Abstract

The Keystone (Colby) reservoir, equivalent to the Queen Formation of Permian (Guadalupian) age, is located in the northwestern part of the Central Basin Platform of the Permian basin, approximately 10 mi from the platform margin. The trapping mechanism is a combination of structure, northwest-southeast-trending anticline, and depositional facies changes. The producing interval is approximately 300 ft thick and occurs at a depth of 3,500 ft.

The reservoir is composed of porous, very fine-grained arkosic sandstones interbedded with generally low-porosity dolomite and anhydritic dolomite. Sandstones are interpreted to have been delivered to the shelf margin by eolian transport and reworked in shallow-water marine to peritidal environments. Sandstones deposited in the shallow-water marine environment are fine-grained and massive, whereas sandstones deposited in the peritidal environment have a higher clay content and abundant anhydrite nodules. The peritidal sandstones commonly are in gradational lateral contact with nonporous nodular anhydrite beds. Interbedded shallow-water marine and peritidal dolomite is generally finely crystalline mudstone with abundant anhydrite nodules and cements. Some of the carbonate beds contain karst breccias and erosion surfaces. These lithologic features, and the absence of fossils, indicate that the shelf was an intermittently exposed hypersaline environment. These facies are arranged in vertically stacked, upward-shoaling parasequences.

The Keystone (Colby) reservoir is vertically divided into five sandstone-dominated units within a 16-mi² study area. Isopach maps of each of these five units, and completion interval data, indicate that large areas of thick sandstones are not open to well bores. Conservative estimates based on porosities measured in cores and estimates of net pay thickness and saturations indicate that more than 15 MMSTB of mobile oil are not accessed by existing well bores in the study area.