Paleokarst and Fracture Overprints in Mid-Continent Carbonates in Evaluation of Horizontal Drilling Potential
WILSON, J. L., New Bedford, TX, R. D. FRITZ,* MASERA Corp., Tulsa, OK, M. ESTEBAN, ERICO PI, London, U.K., and J. W. SHELTON, MASERA Corp., Tulsa, OK
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.
The Arbuckle Group, for example, was dolomitized very early by peritidal and/or freshwater mixing processes. Sequences in which intercrystalline porosity was developed with little or no vuggy porosity usually have low permeability and effective porosity; however, these dolomitic sequences are susceptible to fracturing, and they may produce in structural traps. These fractured sequences in the Arbuckle also are susceptible to karstification associated with multiple unconformities.
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 of the horizontal drilling potential in these carbonates.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)