How much oil and gas is still to be found in one of the Gulf Coast's deep sandstones? In 2023 the U.S. Geological Survey re-assessed the Norphlet Formation under Mississippi, Alabama and Florida and put the undiscovered, technically recoverable amount at a mean of 16 million barrels of oil, 348 billion cubic feet of gas and 19 million barrels of natural gas liquids — well below earlier estimates.
What the Norphlet is
The Norphlet is Late Jurassic (Oxfordian) rock — sandstone, conglomerate and fine-grained sediments laid down on land. It lies 10,000–27,000 feet down, above ancient basement rock, the Eagle Mills Formation and the Louann Salt, and beneath the marine limestones of the Smackover Formation.
When it formed, the Gulf of Mexico was a young rift basin opening as Gondwana pulled away from the rest of Pangaea. Rivers wearing down the southern Appalachians dropped coarse alluvial fans on its northern shore; beyond them, a dry climate and the right winds built a great sand sea (erg) at the foot of the mountains, much like the dune fields of Namibia today. Salt flats and shore environments edged the erg on the south, against the restricted young Gulf. A rising sea eventually reworked the upper sands and buried them under the Smackover carbonates.
Why oil collects there
| Part of the system | In the Norphlet |
|---|---|
| Reservoir | the dune sandstones of the erg, and sands the sea reworked — from under 100 ft to over 700 ft thick. Early chlorite coatings on the grains kept later cement out and the pores open. The coarse fan deposits up-dip are not prospective |
| Source | the organic-rich shales of the overlying Smackover |
| Timing | generation began in the Early Cretaceous; the oil and gas moved soon after into porous, under-pressured sandstone |
| Traps | mostly salt: the Louann Salt rose and moved, bending the sands into closures, and fields line up along regional normal faults where salt-cored anticlines hold the petroleum |
| Type | a clear gradient — oil in the northeast, then natural gas liquids, then dry gas to the southwest, as the formation gets deeper and hotter |
The fairway is a northwest–southeast band across the eastern half of the Louisiana–Mississippi Salt Basins Province. West of the Mississippi River the equivalent rocks are muddy, poorly sorted and often under 100 ft thick, with no reported dune sands, so this assessment leaves them out — unlike earlier ones.
So far
| Wells drilled | up to 400 |
| Produced to date | about 66 million barrels of oil and 6 trillion cubic feet of gas |
| Offshore | oil found in federal waters in the 2000s has given the Norphlet its most productive wells — but there the sands sit on salt blocks that slid 25–30 km sideways, which does not happen onshore, so those finds do not change this model |
Earlier USGS assessments: 1995 split the Norphlet into five assessed units (plus a sixth, not quantified); 2011 estimated a mean 87 million barrels of oil and about 3.5 trillion cubic feet of gas.
Figure 1: the four assessment units, U.S. Gulf Coast. USGS
The four assessment units
| Unit | What it is | Assessed for |
|---|---|---|
| Updip Oil | the landward edge — mostly fan conglomerate with little reservoir quality; cleaner sands near its southwest boundary (the Flomaton gas field) and in small river channels (Excel, Excel North) | oil only |
| Mississippi Interior Salt Basin Oil and Gas | mobile salt that forms diapirs, stocks and "turtle" anticlines, all possible traps | oil and gas |
| Alabama-Florida Oil and Gas | between the oil and gas areas; salt-cored faulted anticlines, fewer than in Mississippi; includes Hatters Pond field (mostly NGL) | oil and gas |
| Alabama-Mississippi Deep Gas | small, mostly Baldwin and Mobile Counties and the state waters of Mobile Bay; the deepest wells, almost all dry gas, from dunes that sank into the salt | gas |
The estimate
Means, fully risked. The range (F95–F5) is the 95-percent to 5-percent chance of at least that much.
| Unit | Oil (MMBO) | Gas (BCFG) | NGL (MMBNGL) |
|---|---|---|---|
| Updip Oil | 3 | 1 | 0 |
| Mississippi Interior Salt Basin | 8 | 54 | 8 |
| Alabama-Florida | 5 | 31 | 11 |
| Alabama-Mississippi Deep Gas | — | 262 | 0 |
| Total | 16 (range 2–46) | 348 (range 53–1,076) | 19 (range 3–63) |
The drop from earlier estimates comes from a smaller assessed area and fewer, smaller expected fields in each unit. Most of the gas is expected in the deep Alabama–Mississippi unit.
Sources
Written from Assessment of Undiscovered Conventional Oil and Gas Resources in the Norphlet Formation, U.S. Gulf Coast Region, 2023, U.S. Geological Survey Fact Sheet 2024–3034, by the Norphlet Formation Assessment Team (John W. Counts, William H. Craddock, Jared T. Gooley, Marc Buursink, Tracey J. Mercier, Cheryl A. Woodall and Christopher J. Schenk); a work of the United States government in the public domain. The input data are in Counts (2024), USGS data release doi:10.5066/P132OJPA. The fact sheet cites, among others, Schenk and Viger (1995), Pearson (2011), Salvador (1987), Mancini and others (2003), Godo (2019), Hunt and others (2017), and Snedden and others (2021).
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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