The San Juan Basin is a large depressed region in northwestern New Mexico bordered on the west by the Defiance Uplift, and the Chuska Mountains, on the north by the San Juan dome, on the south by the Chaco slope and the Zuni Uplift, and on the east by the Naciemento uplift. Cretaceous and Tertiary sedimentary rocks bow down in the San Juan basin into a large, shallow sag approximately 100 miles across.
The sediments that fill the basin were derived from the San Juan Mountains to the north and the southern tip of the Rockies. Both of these mountain ranges were new in Tertiary times and were just beginning to erode. The basin began forming even as the mountains rose and the sediments from that period fill it to a depth of 7000 feet. These strata are easily seen in the canyons cut by drainages to the San Juan River to the north.
The sedimentary rocks that fill this basin contain both source rocks and natural reservoirs for oil and gas, both formed by slow decomposition of plant and animal material within the source rocks. The resevoir rocks of the San Juan basin are Cretaceous and Tertiary sandstones. The San Juan basin gas field is marked by many well sites. The gas is found several thousand feet below the surface, naturally pressurized, eliminating the need for pumping. The gas field extends over a wide area. Well sites in canyons are favored, as the natural erosion has already done some preliminary "drilling" for the wells.
The San Juan Basin was an active seaway connecting the central New Mexico sea with the Paradox Basin in Utah during most of pre-Permian time.
There is a page on gas and oil resources in the San Juan Basin available from the USGS server.
If, like me, you are confused by the terms used to designate geologic time, a chart, based upon one given in Roadside Geology of Arizona by H. Chronic is available.
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