ABSTRACT: Karst-Related Diagenesis and Reservoir Development in the Arbuckle Group, Wilburton Field, Oklahoma

BLIEFNICK, DEBORAH M., ARCO Oil and Gas Company, Midland, TX, and W. C. BELFIELD, ARCO Exploration and Production Technology, Plano, TX

Wilburton field is a multizone reservoir on the southwestern edge of the Arkoma basin. The most recent zone to be declared commercial is the Cambrian-Ordovician Arbuckle Group. Faults, structural position, depositional environment, and diagenetic alterations play a role in controlling reservoir quality, communication of fluids and pressures within the reservoir, and stratigraphic correlations.

The Arbuckle Group in Wilburton field consists of dolomite, calcareous dolomite and minor clastic-rich intervals, chert, and dolomitic limestones. The depositional environment was a low-energy, shallow, subtidal to intertidal setting resulting in deposition of a thick sequence of lime muds. Early diagenesis consisted of pervasive

dolomitization that created a dolomudstone with low (12%) intercrystalline porosity. Other types of secondary porosity (vugs, fractures, breccias) also are important to reservoir behavior.

The regionally extensive Middle Ordovician unconformity, which occurs at the top of the Arbuckle Group, exposed that carbonate surface to meteoric conditions that resulted in formation of karst. The porosity development or enhancement associated with karsting modified depositional textures and their related pore geometries. One such modification was the development of breccias within the Arbuckle caused by freshwater solution, weakening, and collapse of the overlying rock. The three types of breccias present in Wilburton field include in-situ, clast-supported and matrix-supported breccias. Following brecciation, burial cements (rhombic dolomite, baroque dolomite, coarsely crystalline calcite, and coarsely crystalline quartz) precipitated during and after hydrocarbon migration.

Stratigraphically, the Arbuckle section can be divided into two zones. An upper zone, 200-250 ft thick, is characterized by a lack of fracturing and brecciation, and by fluid flow mainly through the matrix or intercrystalline pore system. Porosity development in these intervals extends across the field. The lower zone is characterized by multiple intervals of fracturing, brecciation (all three types), and solution collapse. With the exception of a single matrix-supported breccia, none of these zones can be correlated across the field with any certainty. The Arbuckle is most likely productive where solution has enhanced intercrystalline, fracture, and breccia porosity, and burial cements have failed to completely fill pore spaces. We anticipate that porosity development in Arbuckle car onates in other areas is similarly controlled and should be productive.

AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)