Depositional Setting and Reservoir Characteristics of Lower Queen (Permian, Guadalupian) Sandstones, Keystone (Colby) Field, Winkler County, Texas

VANDERHILL, JAMES B., Mobil Exploration and Producing U.S., Inc., Midland, TX

Keystone (Colby) field is located near the western margin of the Central Basin platform. The field produces oil and associated gas from the Colby Sandstone, an equivalent of the lower half of the Queen Formation (Permian, Guadalupian). A 12.5 mi2 area of the western part of the field was chosen for detailed stratigraphic and sedimentologic study. The Colby consists of interbedded porous reservoir sandstones, tight dolomites, and rare anhydrites. Sandstones are arkosic and very fine grained, with porosities ranging from 5-20%. Effective porosity is largely secondary and the result of dissolution of unstable grains, mainly feldspars. Volumetrically important cements include magnesite, dolomite, and kaolinite. Sedimentary structures are indistinct, consisting mainly of discontinuous, sub orizontal, wavy laminations interpreted to be the result of bioturbation. Most Colby dolomites are wackestones containing peloids, forams, and calcispheres in a dolomicrospar matrix. Abundant cryptalgal laminations and the restricted fauna indicate deposition in shallow subtidal and intertidal settings. Anhydrite beds have nodular "chicken-wire" fabrics commonly associated with sabkha deposition. They are confined to the upper part of the Colby and decrease in number to the west. Comparisons between core lithologies and wireline logs show that sandstones confidently can be identified on sonic logs as those intervals with traveltimes greater than 50 micro sec/ft. Individual sandstones have sheet-like geometries and can be identified throughout the study area. Sandstones generally are not ssociated directly with anhydrite beds. The upper Queen and overlying Seven Rivers formations are dominated by interbedded dolomite and nodular anhydrite with rare sandstones. The association of bioturbated sandstones with shoreline dolomites but not with sabkha anhydrites suggests a shallow-marine rather than eolian sandsheet origin.

AAPG Search and Discovery Article #91011©1991 AAPG Southwest Section Meeting, Abilene, Texas, February 9-12, 1991 (2009)