Abstract

This study reports on a conventional core spanning the Upper Ordovician lowermost Fusselman Formation and the Upper Ordovician Montoya Group from the Pure Resources, Inc., Dollarhide Unit 25-2-S in the northern structural block of Dollarhide Field, Andrews County, Texas. The entire core spans 8212-8545 ft. The present study is exclusively focused on the lowermost Fusselman to Montoya interval from 8400 ft to 8545 ft. The well was completed in the Montoya Group from 8516-8546 ft with an initial porential of 160 BOPD + 3100 BWPD as of late 2000. The Montoya produced approximately 80,000 BO as of June 2003. This completion in a mature field raises the Montoya as a viable, economic completion or reentry target.

Depositional environment, diagenetic sequence, karstification and reservoir quality are described over the Montoya core. The recovered core from 8431 ft to 8545 feet is dominantly dolostone with a distinctive detrital and replacive chert interval and approximately 3 ft of limestone within two thin beds. Peritidal depositional environments predominant in the upper Montoya recovered core, but the stratigraphically lower portion of the core is an open-marine ramp packstone and grainstone that has reservoir quality porosity and permeability. This lower portion of the core is a progradational low to -high energy ramp that includes an ooid shoal complex from 8534.8 ft to 8514.3 ft. This interval is characterized by fair to good oomoldic porosity which did not undergo extensive porosity reductions by secondary diagenesis.

Thin section petrography indicates that the diagenetic sequence includes at least two generations of dolomitization with dissolution of unstable high-Mg calcite or aragonitic allochems. The latest dolomitization event formed planar-e pore-lining dolospar and poikilitic pore-occluding dolospar. This latest dolomitization event was accompanied by, or at least 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.