Oil and gas exploration wells in the Pedregosa Basin
— Thompson, Sam, III, Tovar R., J. C., and J. N. Conley

The area of interest for this paper covers about 110,70C km² (49,500 mil) in the Basin and Range province from Tuc. son, Arizona, to El Paso, Texas (fig. 1). Figure 2 shows 3E petroleum-exploration wells which have penetrated Paleozoic or Precambrian rocks or both in the Pedregosa basic, on the Burro and associated uplifts, and in the southwestern part of the Orogrande basin. Five additional wells which penetrated only Mesozoic rocks are shown in Mexico.

This region may be called the Cuatro Fronteras, as it includes four major political boundaries—the international boundary between the United States and Mexico, and three boundaries between states, Arizona-New Mexico, New Mexico-Texas, and Sonora-Chihuahua. Any regional study of stratigraphic or structural geology, and especially an evaluation of petroleum potential, requires an exchange of surface and sub-surface data across these boundaries. We are indebted to many workers in these areas for supplying us with essential informtion; only some of them are acknowledged specifically in this brief paper.

The inter-regional relationships of the Pedregosa, Orogrande and Permian basins during the Paleozoic are discussed by Greenwood and others (1977). For each major stratigraphic unit an isopach map was given, and the general source and reservoir quality in the frontier basins was compared to that of the correlative unit in the productive Permian basin of south-eastern New Mexico and western Texas.

Table 1 presents a generalized correlation of stratigraphic units in the Pedregosa basin and adjoining areas, an updated but still preliminary evaluation of the petroleum source and reservoir qualities, and the current ranking of exploration objectives. Most of the major objectives are in the Paleozoic, which is characterized by shallow-marine carbonate deposits. The best reservoirs observed so far are in the shelf-margin dolostones of the upper Horquilla (Hidalgo Co., N.M.), the shelf dolostones of the Fusselman (Luna Co., N.M.), the shelf dolostones and sandstones of the Martin and equivalents (Cochise Co., Ariz.), and the shelf dolostones of the El Paso, Concha and Epitaph (New Mexico and Chihuahua). The best source rocks documented so far are in the dark mudstones of the Percha, Paradise and upper Horquilla (basin facies).

Triassic rocks are found in the extreme western part of the Pedregosa area, and Jurassic rocks are found in the eastern part (Chihuahua trough), but both generally are absent over the central part. The Lower Cretaceous Bisbee Group is thick and widespread over most of the area, yet it is absent on the Burro and related uplifts and is not found to the north. Rudist-limestone buildups of the Mura1=U-Bar have the best reservoir potential in the Mesozoic of the Pedregosa area; however, good permeability has not been observed, and the relatively small size of the buildups make them difficult exploration targets. In some areas the Cintura=Mojado sandstones locally exhibit good permeability, but an effective seal and associated source facies are lacking. The Upper (to Lower?) Cretaceous Bear- tooth sandstones and the overlying Colorado shales have simi-lar reservoir-to-source and seal relationships as the productive Dakota-Mancos units of the San Juan Basin; however, marine Upper Cretaceous rocks have not been found south of the Burro and related uplifts in this region.

Laramide and later tectonic and igneous events have had both positive and negative effects on possible accumulations of petrolum in the older rocks (Thompson, 1976). Thermal-alteration indices (based on kerogen and conodont studies) are in the overmature to metamorphosed range near plutonic in-trusive complexes but are in the mature range (where oil may be preserved) a few thousand feet away (Thompson and others, 1977). However, these indications of higher paleo-temperatures, and the measurements of high heat-flow at many localities by Reiter and others (1975), suggest that the potential for dry gas is better than that for oil in the Pedregosa basin. Basin and Range deformation was the most negative event, as extensional faulting disrupted regional traps and caused segments to be uplifted into horsts where flushing by fresh meteoric water is evident (Thompson and Bieberman, 1975). Therefore, the best potential for petroleum exploration lies in the deep grabens beneath the bolson (intermontane) valleys where preservation of oil and gas is more likely.