Arbuckle Reservoirs in Central Kansas: Relative Importance of Depositional Facies, Early Diagenesis and Unconformity Karst Processes on Reservoir Properties

Kansas Geological Survey Open-file Report 98-55

Discussion and Conclusions

Although production strategies typically have been based on karst controlled models associated with the post-Arbuckle (post-Sauk) unconformity, our results from detailed study of 12 cores and preliminary analysis of 20 others in Kansas indicate that matrix properties unrelated to the unconformity are significant and may be the dominant control on reservoir properties and architecture in some reservoirs and in deeper portions of the Arbuckle. The relative lack of karst associated fracture, breccia, and dissolution porosity in the 12 cores was surprising, especially considering that the cores came from flanks or tops of structural highs where karst processes would likely have been most extensive. In fact, only one core, the #11-05 Trusedell from central Rice County showed consistent features suggesting karst influence.

Arbuckle strata in Kansas comprise original shallow subtidal to peritidal carbonate facies that have been overprinted by pervasive, but mostly non-fabric destructive dolomitization(s) and later diagenetic processes. Much of the matrix porosity (intercrystalline, moldic, fenestral, vuggy) is associated with coarse-grained, laminated to bedded facies that are differentially cemented or with algal (stromatolitic) intervals that show differential porosity development likely due to differences in original texture (e.g. mud content) and early diagenesis (e.g. development of fenestral and vuggy porosity during early subaerial exposure events). As evidenced by oil stains, it is apparent that these intervals are significant in their potential for storage of fluids.

Petrophysical properties of the facies at the core plug scale are generally controlled by matrix grain size. Each lithology exhibits a generally unique range of petrophysical properties modified by the presence of fractures, vuggy porosity, and grain size variation within the lithologic class. Petrophysical properties for facies that are a composite of lithologies are scale-dependent and are a function of the proportions and architecture within the facies. All lithologies exhibit increasing permeability with increasing porosity and can be characterized as lying along the same general porosity-permeability trend. Variance in permeability for any given porosity is approximately two orders of magnitude and may be primarily attributed to the influence of several lithologic variables.

The regional stratigraphic and sedimentologic framework for Arbuckle strata is providing an understanding of the relative importance of depositional facies, diagenesis, and unconformity-related karst processes for controlling reservoir architecture and properties in various structural settings. Integrated with quantitative petrophysical data for individual depositional facies and diagenetic features, the comprehensive framework will provide: (1) a predictive capability for identifying favorable reservoir facies that intersect the post Arbuckle (post-Sauk) unconformity on structural highs; (2) improved ability to identify additional horizons deeper within the Arbuckle that have favorable reservoir potential; (3) quantitative data that can guide production strategies and determine if zones are best produced with vertical, horizontal, or targeted infill drilling; and (4) quantitative data that can be used in reservoir simulators to aid in determining production strategies.

Acknowledgments

The following members of the Kansas Geological Survey provided valuable assistance and advice with various aspects of this study: Scott Beaty, Tim Carr, John Charlton, John Doventon, Steve Franklin, Lee Gerhard, Paul Gerlach, Roxanne Gissler, Willard (Bill) Guy, Mike Magnuson, Alex Martinez, David Newell, and Lynn Watney.

Robert Dougherty of Great Bend, Kansas provided well logs and descriptions from his work with the #1 Madsen, in Rice County and Paul Ramondetta provided digital well logs and advice and regarding his work with the El Dorado Field in Butler, County, Kansas. Kathy Edwards digitized the logs. In addition, Robert F. (Bob) Walters published three excellent papers on the Arbuckle in Kansas:

Buried Precambrian hills in northeastern Barton County, central Kansas, 1946, American Association of Petroleum Geologists, Bulletin, v. 30, p. 660-710.

Oil Production from fractured Precambrian basement rocks in central Kansas, 1953, American Association of Petroleum Geologists, Bulletin, v. 37, p. 300-313.

Differential entrapment of oil and gas in Arbuckle dolomite of central Kansas, 1958, American Association of Petroleum Geologists, Bulletin, v. 42, p. 2,133-2,173.

Steinhauff and Franseen are grateful to Bob Walters for meeting with them last fall to share his knowledge of the Arbuckle. Bob passed away last month. He will be missed.

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