Porosity and Saturation Distribution, Production Characteristics, and Facies Geometry at East Penwell San Andres Unit, University Lands, West Texas: ABSTRACT

Abstract

The East Penwell San Andres unit has produced 40 million bbl of an estimated 176 million bbl original oil in place, and approximately 60 million bbl of mobile oil remain in this reservoir. Waterflood operations began in 1969. Production is from the upper San Andres Formation at a depth of approximately 3500 ft on the east flank of a low, broad anticline. This section is composed of an upward-shoaling package of porous subtidal pellet and skeletal grainstones overlain by generally nonporous mudstone and pisolite packstone. The southern portion of the unit contains a thick zone of nonreservoir peritidal sediments and the northern and central portions of the unit contain tidal channels which trend east-west, parallel to both structural and depositional dip. The San Andres is thoroughly dolomitized and contains anhydrite nodules and anhydrite cements partially altered to gypsum. Log data from the upper San Andres section were used to map hydrocarbon distribution in the unit. Porosities were calculated from sonic logs calibrated with core porosities measured using low-temperature and analytic techniques designed to avoid the effects of gypsum dehydration. These data were used to map reservoir thickness, average porosity, average water saturation, the product of average porosity and thickness (/phi/more » x h), and the product of average oil saturation, porosity, and thickness (S/sub 0/ x /phi/ x h).« less