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The Upper Mancos Shale in the San Juan Basin: Three Oil and Gas Plays, Conventional and Unconventional


The Mancos Shale (Upper Cretaceous) covers approximately 12,000 mi2 in the San Juan Basin of northwestern New Mexico and southwestern Colorado. The Mancos is subdivided into two formations, the Upper Mancos Shale and the Lower Mancos Shale. The Upper Mancos is 900 to 1,600 ft thick in the San Juan Basin. The Upper Mancos has been productive of oil and natural gas from sandstones and shales.

The Upper Mancos Shale contains three plays: the Tocito marine bar play, the Naturally fractured Mancos shale play, and the Offshore Mancos shale play. The Tocito play is a conventional oil play productive from sandstones in the lower part of the Upper Mancos. Reservoirs are northwest-southeast trending shoestring sandstones on the southwestern flank of the basin.

The Naturally fractured Mancos shale play is located along the southeastern and northwestern flanks of the basin where Laramide tectonic uplift that formed the present-day basin outline initiated fracturing of the more brittle lithologies within the Upper Mancos. Open fractures formed prolific reservoirs produced by vertical wells.

The Offshore Mancos shale play is located northeast of, or paleo-offshore of, the Tocito marine bars. This is the modern unconventional play. The play extends north from the Tocito marine bars into the basin axis. Reservoir intervals are organic-rich marine shales with laminations and very thin beds of very fine-grained sandstones. Percentage and thickness of sandstone beds decrease to the northeast with increasing distance from the paleoshoreline. Recent exploration is within the Mancos C zone, which constitutes the lowermost 75 to 470 ft of the Upper Mancos Shale. The Mancos C thickens to the northeast. Upper Mancos shales are both source rocks and reservoirs in the Offshore Mancos shale play. Along the southwestern flank of the basin, shales within the Mancos C are thermally immature. Peak oil generation in the Mancos C was attained along a trend just northeast of the Tocito marine bar reservoirs. The dry gas window is present in the deeper northern part of the basin. TOC of Mancos C shales ranges from 0.5 to 3.2%, averaging 1.8%. Kerogens are a mixture of oil prone, gas prone, and inertinitic types with oil-prone types dominant. Production has been marginally economic oil and gas from scattered historic vertical wells. Most recent exploratory wells have been drilled horizontally with production exceeding that attained in vertical wells by an average factor of 9.4.