The Smackover Formation is one of the most heavily developed rock units in the U.S. Gulf Coast, with drilling and production going back to the 1920s. The U.S. Geological Survey has assessed how much undiscovered, technically recoverable conventional oil and gas it still holds across the onshore Gulf Coast basin and state waters of Texas, Louisiana, Arkansas, Mississippi, Alabama and Florida.
The five Smackover Formation assessment units. Image from the USGS fact sheet.
An ancient carbonate shelf
The Smackover was laid down in the Late Jurassic (Oxfordian), when the sea spread widely across the Gulf of Mexico Basin and built a vast carbonate shelf. Geologists recognize two informal parts:
- The lower Smackover, informally called the Brown Dense shale — organic-rich, limy shales resting on the Norphlet Formation. It is the source rock for oil and gas in many reservoirs above it.
- The upper Smackover — grain-rich limestones and packstones that serve as reservoirs.
Though mostly limestone, the formation includes sandstone in some landward areas — central Mississippi, southeastern Arkansas and northeastern Louisiana — carried in by rivers. How porous its carbonates are varies widely with their content of ooids and how much they have turned to dolomite. From east to west the petroleum system also changes with differences in heat flow, the type of organic matter, and thermochemical sulfate reduction, which affects whether hydrocarbons survive.
Drawing the boundaries
Parts of the formation are too hot or too highly pressured to drill with current equipment. Using published reports, operators’ information on drilling limits and three-dimensional basin modeling of burial history, USGS excluded areas beyond technically recoverable temperature and pressure limits — in one unit, the cutoff lines were 260°C or 25,000 pounds per square inch.
It then divided the formation into five assessment units, based on rock types, faults and salt basins, thermal maturity (whether rocks have generated oil or gas), and the influence of the ancestral Mississippi River delta, whose sands could add reservoirs but might also let oil and gas escape or dilute the source rock.
| Assessment unit | Setting |
|---|---|
| Western Updip and Peripheral Fault Zone | inner ramp and carbonate shelf; north of the 1.78% thermal-maturity line where oil cracks to gas |
| Eastern Updip and Peripheral Fault Zone | as above; the only unit where microbialites matter, mainly in southeastern Alabama |
| East-Central Interbedded Sand and Carbonate Shoal | between the two, where the ancestral Mississippi laid down a thick sandstone package |
| Peripheral Fault Zone and Carbonate Shelf | south of the 1.78% line, where gas is generated |
| Downdip Salt Basins | open shelf and outer platform; undrilled, with traps expected around salt structures and sandy debris flows |
What’s left
The mean estimates for all five units combined:
| Resource | Mean | Range (F95–F5) |
|---|---|---|
| Oil | 143 million barrels | 83–227 million barrels |
| Natural gas | 1,084 billion cubic feet | 393–2,317 billion cubic feet |
| Natural gas liquids | 72 million barrels | 26–154 million barrels |
F95 is a 95% chance of at least that amount; F5 a 5% chance.
The units differ sharply. The Eastern Updip unit holds the most oil (mean 76 million barrels) and the Western Updip unit the next most (51 million). The undrilled Downdip Salt Basins hold little oil but the most gas: a mean 414 billion cubic feet in gas accumulations.
Sources
Based on Birdwell JE, Whidden KJ, Paxton ST, et al. (USGS Smackover Formation Assessment Team), "Assessment of undiscovered, technically recoverable conventional oil and gas resources in the Upper Jurassic Smackover Formation, U.S. Gulf Coast region," U.S. Geological Survey Fact Sheet 2023–3046; a work of the United States government in the public domain. Map from the same fact sheet.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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