The Big Wells (San Miguel) oil field in Dimmit and Zavala Counties, southern 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 an otherwise smooth, gulfward-dipping monocline, resulting in a combination stratigraphic and structural trap. The reservoir is relatively tight with average porosity of 10% and permeability of 7 md; wells require fracing to stimulate production. Ultimate recovery is projected to be 30% of the 180 million bbl field.

The reservoir is subdivided into a nonproductive, transgressive upper sandstone and a productive but intensely bioturbated, predominantly deltaic lower sandstone. The tight upper sandstone provides the reservoir seal. Internal architecture of the reservoir is complex, consisting of strike-elongate beach-ridge deposits that merge north into a dip-elongate, digitate channel-sandstone system representing the deltaic entrant into the basin. SP-log facies display mostly strike-parallel orientations; however, resistivity-log facies are more complex and varied, reflecting a high degree of reservoir heterogeneity.

Early production is uninfluenced by the sedimentary fabric of the reservoir. Initial isoproductivity maps display peaks that correspond with faults, proximity to the gas cap, and to a lesser extent, local sandstone thicks. However, during subsequent production, internal architecture strongly influences reservoir yields as the depositionally complex northern half of the field displays lower recoveries than the beach-ridge deposits to the south.

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