--> Abstract: Is Dolomitization a Porosity-Generating or a Porosity-Destroying Diagenetic Process?, by F. J. Lucia and R. P. Major; #90987 (1993).
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LUCIA, F. JERRY, and R. P. MAJOR, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

ABSTRACT: Is Dolomitization a Previous HitPorosityNext Hit-Generating or a Previous HitPorosityNext Hit-Destroying Diagenetic Process?

Dolomitization is often assumed to create Previous HitporosityNext Hit because Previous HitporosityNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit-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-Previous HitporosityNext Hit dolomite to downdip, high-Previous HitporosityNext Hit 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 Previous HitporosityNext Hit reduction, and rocks proximal to the source of dolomitizing fluids were subjected to a greater amount of Previous HitporosityNext Hit reduction than more distal rocks. This observation indicates that dolomitization is a poros ty-destroying process and that the degree of Previous HitporosityNext Hit 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 Previous HitporosityNext Hit is a commonly cited model, we know of no data sets demonstrating that this process occurs in nature. We suggest that the association of Previous HitporosityTop 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.