ABSTRACT

Cheyenne field, Winkler County, Texas, produces oil from shelf-margin lithofacies of the Tansill Formation of late Guadalupian age. The Tansill Formation encountered in the Cheyenne field compares favorably with basin-margin outcrops found in the Guadalupe Mountains of New Mexico and Texas. The Delaware basin margin consisted of (1) outer shelf ooid shoals and back-shoal green-algal flats, (2) a shelf-crest pisoid-tepee complex, and (3) inner shelf evaporitic, stromatolitic, and peloidal sediments during the late Guadalupian.

The Cheyenne field reservoir exhibits varied lithologies and porosites in the Tansill Formation. Fresh-water phreatic diagenesis created moldic porosity in the outer shelf, green-algal lithofacies. Fresh-water vadose conditions existed during low sea level stands preserving intergranular porosity in the pisolitic lithofacies of the shelf-crest tepee complex. Fresh-water vadose diagenesis also helped form burrow-moldic porosity and fenestral porosity in the inner shelf peloidal lithofacies. Periodic hypersaline vadose conditions occluded some porosity with evaporites and aragonitic cements. Dolomitization of the Tansill Formation occurred in the mixing zone between a seasonal fresh-water lens in the shelf-crest sediments and the saline waters of the Permian sea and inner shelf lagoon. As sea level fluctuated the mixing zone migrated laterally. In this way, much of the Tansill Formation was dolomitized, forming finely crystalline dolomite that preserved textural details well but did little to enhance porosity.

Porosity is best developed in the thin beds of the outer shelf, green-algal lithofacies. More importantly, the 194 ft (59 m) of pay in the Cheyenne field results from favorable diagenesis of lithofacies on the shelf margin that were interbedded with the shelfcrest lithofacies by fluctuations of sea level. The arid climate at the time of deposition was essential in maintaining porosity created by early diagenesis and for the absence of abundant vadose cements in the Tansill Formation. The trap in Cheyenne field is depositionally controlled by updip, nonporous, and impermeable sediments of the inner shelf lagoon. Anhydrites of the overlying Salado Formation provide an excellent seal. Interbedded lithologies of the Cheyenne field average 8% porosity and 1 md permeability. The field has recovered 499,950 bbls of oil as of May 1983, with estimated total recoverable reserves of 1,744,987 bbls of oil.

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