Reservoir Characterization of the Drinkard Formation of the Justis Tubb-Drinkard Field, Lea County, New Mexico: A Valuable Tool in Optimizing Field Development
Cory L. Hoffman, Ph.D.
ChevronTexaco, Midland, Texas
The Drinkard formation of the Justis Tubb-Drinkard field in Lea County, New Mexico, consists of cyclic, shallow-water carbonates that developed on the western edge of the Central Basin Platform. On the ChevronTexaco operated leases in this field, an anticline with four-way closure provides the trapping mechanism for the Drinkard reservoir.
The early Permian (Leonardian) Tubb and Drinkard formations have been produced in this field since 1961 with a cumulative production of over 3.2 MMBOE. Over forty years later, the field is still on primary production and is producing 490 BOEPD. Over the past decade, Justis Tubb-Drinkard field development has consisted of an aggressive 20 acre infill drilling program, and more recently, less expensive horizontal re-entries.
These horizontal re-entries have been drilled at various structural positions and at different orientations throughout the field. However, the initial production from each lateral has been unpredictable, ranging from a disappointing 10 BOPD to an economically attractive 80 BOPD, despite targeting zones exhibiting similar porosity development. In order to improve the economics of the horizontal drilling program, it was decided to perform a reservoir characterization study of the Drinkard formation to (1) identify specific zones to target within the Drinkard formation, (2) optimize well path design, and (3) determine the best part of the field to concentrate our drilling efforts.
In 2001, Texaco Exploration and Production, Inc. cored the lower Drinkard formation in the GL Erwin ‘A’ Federal #9, the GL Erwin ‘B’ Federal NCT-2 #11, and the CC Fristoe ‘B’ Federal NCT-2 #26 wells. Of the three cored wells, the GL Erwin ‘A’ Federal #9 is at the lowest structural position and the most basinward, approximately 3000’ west of the other two cored wells.
Each core consists of ~130’ of continuous section of the lower Drinkard with a few feet of the underlying (uppermost) Abo. This interval is equivalent to the published high frequency sequences (HFS) within the Lower Clearfork of HFS L1.6 (Abo), HFS L2.1 (Drinkard) and the base of HFS L2.2 (Drinkard). Each core exhibits 40-45 upward-shoaling cycles, which range in thickness from 1-6’ (sixth-order cycles). Many of the cycles in these shallow-water platform carbonates contain basal organic-rich, silty mudstones overlain by mud- to grain-dominated carbonates capped by tidal-flats.
Two lithologies are seen in the Drinkard Formation – limestone and dolostone. The proportion of limestone within the Drinkard increases away from the structural high. Interestingly, the best reservoir rock (highest porosity and permeability) is not the dolostone as one might expect, but rather the (grain-dominated) limestone.
Core data showed that the interval from the top of the Abo (HFS L1.6) to the HFS L2.1 maximum flooding surface (MFS) consists primarily of low effective porosity, dolomitized, lagoonal mudstones and wackestones, and porous but impermeable cryptalgal laminated boundstones (tidal-flats) throughout the field – eliminating this entire interval as a reservoir target. From the HFS L2.1 MFS to the top of HFS L2.1, the best reservoir rock consists of grain-dominated (limestone) packstones and high energy (limestone) grainstones near the structural high. Although grain-dominated packstones are seen downdip within this interval, they tended to be dolomitized and of lower permeability. At the base of HFS L2.2, open marine (limestone) ooid packstones and high energy (limestone) ooid grainstones form the best reservoir rocks, but are far downdip from the structural high.
Although reservoir characterization of the Justis Tubb-Drinkard field is still ongoing, the three major questions (above) have already been answered to a great degree: (1) based on the structural position of the well and sequence stratigraphic context, only specific porous intervals ranging in thickness from 15’ to 45’ comprise the best reservoir targets for horizontal wells, (2) due to the strong correlation between depositional facies and reservoir quality, the best orientation for horizontal wells is parallel or slightly oblique to the trend of the strike-oriented grainstone bodies, and (3) the almost ubiquitous presence of impermeable mud-rich lagoonal and supratidal facies along the structural high throughout the deposition of the Drinkard Formation effectively removes one-third of the acreage on the eastern portion of the field from consideration for horizontal drilling.
These three findings have explained the production anomalies exhibited by the horizontal wells, and have caused a radical shift in the field development strategy for the Justis Tubb-Drinkard field. Prior to this study, horizontal wells targeted the 100’ thick porosity interval at the base of the Drinkard – meaning that at least 50% (up to 85%) of the length of the horizontal well was drilled in non-reservoir rock. Additionally, the orientation of these horizontal wells was constrained by lease lines, not depositional trends, and as such many horizontals did not stay within the high-quality reservoir rock. This study also explained the poor production seen in the eastern portion of the field where high porosity (yet impermeable) ‘reservoirs’ were targeted on the structural high. Future horizontal wells will target the western side of the field away from the structural high, be oriented sub-parallel to the strike-oriented grainstone bodies, and will only target the 15-45’ thick porous intervals containing a high degree of effective porosity.