Paleokarst and Fracture Overprints in Mid-Continent Carbonates in Evaluation of Horizontal Drilling Potential

Abstract

The Mid-Continent region, especially in Oklahoma and Arkansas, contains thick Paleozoic carbonate sections that are dolomitic and karstic in character. These sections commonly exhibit strong structural overprints, including intense fracturing, due primarily to Pennsylvanian orogenies. Because of their rather wide association with source rocks, these carbonates are thought to represent good potential targets for horizontal drilling. The Cambro-Ordovician Arbuckle Group, the Ordovician Viola Group, the Siluro-Devonian Hunton Group, and the Mississippian Limestone all contain zones that are locally productive. These stratigraphic units are either uniformly tight or they are heterogeneous with complex porosity profiles. In karst terranes both types commonly occur together; both require fracturing to increase porosity and permeability. Both youthful and mature stages of paleokarst are observed in the Arbuckle Group; the best porosity is developed in the youthful stage. These stages can develop microporous, planar porous, or macroporous types of reservoir geometry. All of these may be heterogeneous in nature, requiring fractures to interconnect porous intervals. Horizontal drilling is yet to be proved as a reliable method for increasing production efficiency in Mid-Continent carbonates. An evaluation of diagenetic history, especially karst processes, along with local and regional structural settings, may provide a key for improved understanding ofmore » the horizontal drilling potential in these carbonates.« less