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
Arbuckle Reservoirs in Central Kansas: Relative Importance of Depositional
Early Diagenesis and Unconformity Karst Processes on Reservoir Properties
Arbuckle strata account for about 40% of the produced oil and known reserves in Kansas. Much of the 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. This study is the first known detailed analysis of the facies and reservoir characteristics of the Arbuckle in Kansas. Our study of 12 cores from four counties in central Kansas suggests 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 highly 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 12 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. Early chert replacement, early and later dolomitization, 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 reconnaissance of about 20 other Arbuckle cores suggest that the facies and paragenetic sequence of events described in this paper are characteristic of most Arbuckle strata in Kansas. 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.
Arbuckle strata in Kansas are similar to equivalent strata in other states, in that they comprise shallow subtidal and peritidal carbonate facies and they have been overprinted by the post-Arbuckle (Sauk-Tippecanoe) unconformity. Arbuckle and equivalent Midcontinent reservoirs are generally considered to have favorable reservoir qualities directly related to structural highs that resulted from regional uplifts and prolonged subaerial exposure and development of karstic features. Because of the general acceptance of a fracture-controlled karst model, and because karst processes are concentrated in the upper 30 to 50 feet of the Arbuckle, historical drilling and completion by small operators has generally penetrated only the uppermost portion of much of the Arbuckle.
Despite a prolific production history, Arbuckle strata in Kansas have never been studied in detail, apart from earlier general descriptive studies. Our initial detailed studies and data from other studies on Arbuckle age-equivalent strata in other states (e.g. Knox and Ellenburger) suggest that factors such as depositional facies, intraformational events, and diagenetic processes (e.g. dolomitization) other than those related to the overlying unconformity may be more important than previously considered for controlling reservoir properties and architecture. Our objectives for the complete study are to produce a stratigraphic framework and detailed understanding of depositional and paragenetic history regionally in Kansas and to integrate this with detailed petrophysical analyses to better understand controls on reservoir characteristics of the Arbuckle Group in Kansas.
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Last updated November 2002