Reservoir Diagenesis and Porosity Development in the Upper Ismay Zone,

Pennsylvanian Paradox Formation, Cherokee Field, Southeastern Utah

By

Thomas C. Chidsey

Utah Geological Survey, Salt Lake City, UT

David E. Eby

Eby Petrography & Consulting, Inc, Littleton, CO

The Pennsylvanian (Desmoinesian) Paradox Formation has produced over 50 MMBO in the Blanding sub-basin of the Paradox Basin, Utah and Colorado. The Ismay zone within the Paradox Formation is dominantly limestone comprising equant buildups of phylloid-algal material with locally variable small-scale calcarenite subfacies. Various facies changes and extensive diagenesis have created complex reservoir heterogeneity in Ismay reservoirs. Cherokee field in southeastern Utah produces from an upper Ismay phylloid-algal mound and associated grainstones composed of porous limestone as well as dolomite. The net reservoir thickness is 27 ft over a 320-ac area; porosity averages 12% with 8 md of permeability. Discovered in 1987, the field has produced over 181,000 BO and 3.6 BCFG.

The limestone-mound facies consist of skeletal phylloid-algal bafflestone with anhydrite plugging early pore space. Associated calcarenite facies are composed of skeletal grainstone, with primary interparticle and intraparticle porosity, and early moldic porosity. Some mixing-zone (fresh and salt water) dolomite and dog-tooth spar (meteoric cement) are present. Off-mound facies typically consist of dolomitic packstone/wackestone with peloids, crinoids, and bryozoans. Early dolomitization, late solution-enlarged channels, and anhydrite and bitumen plugging are common.

The typical sequence of diagenetic events consists of: (1) early dolomitization by hypersaline or mixing zone brines, (2) styolitization, (3) late dissolution/micropores, (4) anhydrite replacement, and (5) bitumen plugging. The most significant and unique diagenetic characteristic is extensive microporosity. The intense microporosity likely developed late, along solution fronts by the action of aggressive hydrothermal solutions from depth. Microporosity represents an important site for untapped hydrocarbons and possible targets for horizontal drilling in Cherokee and other Ismay-zone fields in the Paradox Basin.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming