Summary

Arbuckle strata account for about 40% of the produced oil and known reserves in Kansas. Much of the past production has come from the upper 25 feet of the Arbuckle in areas of 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 the facies and reservoir characteristics of the Arbuckle in Kansas has not been performed. Results from our initial study of six cores from several different regions in central Kansas suggest that Arbuckle reservoir characteristics are also strongly related to depositional facies, early diagenesis, and dolomitization. The development of brecciation, fracturing and dissolution related to the post-Arbuckle unconformity is variable and variously created or destroyed porosity.

Five main depositional facies, identified to date, account for more than 85% of the cored interval described in all 6 cores. Listed in approximate order of relative abundance, with the most abundant facies first, these include (1) clotted algal boundstone (2) laminite algal boundstones, (3) peloidal packstone-grainstone, (4) 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. Petrophysical properties of the facies at the core plug scale are generally controlled by matrix grain size. All lithologies exhibit increasing permeability with increasing porosity and can be characterized as lying along the same general porosity-permeability trend. Clotted algal boundstones, peloidal packstone-grainstone, wackestones, mudstones, and shale generally exhibit porosities less than 10% and absolute permeabilities below 0.1md and frequently below 0.01md. These facies also exhibit high to very high irreducible water saturations (Swi) and effective hydrocarbon permeabilities at Swi that are one to several orders of magnitude less than the absolute permeability values. In contrast, laminated algal boundstones with abundant grains exhibit porosities ranging from 10% to 30% and absolute permeabilities ranging generally from 0.1md to 1,500md. Irreducible water saturations within these facies are related to permeability but are generally low and effective hydrocarbon permeabilities range between approximately 30% and 100% of absolute permeabilities. Early chert replacement, early and late dolomitizatiou, early to late brecciation and fracturing have variously created or destroyed porosity. These processes, in conjunction with depositional facies, have created a vertical heterogeneity and resulted in complex compartmentalization within the Arbuckle Group strata.

Our initial studies on the six cores and reconnaissance of others suggest that the facies and paragenetic sequence of events described in this paper are characteristic of most Arbuckle strata in Kansas. Our continuing regional stratigraphic and sedimentologic study is providing a framework for Arbuckle strata, and detailed information on the relative importance of depositional facies, diagenesis and unconformity-related karst processes for controlling reservoir architecture in various structural settings. Integration of depositional facies and diagenetic features with petrophysical properties (including porosity, permeability, capillary pressure, electrical resistivity, and relative permeability) is allowing evaluation of the role that the various facies play within the total reservoir system. It is also resulting in quantitative translation of geologic models into numerical reservoir properties models for reservoir simulation and as an aid in determining production strategies.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah