Abstract

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 belt around the flanks of the basin, the Mancos dips into the subsurface of the basin. The basin is structurally asymmetric with a gentle southern flank and a steep northern flank. Depth to the top of the Mancos exceeds 6,500 ft along the basin axis.

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 has been productive of oil and natural gas from both conventional and unconventional reservoirs. The Upper Mancos Shale is 900 to 1,600 ft thick in the San Juan Basin. It is thinnest along the southwestern flank of the basin and gradually thickens to the northeast.

There are three 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 play productive of oil and associated gas from sandstones in the lowermost part of the Upper Mancos Shale. The reservoirs are northwest-southeast trending shoestring sandstones on the southern flank of the basin that were deposited offshore of and parallel to the paleoshoreline. Most of the reservoirs were discovered in the 1950’s.

The Naturally fractured Mancos shale play is a conventional play located along the southeastern and northwestern flanks of the basin. In these areas, Laramide tectonic uplift that formed the present-day basin outline initiated fracturing of the more brittle lithologies within the Upper Mancos Shale. The open fractures resulted in prolifically productive reservoirs that have been exploited by vertical wells.

The Offshore Mancos shale play is located northeast of the Tocito marine bars. This is the modern unconventional play within the basin. This play extends northeast from the Tocito marine bars into the axial part of the basin. Almost all production and recent exploration is within the lowermost 400 ft of the Upper Mancos Shale (the Mancos C interval). Reservoir intervals are organic-rich marine shales with laminations and very thin beds of very finegrained sandstones and siltstones. Percentage and thickness of sandstone beds decreases to the northeast with increasing distance from the paleoshoreline. Most current and recent exploratory wells have been drilled horizontally and stimulated with multi-stage hydraulic fracturing.

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 Upper Mancos shales are thermally immature. Peak oil generation was attained along a trend just northeast of the Tocito marine bar reservoirs. The transition from the oil window to the wet-gas window occurs 8 to 10 miles northeast of the Tocito marine bar reservoirs. The dry gas window is present in the northern part of the basin. Total organic carbon (TOC) content of Mancos C shales ranges from 0.5 to 3.2%. Oil-prone kerogens are dominant in the majority of samples.