ABSTRACT

The Kerlin field in southwest Arkansas produces from secondary moldic porosity in the Pettet "A" zone of the Lower Cretaceous Sligo Formation. Three associated lithic units were delineated on the basis of a petrographic analysis of cores from approximately 20 wells. The three lithofacies identified are: oolite-skeletal grainstone, oolite-skeletal packstone, and the skeletal packstone facies. The depositional model delineated by the facies distribution indicates an elongate offshore shoaling complex. The oolite-skeletal grainstone facies represents the shoal, and the oolite-skeletal packstone facies represents the fringing spillover facies. The shoaling complex migrated over a basic substrate of the skeletal packstone facies.

Production from the Kerlin field is from approximately five to ten feet of porosity in the oolite-skeletal grainstone facies. However, the adjacent oolite-skeletal packstone facies is nonproductive. Secondary moldic porosity in the oolite-skeletal grainstone facies is the basic porosity type and averages 13 percent in the producing horizon. Porosity was formed in the productive grainstone facies by selective dissolution of the aragonite shell fragments in the vadose and fresh-water phreatic zones. Depositional interparticle porosity, which accounts for only a minor percentage of the total field porosity, was reduced by comentation. Present average permeability in the Pettet zone is 7.78 md.

Original permeability in the nonproductive oolite-skeletal packstone facies was reduced by the increased carbonate-mud content which inhibited the dissolutioning of aragonite skeletal material. Thus, moldic porosity was poorly developed in this facies.