| Abstrakt: |
Secondary fabric-selective porosity was produced experimentally in mineralogically stabilized (100% low Mg-calcite), Pennsylvanian phylloid algal limestones. Plastic-jacketed cylindrical specimens of initially low-permeability phylloid algal biocalcarenites were subjected to pressures simulating burial at 12,000 feet (3,660 m) in a specially designed triaxial stress apparatus which permitted circulation of weakly acidic (pH 6) pore fluid (CO-enriched distilled water) under constant pressure and temperature (75°F:24°C). Petrographic analysis revealed that a series of systematic experiments produced algal moldic porosity by selective dissolution of coarse crystalline, low-Mg calcite, inter- and intraparticle cements and neomorphosed phylloid algae. Experimental dissolution was initiated along inter- and intracrystalline pores (crystal boundaries and cleavage planes) and stylolites. Moldic porosity formed in early stages of experiments, when rates of circulation were slowest; prolonged experimentation and increased circulation rates promoted the development of nonfabric selective pores (vugs and channels). Experimentally-created pore systems are nearly identical to naturally-occurring porosity within phylloid algal limestone hydrocarbon reservoirs of the Four Corners area, U.S.A. These experimental results offer an alternative explanation for the genesis of moldic porosity in limestones. That is, local variations in texture (e.g., intraparticle permeability), rather than mineralogical differences, appear to have controlled experimental fabric-selective dissolution. [ABSTRACT FROM AUTHOR] |
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