Abstract: Fabric-Selective Permoporosity Experimentally Developed in Carbonate Rocks

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

The development of fabric-selective porosity in representative carbonate rocks is being evaluated systematically under controlled laboratory conditions. Jacketed cylindrical specimens are placed in a specially designed triaxial apparatus that permits circulation of pore fluid (typically water charged with CO₂) under constant pressure while subjecting the rock specimen to constant vertical axial pressure, lateral confining pressure, and temperature. Depths up to 15,000 ft (~4,600 m) are simulated by axial pressures to 15,000 psi (~100 MPa), lateral pressures to 10,000 psi (~70 MPa), pore pressures to 9,000 psi (~60 MPa), and temperatures to 300°F (~150°C).

Results obtained thus far indicate that fabric selective secondary porosity can be developed by circulating fluids under conditions characteristic of deep burial, i.e., after pervasive cementation and after mineralogic stabilization have occurred within the carbonate rock. For example, in the experimental development of oomoldic porosity in a tightly cemented oolitic calcarenite consisting entirely of low-magnesium calcite, mineralogy appears to play no role in the selective dissolution of ooids in test samples. Moreover, differences in CO₂ concentration and flow rate of the pore fluid and in test specimen configuration have no observable influence on the dissolution effects, only on the solution rates. Secondary porosity is developed and permeability is enhanced significan ly by initial dissolution of outermost cortical layers of the ooids, development of interconnecting openings where ooids were in contact, and ultimate removal of nuclei and entire ooids to produce oomoldic porosity.

Because differential solution occurs where flow rates vary, pores exposed to a greater flow rate are enlarged preferentially. Hence, what appears to determine whether moldic or interparticle porosity ultimately results is whether flow of pore fluids becomes established first in potentially interconnecting intraparticle porosity (e.g., within cortical layers of ooids) or along interparticle pathways (e.g., between grains of cement).

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC