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The AAPG/Datapages Combined Publications Database
In the early 1970s, Chevron and Amoco began investigating the oil and gas potential of Tertiary basins in western Nevada. Reconnaissance geologic studies focused interest on the large area of the Fallon basin with its numerous reported hydrocarbon shows. The two companies acquired leases and jointly ran seismic and gravity surveys in the northern part of the basin. At a location based on survey results, the Standard-Amoco 1 S.P. Land Co. well was drilled to 11,000 ft (3,353 m) as a stratigraphic test of the Tertiary section.
The oldest rocks in outcrop around the basin are a 12,000 ft (3,658 m) thick section of Upper Triassic to Middle Jurassic marine siltstones and shales interbedded with lesser amounts of sandstone, limestone, and conglomerate. The Mesozoic rocks are intruded by an Upper Jurassic gabbroic lopolith and Cretaceous and Tertiary granitic plutons. An 8,300-ft (2,530 m) thick section of Tertiary volcanic rocks and nonmarine sediments overlies the Mesozoic rocks in outcrop. The Tertiary section is divided into a lower volcanic member, a middle fluviolacustrine and volcanic-derived sedimentary member, and an overlying "capping basalt" unit.
Seismic data show that in the subsurface the northern Fallon basin is bisected by a northerly trending subsurface high. The maximum subsurface section of Tertiary to Recent sediments and volcanic rocks is 6,000 ft (1,829 m) thick west of the structural high and is over 13,000 ft (3,902 m) thick east of the high.
The Standard-Amoco 1 S.P. Land Co. well penetrated highly organic playa-lake sediments from the surface to 6,900 ft (2,103 m). From this depth to 11,000 ft (3,353 m) T.D., the well penetrated subsurface equivalents of the Tertiary outcrop section and bottomed near the base of the lower volcanic member. Oil and gas shows including free oil in vugs at the top of a basalt core at 8,168 ft (2,490 m) were present in the well, but results of formation tests of selected intervals showed that reservoir rocks were absent.
The results of the exploration work show that (1) the northern Fallon basin contains a large volume of highly organic oil-prone source rocks, (2) subsurface temperatures in these rocks are too low to generate significant amounts of oil, and (3) extensional faulting and the formation of basin and range structure over a broad area of western Nevada have occurred in the last 4 to 6 m.y.
The period of marked extension in western Nevada and probably of the basin and range as a whole is approximately time-coincident with the late Neogene offset of the San Andreas, and with the development of most of the oil-producing structures of the west-side San Joaquin Valley and of the Santa Barbara Channel-Santa Clara trough.
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