of the outcrop section in the Sangre de Diablos to the west. Analyses of the 

 formation waters again suggested some flushing action in the shallow forma- 

 tions, although the deeper pre-Pennsylvanian rocks carried waters with high 

 salinities. Perhaps the most encouraging of all the factual data derived from 

 the drilling of this test was the presence of possible reef or near-reef facies in 

 a Devonian and Silurian dolomite section which carried faint oil staining in 

 cores and cuttings. A drill-stem test of this interval indicated excellent porosity 

 and permeability; and although it flowed water, there was a rainbow of oil on 

 the salt water. Amprex's geologists were encouraged, for it seemed quite pos- 

 sible that this location might be near the edge of an oil accumulation. 



After abandoning the No. 1-C, Amprex moved 35 miles east in the north- 

 eastern part of the basin near the northern end of the Excalon Hills and drilled 

 its fourth stratigraphic test, the No. 1-D. This test actually proved the existence 

 of a thick section of Carboniferous rocks in the Arenoso Basin, penetrating 

 more than 7000 feet of Pennsylvanian sands, shales, and dark limestones be- 

 neath a predominantly carbonate Permian section. At its total depth, 12,635 

 feet, this test was in Mississippian rocks. Numerous shows of oil and gas were 

 recorded by the mud logger and noted in the cuttings and cores, but most of 

 the sands and limestones were impermeable. The top of the Mississippian in this 

 test proved to be 8000 feet structurally lower than in the No. 1-C and 4000 

 feet lower than in the No. 1-A, Amprex's first test located more than 50 miles 

 to the south. 



Several significant features of the geology of the Arenoso Basin were 

 disclosed by the drilling of the test wells. The Pennsylvanian section was found 

 to thicken toward the axis of the basin and to exhibit a marked change of facies. 

 The carbonate rocks which characterized the outcropping Pennsylvanian sedi- 

 ments in the Sangre de Diablos and the sections penetrated in both the No. 1-B 

 and No. 1-C test wells did not exceed 600 feet in thickness; lithologic and faunal 

 aspects of these carbonates indicated shelf deposition. The thick section of 

 Pennsylvanian elastics near the basin axis, however, exhibited characteristics 

 of typical basinal deposits, including laminate bedding, restricted planktonic 

 faunas, and dark shales with disseminated pyrite. This change in facies denoting 

 distinct environmental conditions in the Arenoso Basin during Pennsylvanian 

 times, together with the westward convergence of the section, provided a basis 

 for reasoning that a structural hinge belt existed along the strike of the region. 

 Amprex's geologists realized that such a hinge belt is usually a flexure or fault 

 zone which marks the position where the basin actually begins to break or bend 

 down to accommodate an increased thickness of sediments. They knew that such 

 flexures characterize many of the major oil-producing basins of the world, pro- 

 viding favorable structures and reservoir conditions to form major producing 

 trends. They reasoned that a hinge belt should be present in the Arenoso Basin 

 in the zone of thickening where the Pennsylvanian facies changed from shelf 



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