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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. Each closure has from one to three vertical wells, and the wells produced from 0.2 to 2.4 BCFG each from the Devonian.  Production characteristics from each vertical well near the top of closures 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 re-entered one of the vertical wells, which had produced 1.8 BCFG, and drilled a horizontal well within the Devonian interval. Initially, the well produced 100% water, but gas rates began to increase. Currently, the well produces at a relatively constant rate of 800 MCFD and 4500 BWPD. The source of the water production is unknown. It could be from: 1) near well bore and out of 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.


On a separate structural closure identified by interpretation of 3-D seismic data, Abraxas drilled a new horizontal well with 1800 ft of lateral displacement in the Devonian. Previous vertical wells on the same closure produced only 0.5 and 0.7 BCFG form the Devonian. The new well penetrated the Devonian about 70 and 170 ft high to the two previous vertical wells. The new well is producing at relatively constant rates of 8 MMCFD and 12 BWPD. Reservoir modeling is in progress, but the well’s estimated ultimate recovery should be several times that of vertical wells in the Oates SW area.