Continuous Gas in the North Caspian Basin: A USGS Assessment

U.S. Geological Survey Fact Sheet 2018–3051, National and Global Petroleum Assessment. By the North Caspian Basin Province Assessment Team: Christopher J. Schenk, Tracey J. Mercier, Thomas M. Finn, Marilyn E. Tennyson, Phuong A. Le, Michael E. Brownfield, Kristen R. Marra, Stephanie B. Gaswirth, Heidi M. Leathers-Miller and Ronald M. Drake II.

Using its geology-based method, the USGS estimates mean undiscovered, technically recoverable resources of 84.5 trillion cubic feet of continuous gas in the North Caspian Basin Province of Kazakhstan and Russia.

The North Caspian Basin Province (black) and the Paleozoic Subsalt Continuous Gas assessment unit (orange). Base map from the National Park Service. USGS.

One of the deepest basins on Earth

The basin holds 20 kilometres of mostly Paleozoic sediment. Its history is poorly known, because it is so deep and there are few data from its centre.

• Rifting may have begun as early as the Neoproterozoic, but most likely in the Ordovician, with the opening of the Uralian Ocean.
• Renewed subsidence from the Late Devonian to Early Permian, perhaps from back-arc extension, laid down hundreds of metres of organic-rich source rock in the deep basin, ringed by shallow-water carbonate platforms.
• In the Early Permian, terranes colliding along the southern margin partly cut the basin off, depositing up to 5 kilometres of evaporites — salt that partly seals the source rocks beneath.
• As the Uralian Ocean closed, from the late Carboniferous to the Triassic, the Ural fold belt rose and its foreland filled with kilometres of eroded sediment. That burial pushed the subsalt source rocks into the oil window, and in the deep centre into the gas window.

What was assessed

The USGS defined a Paleozoic Composite Total Petroleum System and, within it, a Paleozoic Subsalt Continuous Gas Assessment Unit.

• Source rocks. Deep-water, organic-rich shales of Late Devonian to Early Permian age — though geochemical data are few, since samples come only from the basin's edges. What data exist show Type II kerogen, total organic carbon up to 8 weight percent, hydrogen index up to 400 milligrams of hydrocarbon per gram of organic carbon, and thicknesses of up to several hundred metres. In the centre they are in the gas window below about 7,000 metres.
• Model. Oil generated as the source rocks were buried in the Permian–Triassic foredeep cracked to gas as subsidence continued in the deep, overpressured centre. The gas moved locally into low-permeability sandstones and shales and was partly held there, forming a regional, continuous gas accumulation.
• Uncertainty. Chiefly, how much gas stayed in the subsalt section after it formed and migrated.

Key inputs (minimum / most likely / maximum), based partly on U.S. shale-gas analogues:

Results

F95 is a 95 percent chance of at least the amount shown; F5 a 5 percent chance. Results are fully risked.

Sources

• Schenk, C.J., and others, 2018, Assessment of continuous gas resources of the North Caspian Basin Province, Kazakhstan and Russia, 2018: U.S. Geological Survey Fact Sheet 2018–3051. https://doi.org/10.3133/fs20183051 · https://pubs.usgs.gov/publication/fs20183051
• The map and the input and results tables come from the fact sheet's PDF.
• Ulmishek, G.F., 2001, Petroleum geology and resources of the North Caspian Basin, Kazakhstan and Russia: U.S. Geological Survey Bulletin 2201–B.
• Rewritten in hubnx's own words.