Montoya (Upper Ordovician) Core Description, Depositional Lithofacies, Diagenesis and Thin Section Petrography: Pure Resources, Inc., Dollarhide Unit 25-2-S, Dollarhide Field, Andrews County, West Texas

BEHNKEN, FRED H., FHB Stratigraphic Services P.O. Box 7824, Midland, Texas 79708-7824 [email protected]

This study reports on an Upper Ordovician to Lower Silurian core from the Pure Resources, Inc., Dollarhide Unit 25-2-, Andrews County, Texas. The core spans 8212-8545 ft. This paper focuses on the lowermost Fusselman to Montoya interval from 8400 - 8545 ft. Completed as a Montoya producer, the well had an initial production of 160 BOPD + 3100 BWPD in late 2000. The Montoya produced approximately 80,000 BO as of June 2003.

Depositional environment, karstification and reservoir character are described. The Montoya core from 8431 - 8545 feet is dominantly dolostone with a detrital, replacive chert interval and approximately 3 ft of limestone within two beds. Peritidal depositional environments dominate the upper Montoya recovered core, however, the lower core is a dominated by open-marine, ramp packstone and grainstone that include an ooid shoal complex from 8534.8 - 8514.3 ft. This interval has fair to good oomoldic porosity, largely unaffected by secondary diagenesis. This ooid shoal complex has significant lateral extent and serves as a prolific producer in other more recent completions.

Montoya thin section petrography indicates at least two generations of dolomitization following dissolution of unstable high-Mg calcite or aragonitic allochems. The latest dolomitization event precipitated pore-lining planar-e and poikilitic pore-occluding dolospar. This dolomitization event was accompanied, or followed closely, by precipitation of pore-bridging tabular to acicular anhydrite. Late generation calcispar reduced reservoir porosity and permeability by occluding moldic, vuggy and some fracture porosity.