Effect of Isolated Biomoldic Porosity on a Pennsylvanian Carbonate Reservoir in North Texas

Hughbert Collier

Log analysis of a Canyon Group (Pennsylvanian) carbonate buildup in a north Texas well indicates a 20-30% water saturation. Oil saturations from two cores ranged from 15 to 32%; however, the well produced only water. Production tests confirm the perforated intervals as the source of the water. Geologic analyses of two cores taken from the most promising intervals in this carbonate buildup explain the lack of hydrocarbon production. The reservoir facies is a fusulinid packstone with 12-17% biomoldic porosity (dissolved fusulinids), but less than 1 md of permeability. The low permeability is due to the selective dissolution of fossils, which are isolated in a micrite matrix. The matrix has scattered intercrystalline microporosity. Horizontal stylolites and scattered short f actures provide limited permeable pathways.

Apparently, oil migrated into the rock, filling first the intercrystalline matrix porosity and then the isolated molds. Oil production updip in the Canyon carbonate rocks or the rupturing of the seal has now allowed water to displace the oil in the fractures and stylolites, and has destroyed the intercrystalline porosity. The molds remain filled with immobile oil.

A standard logging suite (dual induction, SP, gamma ray, microlog, density, and neutron) shows low water saturations and indicates good permeability. This logging suite is inadequate to evaluate a reservoir with biomoldic porosity. The addition of sonic and EPT logs, however, indicates the presence of moldic porosity and determines higher water saturations. Sonic porosities derived from the Wyllie time average equation are four to five porosity units lower than the density-neutron porosities. Sonic porosities increase the water saturations to 40-45%. EPT-derived "w" values give even higher water saturations.

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