Arbuckle strata account for about 40% of the produced oil and known reserves in Kansas. Much past production has come from the upper 25 feet of the Arbuckle in areas with structural highs and regional uplifts related to basement structural elements that were enhanced by karstic processes related to the overlying unconformity. A detailed analysis of facies and reservoir characteristics of the Arbuckle has not been performed. Results from our study of more than a dozen cores from several regions in central Kansas suggest that Arbuckle reservoir characteristics are strongly related to depositional facies, early diagenesis; and dolomitization. Development of brecciation, fracturing and dissolution related to the post-Arbuckle unconformity is variable and alternately created or destroyed porosity.

Five main dep6sitionalfacies account for more than 85% of the cored intervals described from eleven cores. Listed in order of decreasing relative abundance, these include (1) clotted algal boundstone, (2) laminite algal boundstone, (3) peloidal packstone-grainstone and (5) wackestone-mudstone. Intraclastic conglomerate and breccia, cave fill shale, depositional shale, and chert account for the remaining 15% of the total rock. Matrix and grain size are the controlling petrophysical properties of the facies at the core plug scale. All lithologies exhibit increasing permeability with increasing porosity and can be

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characterized as lying along the same general porosity-permeability trend. Clotted algal boundstones, pelodial packstone-grainstone, wackestones, mudstones, and shale generally have porosities of less than 10%. Absolute permeabilities are generally below 0.1 md and frequently below 0.01 md. The facies also exhibit high to very high irreducible water saturations (Swi), and effective hydrocarbon permeabilities at Swi are one several orders of magnitude less than the absolute permeability values.

In contrast, laminated algal boundstones with abundant grains have porosities ranging from 10% to 30% and absolute permeabilities ranging generally from 0.1 md to 1,500 md. Irreducible water saturations in these facies are related to permeability but are generally low. Effective hydrocarbon permeabilities range between 30% and 100% of absolute permeabilities. Early chert replacement, early and late dolomitization, and early to late brecciation and fracturing have variously created and destroyed porosity. These processes, in conjunction with depositional facie haw created a vertical herogeneity resulting in complex compartmentalization of Arbuckle Group strata.

While much attention has been directed previously at karst features, results from this study indicate that matrix properties also play an important and even dominant role in some reservoirs. Arbuckle strata have experienced pervasive but mostly non-fabric destructive dolomitization(s), allowing inference of depositional facies and the paragenetic sequence of events that affected these strata. The striking feature in the cores is the abundance (>50%) of matrix porosity (intercrystalline, moldic, fenestral, vuggy) throughout the length of the cores related to depositional facies, early diagenesis, and dolomitization, unrelated to the upper post-Sauk subaerial exposure surface.

The facies and paragenentic sequence of events described here are characteristic of most Arbuckle strata in Kansas. Although production strategies typically have been based on karst-controlled models associated with the post-Sauk unconformity, study of a dozen cores and reconnaissance of others in Kansas indicate that matrix properties unrelated to the unconformity are significant and may be the dominant control on reservoir properties and architecture. The relative lack of karst-associated fracture, breccia, and dissolution porosity in the cores was surprising, especially considering that the cores come from the flanks or tops of structural highs where karst processes would likely have been most extensive.

Arbuckle strata in Kansas comprise original shallow water subtidal to peritidal carbonate facies that have been overprinted by pervasive, but mostly non-fabric destructive dolomitization(s) and late diagenesis. Much of the matrix porosity (intercystalline, 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). As evidenced by oil stains and (in many cases) a prolific production history, the intervals apparently are significant in their potential as reservoirs.

The regional stratigraphic and sedimentological framework of 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-Sauk unconformity on structural highs; (2) improved ability to identify additional horizons deeper in 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 target infill drilling; and (4) quantitative data that can be used in reservoir simulations to aid in determining production strategies.

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