New Mexico Geological Society Annual Spring Meeting
April 7, 2017

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

Ronald F Broadhead

New Mexico Bureau of Geology & Mineral Resources, 801 Leroy Place, Socorro, NM, 87801,

The Mancos Shale (Upper Cretaceous) covers approximately 12,000 mi2 in the San Juan Basin of northwestern New Mexico and southwestern Colorado. From its outcrop around the basin flanks, the Mancos dips into the subsurface of the basin. The northwest-southeast trending axis is located in the northern part of the basin where depth to top of the Mancos exceeds 6,500 ft.

The Mancos is subdivided into two formations, the Upper Mancos Shale and the Lower Mancos Shale which are separated by an unconformity. The Upper Mancos Shale 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 from shales.

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

The Naturally fractured Mancos shale play is located along the southeastern and northwestern flanks of the basin. In these areas, Laramide tectonic uplift that formed the present-day basin outline turned strata upward and initiated fracturing of the more brittle lithologies within the Upper Mancos. Open fractures formed prolifically productive 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 within the Upper Mancos Shale. This play extends northward from the Tocito marine bars into the axial part of the basin. Reservoir intervals are organic-rich marine shales with laminations and very thin beds of very fine-grained sandstones and siltstones. Percentage and thickness of sandstone beds decrease to the northeast with increasing distance from the paleoshoreline. Almost all recent exploration is within the Mancos C zone, which constitutes the lowermost part of the Upper Mancos Shale. The Mancos C is 75 to 470 ft thick. The Mancos C is thinnest along the southwestern flank of the basin and thickens to the northeast.

The Upper Mancos shales are both the source rocks and the reservoirs in the Offshore Mancos shale play. Along the southwestern flank of the basin, the 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. Total organic carbon (TOC) of Mancos C shales ranges from 0.5 to 3.2% and averages 1.8%. Kerogens are a mixture of oil prone, gas prone, and inertinitic types with oil-prone kerogens dominant.

Shales of the Offshore Mancos shale play have produced subeconomic to marginally economic volumes of oil and gas from scattered historic vertical wells. Most recent exploratory wells have been drilled horizontally with production exceeding that attained in vertical wells.

2017 New Mexico Geological Society Annual Spring Meeting
April 7, 2017, Macey Center, New Mexico Tech campus, Socorro, NM