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

R. P. Major, G. W. Vander Stoep

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 3,500 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 i 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 (^phgr × h), and the product of average oil saturation, porosity, and thickness (S_o × ^phgr × h).

Reservoir thickness, average porosity, and ^phgr × h increase downdip (eastward) and are relatively low in the southern portion of the unit. The map presents a complex S_o × ^phgr × h pattern, resulting, in part, because lower oil saturation downdip is compensated by increased reservoir thickness. In general, the southern portion of the unit has ^phgr × h and S_o × ^phgr × h values corresponding to a thick zone of peritidal sediments

The central and northern portions of the unit contain two zones of high S_o × ^phgr × h which trend approximately parallel to dip, following the trend of tidal channels.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.