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The San Juan basin of New Mexico and Colorado is an asymmetrical basin formed during the Late Cretaceous and early Tertiary. The structurally deepest part of the basin is near the Colorado-New Mexico border. Presently, the only applications for the incentive price for gas produced from tight sandstones involve reservoirs of Cretaceous age. These reservoir rocks were deposited in open shelf to lower delta-plain environments associated with the Western Interior Cretaceous sea which transgressed and regressed the area in a general southwest-northeast direction. Coastal-barrier sandstones are significant gas producers and typically exhibit a strong marine influence. These sandstones are represented by long, linear northwest-southeast reservoir trends.
The tight gas applications for the San Juan basin, to date, concern reservoirs which can be placed into a spectrum with two end-member types. One type is characterized by continuation of mappable sand trends which extend from economic producing areas. Even though reservoirs of this type generally look similar to those in economic wells, they require production testing to determine their economic potential. This unpredictability is caused by the presence or absence of natural fractures. The other type of tight gas reservoir is related to the thinning of the good reservoir section or the increase in content of silt and clay-size material within the productive interval. This type of reservoir looks poor on well logs because of interbedding of siltstones and shales with thin, good reservo r sandstones and/or the presence of considerable silt and clay in the sandstones themselves.
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