About This Item
Share This Item
Secondary fabric-selective porosity was experimentally produced in Pennsylvanian phylloid algal limestones from the outcrop in southeastern Kansas and from the subsurface in southeastern Utah (Ismay field, Paradox basin). Plastic-jacketed cylindrical specimens of well-cemented limestones were subjected to pressures simulating burial at 12,000 and 15,000 ft in a specially designed triaxial apparatus that permitted circulation of weakly acidic (pH 6) pore fluid (CO2-enriched distilled water) under constant pressure and temperature. Thin sections revealed that the experiment produced algal moldic pores by selective dissolution of coarse low-magnesian calcite cement and pseudosparite. Dissolution was initiated along intercrystalline and intracrystalline pores (crys al boundaries and cleavage planes). Experimental pore systems were identical to natural porosity in Ismay reservoirs. Moldic porosity formed in the early stages of tests when flow rates were slowest; prolonged experimentation and more rapid rates of fluid circulation promoted the formation of vugs and channels.
These experiments document, for the first time, a potential for moldic-porosity formation during late diagenesis in deeply buried, mineralogically homogeneous phylloid algal limestones; thereby, extending the known depth range for porosity in late Paleozoic algal carbonates. Reservoir-quality algal moldic porosity may, therefore, exist within deep, as yet undrilled, parts of basins where mesogenetic decarbonatization has been operative.
End_of_Article - Last_Page 248------------