LUCIA, F. JERRY, and R. P. MAJOR, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

ABSTRACT: Is Dolomitization a Porosity-Generating or a Porosity-Destroying Diagenetic Process?

Dolomitization is often assumed to create porosity because porosity in carbonate reservoirs is commonly observed to be preferentially associated with dolomite. If dolomitization proceeds by mole-for-mole replacement of CaCO<3> by (Mg,Ca)CO<3>, approximately 12 percent porosity will be created because dolomite is more dense than calcite. If, however, dolomitization proceeds with a net addition of CO<3> to the site of dolomitization, then dolomitization can be a porosity-reducing diagenetic process.

Plio-Pleistocene dolomitized foreslope grainstone on Bonaire, Netherlands Antilles, exhibits a progressive increase in dolomite unit cell dimensions, calcium content, and strontium content from updip, low-porosity dolomite to downdip, high-porosity dolomite and limestone. Replacement dolomite is less stoichiometric and contains more strontium than does dolomite cement. These rocks were dolomitized by marine-derived hypersaline brines that moved downdip from a superjacent sea-marginal lagoon or saline lake. Dolomitization involved a net import of CO<3> that resulted in porosity reduction, and rocks proximal to the source of dolomitizing fluids were subjected to a greater amount of porosity reduction than more distal rocks. This observation indicates that dolomitization is a poros ty-destroying process and that the degree of porosity destruction can be calibrated to the flow path of dolomitizing fluids.

Although mole-for-mole replacement of CaCO<3> by (Mg,Ca)CO<3> with concomitant creation of porosity is a commonly cited model, we know of no data sets demonstrating that this process occurs in nature. We suggest that the association of porosity with dolomite in carbonate reservoirs indicates that these reservoirs are dolomite because they are porous rather than porous because they are dolomite.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.