LUCIA, F. JERRY, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

ABSTRACT: Origin and Petrophysics of Carbonate Rock Fabrics

The origin of carbonate fabrics has been studied from numerous perspectives including sedimentation, diagenesis, geochemistry, hydrology, and stratigraphy, with the goal of understanding earth history. The results of these studies have been applied to the search for and exploitation of petroleum reservoirs. Effective exploitation of petroleum reservoirs, however, requires knowledge of the distribution of petrophysical properties, porosity, capillarity, and permeability. This requirement has generated research aimed at understanding the development of pore sizes in carbonate rocks and has resulted in unique conclusions about the origin of carbonate rocks including (1) carbonates are born with high porosity and lose porosity gradually over a long period of time, (2) dolomitization does not make porosity but mimics the porosity of precursor limestone, (3) dissolution to form vuggy porosity does not increase overall porosity because the dissolved carbonate is precipitated locally.

One control on pore size is the size and shape of the depositional grains or crystals. Pore sizes in mud-dominated sediments are much smaller than those in grain-dominated sediments, and the relationship between this simple fact and permeability has been demonstrated by Ennis and Sawatsky, who show that grainstones and packstones have 100 times more permeability than wackestones even though wackestones have higher porosity. A second control on pore size is the amount of interparticle porosity. In simple limestones, interparticle porosity is reduced by cementation and compaction, and pore size is reduced as cementation and compaction increase. Permeability is related to pore size: as pore size is reduced, permeability is also reduced. This fact gives rise to a predictable relationship between interparticle porosity, permeability, and grain size and sorting. Fabric and porosity data from passive-margin carbonate units ranging from Holocene to Ordovician indicate a very slow rate of porosity loss.

The origin of porosity in dolostones has long been debated. Porosity studies have demonstrated that dolomitization of grain-dominated limestones usually does not change the pore size or the petrophysical properties. However, dolomitization of a mud-dominated limestone may change the pore size considerably if the dolomite crystal size is larger than the carbonate mud size. Contrary to conventional wisdom, dolomitization does not create but occludes porosity. The reduction in pore space by dolomitization reduces the pore size and permeability. This fact gives rise to a predictable relationship, similar to simple limestones, between interparticle porosity, permeability, precursor grain size, and dolomite crystal size.

Vuggy porosity is defined as pore space that is not between particles and is divided into two types on the basis of its effect on permeability. Separate-vug porosity is principally found within grains or crystals and has little effect on permeability because it is poorly interconnected. Touching-vug porosity is normally larger than the particles and has a well-interconnected pore system that is independent of interparticle porosity. Separate vugs may be primary intrafossil porosity or grain molds produced by selective dissolution. However, porosity is not gained by the dissolution of grain molds because the dissolved carbonate is precipitated in the adjacent interparticle pore space. Karst systems are a common source of caverns, fractures, and breccias typical of touching-vug pore systems in carbonates. Reservoir studies have suggested that caverns and breccias are commonly infilled with internal sediment and cement, and that fractures compose the principal pore system.

AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures