Off the Beaten Path: An Update on the Mancos Shale Oil Window

In an era when industry attention is concentrated on the Permian and other major basins, the Mancos Shale oil window in New Mexico remains an overlooked but active contributor. Though small in scale and currently focused on a single primary landing zone, the play continues to deliver consistent well results with some variability that warrants monitoring. This article offers a status update on recent development trends and observed performance.

Geologic Setting

The Mancos Shale is a Late Cretaceous marine unit deposited in the Western Interior Seaway. In the San Juan Basin of northwestern New Mexico, it lies between the Gallup Sandstone and the overlying Mesaverde Group and includes multiple interbedded shales, siltstones, and fine sandstones. While the Mancos has generated gas across a broader area, the oil window is relatively limited, located along the basin’s southern flank.

The most active development is occurring in the Upper Mancos, particularly in the interval commonly referred to as the Mancos C. Within that, the C2 bench is the dominant target. TOC values typically range from 1% to 3%, and thermal maturity in this area places it within the window for liquid hydrocarbons.

Development History

Horizontal development in the oil-prone portion of the Mancos began around 2012 and averaged about 30 wells per year for much of the following decade. Most early wells featured one-mile laterals. Drilling activity slowed in 2020, 2021, and 2023, but rebounded in 2022 and 2024, when annual well counts reached approximately 50. Based on the current pace, 2025 is likely to finish in a similar range.

Lateral lengths have increased in recent years, with most new wells extending 1.5 to 2.0 miles. Despite advances in completion design, production performance across vintages has remained relatively consistent, without a clear step change over time.

Production Performance

Our review of approximately 300 wells with at least 12 months of production history shows an average recovery of 47 barrels of oil per foot of lateral. However, performance varies widely, The P10/P90 is greater than 5, illustrating the impact of geologic variability.

Our play-wide type curve indicates an average IP of 73 bopd/1000 feet of lateral with an initial secant decline of 75% and a b-factor of 1.0 to 1.1. Most wells drilled to date have nominal well spacing in the range of 2 to 3 wells per section, though some wells have been developed on tighter spacing (in our methodology, nominal well spacing is determined by proximity of the nearest offsets, not the actual number of wells in a drilling unit). At well densities up to 6 WPS, we observed only modest degradation in performance; the sample size is small, however.

A Niche Role in the Unconventional Landscape

The Mancos Shale oil window is not positioned to rival the Permian in size or pace, nor does it offer multiple stacked landing zones. But it continues to provide attractive well results on average and illustrates how some operators are successfully developing unconventional resources outside of the headline plays.

References and Further Reading

1.      Geology and Oil and Gas Assessment of the Mancos-Menefee Composite Total Petroleum System, San Juan Basin, New Mexico and Colorado

2.      Mancos Shale Play in New Mexico

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

4.      The Offshore Mancos Play in the San Juan Basin: Productive Carrier Beds within the Mancos Petroleum System

5.      LOGOS Energy Announces Record-Breaking Production From Mancos Shale Well

6.      Mancos Shale: Great Prospects Still Ahead

7.      Description and Comprehensive Analysis of the Mancos Shale, Uinta Basin, U.S.A.

8.      Mancos Shale, Uinta Basin, Utah: Resource Potential and Best Practices for an Emerging Shale Gas Play

Testimony Before the U.S. House Committee on Natural Resources Regarding the Mancos Shale and Navajo Allottees