Undiscovered Oil and Gas in the Mancos-Menefee and Todilto Systems, San Juan Basin

The U.S. Geological Survey has made a geology-based estimate of the undiscovered, technically recoverable oil and gas — conventional and continuous (unconventional) — in two total petroleum systems (TPS) of the San Juan Basin Province, New Mexico and Colorado: the Mancos-Menefee Composite TPS and the Todilto TPS beneath it. The USGS last assessed them in 2002, as part of a broad assessment of the basin's Jurassic and Cretaceous reservoirs.

The answer: a mean of 12 million barrels of oil and 27 trillion cubic feet of gas.

The petroleum systems

Todilto

The Middle Jurassic Todilto Limestone Member of the Wanakah Formation holds organic-rich limestone beds, laid down in a marine-lake setting and thermally mature in parts of the basin. Beneath it, the Entrada Sandstone is an extensive wind-blown sand deposit with dune topography. A little oil migrated down from the Todilto into the Entrada's dune crests, forming local conventional oil accumulations.

Mancos-Menefee

The system's Cretaceous reservoirs run from the base of the Dakota Sandstone to the lowest part of the Cliff House Sandstone of the Mesaverde Group, deposited as the Cretaceous Western Interior Seaway advanced and retreated. Its reservoirs:

1. Dakota Sandstone;
2. Gallup Sandstone;
3. Mancos Shale, with the Tocito Sandstone Lentil and El Vado Sandstone Member;
4. the Mesaverde Group.

• The Mancos Shale is a main source of the basin's oil and gas: marine shale with Type II organic matter, siltstone and fine sandstone, more than 2,000 feet thick in places. Shoreface sandstones in its upper part — the Tocito and El Vado — are important reservoirs.
• The overlying Menefee Formation of the Mesaverde Group — river-channel sandstones, carbonaceous shale and coal — may also have generated oil and gas.
• Both were buried deep enough to generate oil and gas from the Eocene to the Miocene; hydrocarbons from the Mancos, perhaps with some from the Menefee, moved into neighbouring reservoirs.

These rocks have mostly produced from vertical wells, often drawing on several zones at once; in recent years horizontal drilling has come in, especially in the Mancos Shale.

Assessment units

Units were drawn mainly by the extent of formations in the basin and differences in gas-to-oil ratios.

Figure 1. The Entrada Sandstone Conventional Oil AU, the Dakota-Lower Mancos Conventional Oil and Gas AU, and the Northeastern and Southwestern Dakota-Lower Mancos Continuous Gas AUs. Base map from the National Park Service.

Figure 2. The Gallup Sandstone Conventional Oil AU, the Upper Mancos Conventional Oil AU and the Upper Mancos Continuous Gas AU.

Figure 3. The Mesaverde Updip Conventional Oil AU (not assessed), the Northeastern and Southwestern Mesaverde Continuous Gas AUs and the Menefee Coalbed Gas AU.

Results

One unit, the Mesaverde Updip Conventional Oil AU, was defined but not quantitatively assessed. Estimates are fully risked; the probability column is each unit's assessment-unit probability as the fact sheet gives it.

F95 is the amount with a 95-percent chance of being there; F5, a 5-percent chance. The continuous units hold 26,909 billion cubic feet of the gas; the conventional units hold all the oil.

Sources

• Marra, K.R., Schenk, C.J., Mercier, T.J., Leathers-Miller, H.M., Tennyson, M.E., Finn, T.M., Woodall, C.A., Brownfield, M.E., Le, P.A., and Drake, R.M., II, 2020, Assessment of undiscovered oil and gas resources in the Mancos-Menefee Composite and underlying Todilto Total Petroleum Systems of the San Juan Basin Province, New Mexico and Colorado, 2020: U.S. Geological Survey Fact Sheet 2020–3049. https://pubs.usgs.gov/publication/fs20203049
• The maps and the per-unit results come from the fact sheet's PDF, which the web version lacks.
• Rewritten in hubnx's own words.