Abstract: Experimental Development of Permoporosity in Carbonate Rocks

Fred A. Donath, Albert V. Carozzi, Lester S. Fruth, Jr., David W. Rich

Permoporosity has been developed experimentally in typical oolitic biocalcarenite with sparite cement (Ste. Genevieve, Mississippian, Illinois basin) under controlled conditions in the laboratory. Thin-section examination and scanning-electron microscopy revealed no open pore spaces initially, although water saturation under vacuum indicated an initial 10-percent intercrystalline porosity.

Jacketed cylindrical specimens were placed in a specially designed triaxial apparatus that permits circulation of pore fluid (water saturated with CO₂) under constant pressure while subjecting the specimen to constant axial pressure, lateral confining pressure, and temperature. Axial pressures to 15,000 psi (~100 MPa), confining pressures to 10,000 psi (~70 MPa), pore pressures to 9,000 psi (~60 MPa), and temperatures to 300° F (~150°C) were selected to simulate burial depths to 15,000 ft (~4,600 m). Porosity clearly was enhanced by circulating a pore-fluid volume as small as 150 times the initial pore volume of the specimen at the following typical conditions: 8,700 psi axial, 5,800 psi lateral, and 525 psi pore pressure, 176°F, equivalent to approximately 9 000 ft (2,750 m) depth.

Extensive corrosion and development of permoporosity took place on surfaces normal to the direction of fluid flow. SEM examination revealed both a general etching and preferential etching along cleavage planes, intracrystalline-glide planes, and crystal boundaries within the interstitial sparite cement. The effects of solution were observed along these features under the petrographic microscope, as well. Within the fibroradiated concentric rings of the ooids, SEM examination revealed complete solution of individual crystals leaving a porous aggregate of juxtaposed, sharply defined rhombic crystals. This effect is expressed in thin section by a marked local lightening in color of the concentric rings.

The influence of different gases and brines on the development of permoporosity in various petrographic varieties of carbonate rocks and carbonate-cemented sandstones is being evaluated in a continuing program of research.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA