Reservoir Facies, Pore Characteristics, and Flow Units: Lower Permian Chase Group, Guymon-Hugoton Field, Oklahoma
Authors: Siemers W. T. , Ahr W. M. 
Published: December 2007

Abstract
The Panhandle-Hugoton field in southwestern Kansas and the panhandles of Oklahoma and Texas is the largest gas field in North America. The field was discovered decades ago; consequently, most published geological studies of the reservoir are limited to generalized descriptions of regional stratigraphy and structure and do not incorporate more recent reservoir techniques. This paper presents a synthesis of detailed petrologic studies of cores and well log data and provides: (1) a discussion of the sedimentologic and diagenetic histories of the Chase Group and their control on the character of the Guymon-Hugoton reservoir, (2) evidence that the Guymon-Hugoton reservoir is comprised of laterally continuous flow units separated by equally continuous barrier layers, and (3) conclusive evidence that the layered, non cross -flow Guymon-Hugoton reservoir can be drained effectively by the present 640-acre well spacing pattern.
The Chase Group in the Guymon-Hugoton field, consists of interlayered carbonates, siliciclastics, and evaporites deposited in repeated, regionally continuous, shallowing upward sequences on a gently dipping, low-relief, shallow-marine ramp. The repeated sequences consist of successively shallower water, marine carbonates (chiefly dolostones) and siliciclastic reservoir rocks capped by shaly redbeds and paleosols. Reservoir pore types, which include intergranular, moldic, intercrystalline, and solution enhanced forms of these types, were formed diagenetically by alteration that occurred soon after deposition and was controlled indirectly by the cyclical, shallowing-upward pattern of sedimentation. Percent porosity and pore character vary between, and to some extent within, carbonate layers, depending on their depositional and diagenetic histories. The pore types within all carbonate layers, however, are interconnected by a well-developed intercrystalline pore network that formed as a result of pervasive, area-wide dolomitization. Interlayered redbeds and paleosols that mark the end of each depositional cycle are composed of siliciclastic mudstones, shales, and argillaceous carbonates which, because of their areal continuity, high threshold entry pressures, and low permeabilities, act as impermeable barriers to vertical fluid flow.
Because of (1) the continuity of the interlayered dolostone and shale intervals, (2) the areally continuous intercrystalline pore network within each dolostone layer, and (3) the isolation of the dolostone layers by the impermeable barrier units, each dolostone layer is a laterally continuous, well-drained flow unit. Independent engineering studies and production data demonstrate that individual flow units (1) have different permeabilities and gas volumes, (2) are not in pressure communication, and (3) exhibit different depletion characteristics. Moreover, replacement well studies show that no virgin pressures have been found in the Guymon-Hugoton field by recent drilling, indicating that the current 640-acre well spacing pattern effectively drains the reservoir. Production and engineering data, therefore, are good indicators of the multilayer, non cross-flow character of the reservoir and are shown here to be direct results of the depositional and diagenetic processes which characterized the Guymon-Hugoton region during Early Permian time.