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AAPG Bulletin

Abstract


Volume: 31 (1947)

Issue: 4. (April)

First Page: 731

Last Page: 771

Title: Reservoir Characteristics of Rattlesnake Oil and Gas Field, San Juan County, New Mexico

Author(s): H. H. Hinson (2)

Abstract:

The Rattlesnake field is in the Navajo Indian Reservation about 7 miles southwest of Shiprock, in San Juan County, northwestern New Mexico. The field is near the northwest edge of the San Juan Basin.

The thickness of the sedimentary rocks penetrated by wells in the field is approximately 7,500 feet. The Mancos shale of Upper Cretaceous age is at the surface and the Ignacio quartzite of Cambrian age is the oldest formation reached. The Dakota sandstone of Upper Cretaceous age and the Hermosa formation of Pennsylvanian age contain oil pools at depths of approximately 800 and 6,700 feet, respectively. The Ouray-Leadville limestone of Mississippian and Devonian age contains a pool of helium-bearing natural gas at a depth of about 6,950 feet. This natural gas is exceptional in that it contains about 73 per cent nitrogen and 7.6 per cent helium.

The Rattlesnake structure is a northwest-southeast trending anticline, having a series of minor highs along its axis. The anticline has greater structural relief with depths, and the lower beds thin over the top of the structure. The dip of the older rocks is much greater on the west flank of the structure than on the east flank. The axial plane of the anticline is progressively farther west with depth, and the apex of the anticline is progressively farther south with depth.

Oil was discovered in the Dakota sand in February, 1924. To the end of 1945, a total of 4,321,753 barrels of oil had been produced from the Dakota. This oil may have a gravity as high as 76° API at the well, but quickly weathers to a gravity of about 60° to 64° API. In all, 115 wells have been drilled for production from the Dakota sandstone. Oil has been found in each of the three sandstone members of the formation.

A total of 489,563 barrels of oil was produced from the Hermosa formation by two wells. The discovery well was completed in June, 1929. The last oil was produced from the Hermosa formation in March, 1940. The gravity of the oil was about 40° API. The oil was found in the lower part of the formation, and data indicate that the production from the two wells was from two different porous zones.

Helium-bearing natural gas was first found in the Ouray-Leadville formation in June, 1942, but the first gas well was not completed until May, 1943. Both solution porosity and porosity in the interstices between coarse dolomite crystals have been found in this formation. Because of the high helium content of the gas, this gas pool is being held as a helium reserve by the Bureau of Mines, United States Department of the Interior.

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