Abstract

Mississippian strata of southeastern New Mexico are Kinderhookian to Chesterian in age (Figures 1, 2). In Eddy, Lea, and southern Chaves Counties, depth to the top of the Mississippian ranges from 5,500 ft in the northwest to 17,000 ft in the southeast. Lower Mississippian (Kinderhookian and Osagian) strata are 0 to 800 ft thick (Figure 3) and are comprised of marine limestones and minor shales and chert. Upper Mississippian (Meramecian and Chesterian) strata are 0 to 600 ft thick (Figure 4) and are comprised of shallow marine limestones and shales. Within the Upper Mississippian section, there is a sharp transition from shelf deposits dominated by limestones in the north and the basinal Barnett Shale in the south (Figure 5). On the northern shelf, Upper Mississippian strata are truncated to the north under the uniformity at the base of the Pennsylvanian (Figure 6) The Mississippian carbonates constitute one of the least developed and perhaps least-well understood targets in southeastern New Mexico and have yielded a cumulative 47 BCF gas and 1.4 MMBO from 45 reservoirs in Eddy, Lea, and southern Chaves Counties (Figure 8).

Four plays are identified in the Mississippian carbonates of southeastern New Mexico: (1) Chester shallow marine limestones in the Tatum Basin on the northern shelf; (2) Meramec shallow marine limestones in the Tatum Basin on the northern shelf; (3) Upper Mississippian limestones interbedded with Barnett shales just south of the shelf-basin transition; and (4) small and widely disseminated reservoirs in the Lower Mississippian limestone, a low-permeability carbonate play. Chester shallow marine limestones have produced a cumulative 34 BCF gas and 391 MBO from 15 reservoirs (Figure 9), with 50% of the gas produced from the Austin reservoir alone (see Figure 8 for location of the Austin reservoir). This play occurs in the Tatum Basin where Upper Chesterian strata were preserved prior to deposition of the Lower Pennsylvanian section. Porosity in most reservoirs is less than 10% but well logs mostly indicate good reservoir permeability (Figure 10). Reservoir thickness ranges from 5 to 60 ft and depth to production ranges from 11,000 t 13,500 ft. Completion techniques range from natural completions to acidization to artificial fracturing. This is the best developed of the Mississippian plays; most reservoirs have 2 or 3 productive wells.

Meramec shallow marine limestones have produced a cumulative 3.1 BCF gas and 86 MBO from 3 reservoirs (Figure 11). The play occurs in the Tatum Basin. Porosity varies from 4 to 12% and well logs indicate mostly good reservoir permeability (Figure 12). Reservoir thickness ranges from 10 to 80 ft and depth to production ranges from 11,100 to 12,500 ft. Completion techniques range from acid jobs to artificial fracturing.

Upper Mississippian limestones interbedded with the Barnett Shale occur just south of the Upper Mississippian shelf margin (Figure 13). This play has produced 3.6 BCF gas and 48 MBO from 9 reservoirs. Porosity is less than 6%. Well logs indicate permeability is variable, ranging from almost nil to high, but mostly appears to be low (Figure 14). Reservoir thickness varies from 30 to 40 ft and depth to production ranges from 9,700 to 12,700 ft. Completion techniques range from natural, unstimulated completions to acidization. Most reservoirs have 1 or 2 productive wells with development limited.

The Lower Mississippian limestone play has produced 3.8 BCF gas and 881 MBO from 14 reservoirs (Figure 15). Porosity of reservoirs is low, generally ranging from 4 to 9%. Well logs indicate permeability is low (Figure 16). Reservoir thickness ranges from 10 to 40 ft and depth to production ranges from 6,800 to 13,300 ft. Completion techniques include natural completions as well as acidization and artificial fracturing. Most reservoirs have only one producing zone. For most reservoirs, the Lower Mississippian limestone discovery was a bailout zone for unsuccessful wells that targeted deeper strata or an uphole completion in a well that abandoned production in deeper zones. The discovery well is rarely offset.