ABSTRACT: Cave/Pillar Definition in the Ordovician Ellenberger Reservoir by 3-D Seismic, Pegasus Field, Midland and Upton Counties, Texas

PURVES, W. J., and J. E. BURNITT, Mobil-MEPUS, Midland, TX, L. R. WEATHERS, Mobil-MEPUS, Houston, TX, and L. K. WIPPERMAN, Mobil-MEPSI, Dallas, TX

The Pegasus field, a giant, multipay field discovered by Mobil in 1949, lies in the southwestern part of the Midland basin. The field occurs at the eastern margin of the early to middle Paleozoic Tobosa basin and structurally sits on one of a series of en echelon, northerly trending highs that parallels the eastern margin of the Central Basin platform. In this area, the Ellenburger Formation was exposed to extensive karstification from a post-Ellenburger Middle Ordovician low sea level stand (Sauk Sequence). The Ellenburger reservoir was described as a laterally extensive cave system and was considered depleted and scheduled for abandonment. A new reservoir model was considered; namely a joint-controlled, rectilinear, laterally restrictive, variable-depth, cave system. A 33 sq. miles -D seismic program was acquired to test the new cave model. Amplitude variations sensitive to rock property variations between dense cave pillars and solution-collapse breccias and cave fill were volumetrically mapped on Intergraph, Landmark, and Silicon Graphics/Voxelview work stations. Wells that have been drilled into solution collapse breccia cave complexes are prolific and produce in excess of 2 million barrels of oil per well. The laterally restricted caves are typically discontinuous rectilinear segments 1.5 mi long by variable widths up to 6.5 mi and 150-200 ft high.

This 3-D seismically defined laterally restricted cave/pillar reservoir model suggests untapped reserves remain, allows a reduced-risk means to locate and drill cave complex areas in the upper Ellenburger, and suggests that a deeper undrilled cave system lies below the present system.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)