A review of the thermal maturity and hydrocarbon potential of the Mancos and Lewis shales in parts of New Mexico, USA

This study reviews specific public geochemical data (mainly TOC and Rock-Eval Pyrolysis parameters) of the Mancos Shale and the Lewis Shale located in an area to the south of the San Juan Basin, New Mexico. Both shale formations are immature within the study area proper and surrounding acreage. Maturity increases in a southeastern direction within the study area and is the only location where the Mancos Shale has thermogenic gas generating potential. The quality of the organic matter (kerogen) reported for the Mancos and the Lewis shales is mostly of Type III (gas-prone) with Type II being less dominant. The thicknesses of both the Mancos and Lewis shales are adequate to generate and store large volumes of natural gas in the thermally mature areas. The study area is also faulted, which may provide opportunities for natural fracturing to have formed. Furthermore, silt and sand layers within the Mancos Shale could provide permeability pathways for the gas, in a similar fashion to the Lewis Shale in the San Juan Basin. The part of the study area that belongs to the Albuquerque Basin contains thick sections of the Mancos Shale but at great depths (> 13,000 ft or > 4 km). However, it might be possible to find prospective areas near the margins of the basin, particularly if there is an active hydrologic system that operates close to the shale that may be responsible for late-stage biogenic gas generation.

► The Mancos in the study area is immature. Maturity increases to the southeast.
► The quality of the organic matter (kerogen) is mostly of type lll (gas-prone).
► Shale thickness is adequate to generate large volumes of gas in mature areas.
► The sandy/silty character of the Mancos could provide permeability pathways.
► Active hydrologic system at the basin margin could help biogenic gas generation.