This discussion is presented in the belief that through its understanding similar fields may be discovered. Favorable economics exist because of shallow depths of the producing sandstone. Reliable field records, a high density of core analyses, and good mechanical logs are of considerable aid in establishing the geologic history of this stratigraphic trap with its hydrodynamic, faulting, and folding complications.

The field is located on the northwestern side of the San Juan basin. Although the Upper Cretaceous producing sandstone is present in Colorado, commercial production is found only in New Mexico. The discovery well, Little No. 2-27 Navajo, was completed in November,

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1962, at a depth of 1,240 ft. Cumulative production is more than 1,000,000 barrels and initial potentials have ranged up to 665 BOPD. Depth of the standstone ranges from 1,134 to 1,781 ft. and averages less than 1,500 ft. Dry-hole cost averages less than $5,500; completed wells cost $15,000-$20,000, and pay-out periods can be as short as 4 months. The productive sandstone ranges from 3 to 13 ft. in thickness, has a porosity of 9-24%, and permeability of 1-872 md. Original bottom-hole pressure was 465 psi.

The producing body is an elongate, northwest-trending marine sandstone accumulation of Niobraran age. It is 25 mi. long and averages 0.5 mi. in width. The sandstone was deposited in erosional troughs formed on the Niobrara-Carlile unconformity. The unconformity rises toward the southwest across the basin. Usually sandstones are found in troughs formed by longshore currents or at the base and seaward edges of cliffs at the unconformity. Detailed isopach mapping shows irregularities along the trough.

Subsequent Laramide folding and faulting and post-Laramide erosion have caused a variety of fluid accumulations within the stratigraphic trap. Zones of fresh water, oil and brackish to salt water, oil, and gas caps occur at different positions along the trend of the sandstone. In Colorado the sandstone dips southeast for 12 mi., from an outcrop elevation of 5,400 ft. to a deep syncline at the Colorado-New Mexico boundary where the oil-water contact is at 3,900 ft. Southeast of the syncline, the sandstone rises 400 ft. in 2½ mi. onto the faulted asymmetrical Blue Hills anticlinal nose and finally descends gradually southeastward into the San Juan basin. A normal fault that divides the sandstone into separate reservoirs is located almost 1 mi. northwest of and essentially parallel ith the axis of the plunging Blue Hills anticline. This fault causes the gas-oil contact to be 85 ft. higher on the upthrown (northwestern) side.

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