tight, nonproductive upper sandstone seals the reservoir, which consists of four major internal facies. A dip-oriented distributary system carried sediment into the basin. Sediment was then transported along strike (dominantly to the southwest) by longshore drift. Wave action reworked the sands into a beach-ridge plain. The resultant distributary and 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 and shore-zone system.

The northward transition from thicker and cleaner sandstones of the beach-ridge plain to argillaceous sandstones within and adjacent to the distributary system strongly affects oil recovery from the field. Reservoir permeability and induced-fracture half-lengths (the distance from the well bore to the fracture terminus or half the total length of induced fracture, 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% in the southern half of the pool and drop to 20-30% in the north. However, recovery of movable (nonresidual) oil is highly efficient. About 88% 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.