How the States Rate
Proved Reserves - Southeastern
US as of 2001
Source:US Energy Information Administration's US
Crude Oil, Natural Gas, and Natural Gas Liquids Reserves, November 2001
The productive Illinois Basin, deepest in south central Illinois, continues into western Kentucky and extreme north-central Tennessee. Oil and gas have been produced from Ordovician through Pennsylvanian-age strata in the Illinois Basin. This area produces mainly from Pennsylvanian and Mississippian sandstones and porous Mississippian limestones. The Pennsylvanian reservoirs are mainly sandstones that formed after tectonic forces uplifted the Illinois Basin during the Mississipian, after which the basin once again began to subside. During this time, rivers and streams poured into the basin, and channel sandstones and other river and shallow sea sediments were deposited. These shallow sea deposits make great reservoir rocks, as they often produce highly porous sandstones. Anticlines and pinchouts are the rule for trapping hydrocarbons in this part of the Illinois Basin.
The Florida peninsula has produced oil since1943, though in very small quantities, from the Cretaceous Sunniland Formation at depths of 11,000 to 13,000 feet (3350 to 3960 meters). Many of these carbonate rocks would act as high quality reservoir rocks, but there are very few source rocks, and therefore, nothing to supply these reservoirs with oil. Small amounts of oil are still being produced today in a belt of porous Cretaceous limestones from west of Miami to near Fort Myers, but are marginally profitable.
The Black Warrior Basin occupies northwestern Alabama and northeastern Mississippi. The first petroleum discovery in the region was made in 1909, when natural gas was struck in rocks of Pennsylvanian age in northwestern Alabama. Most of these traps are structural in nature, and contain both oil and natural gas.
While the oil and gas fields of the Appalachian Basin are mostly "mature" (major new discoveries are generally a thing of the past), the Gulf Coast of Mississippi, Alabama and far western Florida continues to have new discoveries. Here, reworked sediments from the current and ancestral Mississippi River dip into the Gulf Basin, with younger sediments overlying Mesozoic sediments. These sediments stack like shingles on a roof onto the Continental Shelf. In very deep sections of the subsurface, Jurassic salt deposits may have at one time been more than a mile (1.6 km) thick. Locally thick occurrences of this salt are found in the Mississippi Interior Salt Basin, which extends from central Louisiana through central Mississippi and southwestern Alabama. Salt is less dense than the rocks which surround it. With heat and pressure that results from burial at great depths, salt can "bubble up" towards the surface. This migration can bend the rocks above, forming anticlines, which may act as hydrocarbon traps. This is the case in the South Carlton field in southwestern Alabama and dozens of fields in central Mississippi, with oil and gas production mostly in Cretaceous sandstones. In the case of extreme salt movement, these salt flows can actually break through the rocks above. Known as "piercement domes", these occurrences are more frequent as one heads south and west into the coastal salt dome province of Louisiana, but they also occur in the Mississippi Interior Salt Basin.
On top of this salt rests sand which formed during a time of high aridity in the region during the Jurassic period. Wind blown dunes in this sand reach heights of up to 800 feet (~250 meters), and can be up to 8 miles (~13 km) long, bringing to mind the conditions seen today in the Sahara Desert of North Africa. This sand, known as the Norphlet Formation, is generally 15,000 feet (4,575 meters) below the surface, and has been a very productive reservoir rock in areas of central and southern Mississippi and Alabama. Immediately above this formation is the Smackover Limestone, a very productive Jurassic-age reservoir rock located in the subsurface from Texas and Arkansas all the way east to Alabama. Another important reservoir here is the Wilcox Formation, a sandstone which was deposited in the ancestral Gulf of Mexico during the Eocene when the shoreline was much further inland than today. The Wilcox is a known reservoir rock throughout much of the Gulf Coastal Plain, extending parallel to the coastline into southern Texas.
Even further south, exploration wells are now being drilled off the coast in water that is greater than one mile deep. The largest Gulf of Mexico oil field to date was found in 1999, with the discovery of an estimated 1 to 3 billion barrels of oil 155 miles (250 km) due south of the Mississippi coastline. The field, known as Thunder Horse, has Miocene aged reservoir sands. Thunder Horse field alone has perhaps ten times more oil than is estimated to be located in the entire Southeast Region. As the technology of offshore drilling continues to improve, the trend of looking for oil in deeper and deeper waters of the Gulf of Mexico will continue into the foreseeable future.
Since about 1980, large reserves of natural gas have been exploited in tandem with coal seams. This natural resource, known as "coal-bed methane", forms at the same time as the coal it is associated with. Since this gas can be a safety hazard in underground mines, coal-bed methane methane production in mining areas, helps to eliminate a potential hazard as well as provide an energy resource. As of the year 2000, 7% of the country's yearly natural gas production came from coal bed methane. Since nearly half of United States coal comes from the Southeast, it follows that natural gas in relation to coal has become increasingly important in the coal fields of the Appalachian Basin. The Black Warrior Basin in northern Alabama and Mississippi, and Southwest Virginia Coalfield have hundreds of coal-bed methane wells currently in operation.
North Carolina, South Carolina, and Georgia have yet to yield a drop of oil or gas from the rocks below. Much of these states are underlain with Precambrian metamorphic and igneous rocks. These rock types do not contain petroleum.
The Paleontological Research Institution