F. Jerry Lucia
Bureau of Economic Geology
John A.& Katherine G. Jackson School of Geosciences
The University of Texas at Austin

ABSTRACT
The geologic history of carbonate pore space in the Permian Basin has been difficult to describe because of the lack of information from young carbonates. In the past, studies focused on understanding how Permian Basin reservoirs formed were restricted to using core and outcrop samples from the Permian Basin and surrounding outcrops. These samples are of Paleozoic age and typically have little matrix porosity and permeability. The results of these studies have led to a number of misconceptions, such as (1) porosity is formed by dissolution because carbonates are highly soluble in acidic waters, (2) porosity is formed by dolomitization because dolostones are commonly more porous than adjacent limestones, or (3) all pore space was occluded early because of the lack of compaction.

Charting the evolution of pore space through time requires knowing the diagenetic history from the time of deposition until the present. For the Permian Basin this time period is 200–600 m.y., and trying to describe this long period of diagenesis using Paleozoic samples has proven difficult. What is needed is direct information about the fabrics at various stages of development. Information on the porosity and permeability of carbonate sediments has been available for some time, and data from limestones and dolostones of Pleistocene, Tertiary, Cretaceous, and Jurassic ages and becoming more available. These data show that porosity destruction is a geologically slow but steady process. Tertiary carbonates with 30–40 percent porosity and Cretaceous and Jurassic carbonates with 20–30 percent porosity are not uncommon. The data also show that young dolostones tend to have porosity values that are similar to or much lower than their adjacent limestones. Fabric observations of young carbonates and Permian Basin reservoirs suggest that (1) many Permian and Pennsylvanian reservoirs have limestone fabrics similar to those of young carbonates and have lost porosity through simple compaction and cemention, (2) porosity in Permian dolostone reservoirs was inherited from the precursor limestone, reduced by overdolomitization, and preserved from extensive compaction, and (3) Devonian chert reservoirs are porous because the chert replaced a highly porous mud-dominated sediment and inhibited compaction.