Controls on Reservoir Quality in Platform Interior Limestones Around the Gulf of Mexico:

Example from the Lower Cretaceous Pearsall Formation in South Texas

By

LOUCKS, ROBERT G.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Reservoir quality in platform interior limestones around the Gulf of Mexico (GOM) is controlled by distribution of grainstone and grain-dominated packstone and by diagenesis of original aragonite-rich sediments. Pearsall Formation limestones were used as a shallow-buried analog to evaluate these reservoir-quality controls.

Rocks having the highest reservoir quality were deposited in large shoaling complexes that produced mud-poor sediments. Permeability correlates inversely with lime-mud content. Rocks containing more than 20% mud tend to have permeabilities less than 1 md. Pore networks consist of interparticle and moldic pores with some micropores within grains. However, moldic pores and micropores do not contribute to permeability; they form a poorly connected, ineffective pore network. Moldic pores are the result of the dissolution of aragonite grains whose perimeters are preserved by micrite rims. In point-counted samples, moldic pores compose nearly 40% of the pore system. Amount of cementation also has a major influence on reservoir quality. Between 56% and 87% of the postcompaction porosity is plugged by cement.

Platform interior, GOM limestone reservoirs commonly have high porosities (20% – 30%) but only moderate permeabilities (up to several hundred millidarcys). If all the porosity were from interparticle pores, then the permeability could have reached more than 1,000 md. The high concentration of ineffective moldic pores lowers permeability. The controls on reservoir quality in the Pearsall limestones are applicable to other platform interior grainstone deposits around the GOM. Application of this model to deeper buried limestones must consider the effects of additional burial diagenesis and alternate early diagenetic history.