LUCIA, F. JERRY, Bureau of Economic Geology, University of Texas at Austin, Austin, TX
ABSTRACT: Dolomitization: A Porosity-Destructive Process
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 was a mole-for-mole replacement process because he measured 12.5% porosity in Tyrolian dolomites, 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. It is time to nail the coffin shut on this myth.
To test the effect of dolomitization on porosity requires knowing the porosity of the precursor limestone. Limestones in lower Paleozoic carbonate reservoirs are typically dense, whereas porosity is commonly found in dolostones. The explanation for this fact has been that dolomitization creates porosity. However, this observation does not hold for young carbonates. Limited data indicate that Holocene dolomites have 50 to 60% porosity, similar to adjacent lime sediments. Recently reported data from the Neogene of the Bahama Banks show dolostones with 35% porosity and adjacent limestones with 40% porosity. The study of a Pliocene-Pleistocene carbonate in Bonaire, Netherlands Antilles, demonstrated that the limestones are more porous than the adjacent dolostones. The limestones average 25% porosity, whereas the dolostones average 11% 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, suggesting porosity destruction through dolomitization.
Fabric studies show that large dolomite crystals replace not only CaCO3 but also microporosity. Replacement dolomite crystals are therefore part replacement and part pore filling. The pore-filling carbonate is mostly from the dissolution of CaCO3 between the dolomite crystals and a small amount from the dolomitizing water. As dissolution progresses, porosity is increased in the intercrystalline volume and decreased in the crystal volume. However, more volume is added to the crystals than pore space occluded, resulting in an overall loss of porosity through dolomitization.
These studies suggest that the porosity in dolostone is inherited from the precursor limestone and not created by a mole-for-mole replacement mechanism. Indeed, Robert L. Folk has reported on a dense dolomite found replacing a marbleized Pleistocene limestone in Italy; dolomitization did not create porosity. Dense lower Paleozoic limestones were most likely porous and permeable at the time of dolomitization. Limestones lost porosity through compaction and cementation, whereas dolostones resisted compaction and retained much of their porosity. These studies suggest that instead of creating porosity, dolomitization reduces porosity and preserves porosity from being destroyed by compaction.
AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures