--> ABSTRACT: Heterogeneous Carbonate Buildups in the Blanding Sub-Basin of the Paradox Basin, Utah and Colorado: Targets for Increased Oil Production Using Horizontal Drilling Techniques, by David E. Eby and Thomas C. Chidsey; #90906(2001)

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David E. Eby1, Thomas C. Chidsey2

(1) Eby Petrography & Consulting, Inc, Littleton, CO
(2) Utah Geological Survey, Salt Lake City, UT

ABSTRACT: Heterogeneous Carbonate Buildups in the Blanding Sub-Basin of the Paradox Basin, Utah and Colorado: Targets for Increased Oil Production Using Horizontal Drilling Techniques

Shallow-shelf carbonate reservoirs in the Pennsylvanian (Desmoinesian) Paradox Formation (primarily the Ismay and Desert Creek zones) have produced over 50 million bbls of oil in the Blanding sub-basin of the Paradox basin, Utah and Colorado. The Ismay zone is dominantly limestone comprising equant buildups of phylloid-algal material with locally variable small-scale subfacies. The Desert Creek zone is dominantly dolomite comprising regional nearshore shoreline trends with highly aligned, linear facies tracts. Various facies changes and extensive diagenesis have created complex reservoir heterogeneity within these two diverse zones.

Two representative fields with good conventional core coverage were selected for detailed study for potential horizontal infill drilling: Cherokee (Ismay) and Bug (Desert Creek) fields in southeastern Utah. Cherokee field is a phylloid-algal buildup capped by anhydrite that produces from porous algal limestones and dolomites. The net reservoir thickness is 27 feet over a 320-acre area; porosity averages 12% with 8 md of permeability in vuggy and intercrystalline pore systems. Bug field is an elongate, northwest-trending carbonate buildup. The producing units vary from porous dolomitized bafflestone to packstone and wackestone. The trapping mechanism is an updip porosity pinchout. The net reservoir thickness is 15 feet over a 2,600-acre area; porosity averages 11% with 25 to 30 md of permeability in moldic, vuggy, and intercrystalline networks.

The reservoir quality of these fields has been affected by multiple generations of dissolution, anhydrite plugging, and various types of cementation which act as barriers or baffles to fluid flow. Production may be increased through evaluation of conventional cores, reservoir modeling, and the application of horizontal (possibly multi-lateral) drilling from existing wells.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado