Chert Reservoir Development in the Devonian Thirtyone Formation: Three Bar Field, West Texas

Chert reservoirs of the Lower Devonian Thirtyone Formation represent a substantial part of the hydrocarbon resource in the Permian Basin. More than 700 million barrels of oil has been produced from these rocks, and an equivalent amount of mobile oil remains. Effective exploitation of this sizable remaining resource, however, demands a comprehensive understanding of the complex factors that have contributed to reservoir development. Analysis of Thirtyone Formation chert deposits in Three Bar field and elsewhere in the Permian Basin reveals that reservoirs display substantial heterogeneity that results from an interplay of depositional, diagenetic, and structural processes. Large-scale reservoir geometries and finer scale, intrareservoir heterogeneity are attributable primarily to original depositional processes. These cherts, which were deposited by relatively deep water sediment gravity processes along a north-trending depocenter, exhibit relatively continuous stratal geometries, especially in the area of Three Bar field, which contains one of the largest of these reservoirs. Porosity development in these rocks is due principally to variations in rates and end products of early silica diagenesis. Because this diagenesis was partly a function of depositional facies architecture, porosity development follows original depositional patterns.In reservoirs such as Three Bar field, where the Thirtyone Formation has been unroofed by Pennsylvanian deformation, meteoric diagenesis has added heterogeneity by causing dissolution of chert and carbonate, especially in areas of higher density fracturing and faulting and along truncated reservoir margins. Structural deformation has also controlled heterogeneity directly, particularly in reservoirs under waterflood. High-density fracture zones create preferred flow paths that result in nonuniform sweep through the reservoir. Faulting locally creates fault compartments by offsetting reservoir flow units.Three Bar field exhibits many of the major styles of heterogeneity that contribute to inefficient recovery in the Thirtyone Formation. As such, the processes and models defined here help explain causes of heterogeneity in all Thirtyone chert reservoirs in the Permian Basin and facilitate recovery of the sizable hydrocarbon resource remaining in these rocks.
Stephen C. Ruppel
Susan D. Hovorka

Ruppel, S. C., and Hovorka, S. D., 1995, Chert Reservoir Development in the Devonian Thirtyone Formation: Three Bar Field, West Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 230, 50 p.

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The University of Texas at Austin, Bureau of Economic Geology
Report of Investigation