Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Geological Society of London Workshop titled the "Geometry and Petrogenesis of Dolomite Hydrocarbon Reservoirs," London, UK, Dec. 34
Origin and Petrophysics of Dolostone Pore Space
F. Jerry Lucia
Dolomitization is always a controversial subject, and porous dolostone has always been at the heart of the controversy. Since Elie de Beaumont concluded in 1837 that dolomitization created 12 percent porosity on the basis of volume changes calculated from the mole-for-mole replacement equation, dolomitization has been touted as a mechanism for creating porosity from dense limestone. However, in the past 50 years a large body of data has been collected demonstrating that dolomitization does not create but occludes porosity. There is no relationship between porosity and permeability in dolostone, however, and dolomite crystal size and the precursor fabric are key elements in predicting the permeability of dolostones.
The porosity and fabric of the precursor limestone are required to test the effect of dolomitization on porosity and permeability. Research aimed at understanding the development of pore space in limestones has shown that (1) carbonates are born with high porosity and lose porosity gradually over a long period of time, (2) mud-dominated fabrics compact more readily than grain-dominated fabrics, (3) dissolution of limestone to form vuggy porosity does not increase overall porosity because the dissolved carbonate is precipitated locally. Permeability of limestones is controlled by the size and shape of the depositional grains or crystals. A second control on permeability is the amount of interparticle porosity. This fact gives rise to a predictable relationship between interparticle porosity, permeability, and grain size and sorting.
Comparisons of precursor limestone fabrics with dolostone fabrics suggest that dolomitization does not make porosity but mimics and reduces the porosity of precursor limestones. Limited data indicate that Holocene dolomites have 50 to 60 percent porosity similar to adjacent lime sediments. Data from the Neogene of the Bahama Banks show dolostones with 35 percent porosity and adjacent limestones with 40 percent porosity. The study of a Plio-Pleistocene carbonate in Bonaire, N.A., demonstrated that the precursor limestones are more porous than the dolostones. The limestones average 25 percent porosity, whereas the dolostones average 11 percent porosity. Porosity studies of the Jurassic Arab D formation show that dolostones are no more porous than associated grain-dominated limestones and many dolostones are dense. These studies suggest that the porosity in dolostone is not created by a mole-for-mole replacement mechanism. Instead dolostone porosity is (1) inherited from the precursor limestone and (2) occluded by the process of overdolomitization. Paleozoic dolostones, however, are commonly more porous than juxtaposed limestones. The explanation for this observation is that the limestones have lost porosity through compaction and cementation, whereas dolostones resisted compaction and retained much of their porosity.
Permeability studies have demonstrated that dolomitization of grain-dominated limestones usually does not change the porosity-permeability relationships. Instead, the precursor fabric controls pore-size distribution. However, the dolomite crystal size of a dolomitized mud-dominated limestone may be larger than the carbonate mud size, improving the porosity-permeability relationship substantially. The reduction in pore space by overdolomitization reduces the pore size and permeability. This fact gives rise to a predictable relationship between interparticle (grains or crystals) porosity, permeability, precursor grain size, and dolomite crystal size.