Exploiting the Devonian Reservoir in Oates SW Area, Western Delaware Basin, Texas

Lee T. Billingsley

The Oates SW field area is located in southwestern Pecos County, Texas, and it is in the southwestern portion of the Delaware basin. The dominant producing reservoir in the area is simply called Devonian, but it is probably Devonian-aged chert in the Thirtyone Formation. Regional structure is NE dip into the basin with some NW-SE trending faults. The Oates SW area is flanked by large structural closures that have produced gas from the Devonian and adjacent reservoirs. These fields include: Perry Bass, 26 BCF from nine wells; Oates NE 266 BCF from 25 wells, and Pikes Peak, 48 BCF from eight wells.
       In contrast to surrounding fields, Oates SW consists of four small structural closures that vary in size from about 320 to 1280 acres. Abraxas acquired 3-D seismic data to refine the structural interpretation and guide potential horizontal well bores. Each closure has from one to three vertical wells, and the wells produced from 0.2 to 2.4 BCFG each from the Devonian chert. Production from each vertical well near the top of closures roughly correlates to Devonian reservoir quality determined from log analysis. However, a comparison between calculated original gas-in-place and actual production for each closure indicates a relatively low recovery factor. All the vertical wells exhibited high rates of water production late in their productive history.
       Abraxas has drilled three horizontal well bores within the Devonian chert on separate closures. Results span the spectrum of potential outcomes. The best well has produced at a constant rate of 8 MMCFD and the worst well only makes 150 MCFD. The third well is a re-entry of a vertical well, which had produced 1.8 BCFG. Abraxas drilled horizontally within the Devonian chert. Initially, the well produced 100% water, but gas rates eventually increased. Currently, the well produces at relatively constant rates of 700 MCFD and 4000 BWPD. The source of the water production is unknown. It could be from: 1) near well bore, but not from the Devonian interval, 2) micro-fractures within the Devonian, which are connected to deeper water sources like the Ellenburger, or 3) near well bore water coned upward during production form the vertical well bore.
       Detailed correlation of logs indicates an unconformity at the top of the Devonian chert. Consequently, the porous Devonian chert interval is thinner in structurally high wells. This interpretation may explain the variable reservoir quality of the three wells, based on the projected trajectory of the horizontal well bores.
       As with other horizontal Devonian chert fields, results are highly variable from well-to-well. Overall the economics of the play in Oates SW have been favorable, but much still needs to be learned in order to repeat the success.

AAPG Search and Discover Article #90065©2007 AAPG Southwest Section Meeting, Wichita Falls, Texas