Undiscovered Gas in the Deep Tuscaloosa Sandstones of Southern Louisiana

The U.S. Geological Survey (USGS) has made a geology-based assessment of undiscovered, technically recoverable conventional gas in Upper Cretaceous Tuscaloosa Group sandstones lying deeper than 19,000 feet beneath southern Louisiana, in the Western Gulf Basin Province. The mean estimate is 14,785 billion cubic feet of gas.

A single assessment unit (AU), the Deep Tuscaloosa Conventional Gas AU, was defined within the Upper Jurassic–Cretaceous–Tertiary Composite Total Petroleum System. It lies basinward of, and is independent from, the Downdip Tuscaloosa Conventional Gas AU assessed in 2007: that assessment covered shallower Tuscaloosa sandstones in traps formed mainly by growth faults, a different geologic model, so the two units do not overlap and this is not an update of the earlier work.

The Deep Tuscaloosa Conventional Gas assessment unit in the Western Gulf Basin Province. Base map from the U.S. Department of the Interior, National Park Service. Credit: U.S. Geological Survey.

How the Tuscaloosa formed

Early in the Late Cretaceous, a relative fall in sea level, probably tied to local uplift, increased erosion across the northern Gulf Coast and carved the mid-Cretaceous unconformity. As the shoreline moved seaward, river and delta sediments built out over newly exposed, carbonate-dominated Lower Cretaceous rocks. These sandstones and shales, with muds laid down when sea level was relatively higher, make up the Tuscaloosa Group, found across much of the eastern Gulf Coast in settings from rivers on land to the slope and basin floor. In southern Louisiana its sandstone is thickest in the centre of the state, near what may have been the ancestral Mississippi River.

The geologic model

Research indicates reservoir quality, source rock maturity and migration are all adequate for gas to occur in the unit. The gas below roughly 19,000 feet is dry and cannot be tied to a source rock by established methods; shallower Tuscaloosa gas, landward in central Louisiana and western Mississippi, comes from the Tuscaloosa marine shale, which rests on the productive sandstones.

The key questions are whether gas has been retained, and how many traps there are and how big. The traps are presumed to be tied to structures made by the Jurassic Louann Salt, in place or pushed out. Salt movement creates structures — keels, welds, diapirs and counter-regional dipping feeders — that bound mini-basins of sediment. These are clear in offshore seismic data, which are far better than onshore data, but the limited onshore data show similar deep, salt-related structures. The spacing of these structures governed the estimated number of gas fields, and field sizes were constrained by known Tuscaloosa production within and outside the unit and by regional analogues.

The reservoirs range from distal delta to channelised and confined intraslope settings. Well logs show 10 feet to more than 650 feet of porous sandstone, thickest along the delta axis in central Louisiana. Porosity averages 20 percent and permeability 100 millidarcies — unusually high at these depths, thanks to chlorite coatings on the sand grains that hold back the growth of quartz cement.

Where the unit's boundaries lie

The unit's northern boundary meets the southern boundary of the 2007 unit. Its eastern and southern boundaries run to the edge of state waters, since Tuscaloosa sandstones were deposited across the east of the unit and fed sediment to the eastern Mississippi Canyon offshore. In southwestern Louisiana the sandstones are thin to absent, so the western boundary follows where sandstone is known, from drill cuttings or interpreted from well logs and seismic data; most of southwestern Louisiana is outside the unit.

Results

The fully risked estimate for the one unit is a mean of 14,785 billion cubic feet of gas, with a median (F50) of 13,353. The F95 estimate — a 95 percent chance of at least that much — is 5,432 billion cubic feet, and the F5 estimate 29,104. No natural gas liquids are expected.

Sources

Based on "Assessment of Undiscovered Conventional Gas Resources in the Deep Tuscaloosa Group Sandstones of the Western Gulf Basin Province, U.S. Gulf Coast Region, 2021," by the USGS Tuscaloosa Group Assessment Team (Matthew D. Merrill and others), U.S. Geological Survey Fact Sheet 2022–3067 (September 2022), published by the U.S. Geological Survey; rewritten in hubnx's own words.

• The map comes from the fact sheet's PDF: https://pubs.usgs.gov/fs/2022/3067/fs20223067.pdf
• The fact sheet draws on Pitman and others (2007), Galloway (2008), Snedden and others (2016), Hackley and others (2020), Dutton and others (2018) and other studies.