Is There Oil in Your Backyard?
Most oil and gas discoveries in the southeastern United States are found in three distinct geologic provinces - the Appalachian Basin (extending from western New York all the way through to northern Georgia and Alabama) the Gulf Coastal Plain, and the Illinois Basin. Think of a basin as a broad region where sediments were deposited in the geologic past, even if no sediments are being deposited there today. For example, the Appalachian Basin is currently high above sea level, located largely in mountainous terrain, and is no longer a basin. 300 million years ago, however, thick accumulations of mud, sand, and organic material settled at the bottom of a shallow sea that was once located there. It is the burial of substantial accumulation of organic material for millions of years that gives us coal, oil, and natural gas today.
The Appalachian Basin was the site of much early oil exploration. In the early and mid-1800's, drillers searching for salt in the Appalachians sometimes struck oil by accident. (In those days, there was still abundant oil located at shallow layers below the surface. Many people only needed to drill 40 or 50 feet (~12 to 15 meters) before they accidentally struck oil. All that oil has been long since found, so this is a thing of the past). The first production of oil came from Wirt County, in west-central West Virginia in 1860. This was only one year after the famous Drake well of Titusville, Pennsylvania, commonly recognized as the first successful well purposefully drilled for oil (see link). This region peaked around 1900 as a culmination of the first major oil boom in America. (As a matter of fact, West Virginia actually lead the nation in oil production for one year, in 1899.) The second oil boom began with the discovery of Spindletop in 1901 along the Gulf Coastal Plain of Texas. The Gulf Coast in the Southeast has not been nearly as productive as the gulf Coast of Louisiana and Texas to the west, but shows some similar characteristics as a petroleum producer. These two basins (Appalachian and Gulf Coast), along with a section of the Illinois Basin in Kentucky, and the Mississippi Interior Salt Basin and Black Warrior Basin of Mississippi and Alabama, have been the main focus of oil and gas exploration and production in the Southeast.
While the rocks of the Appalachian Basin are markedly more flat-lying than the adjoining Valley and Ridge province, some of these sedimentary rocks are dissected with folds (forming anticlines) and faults, providing the opportunity to trap oil and gas. If there is source rock present, the possibility generally exists for oil and gas to be trapped at the crest of these anticlines. This is especially the case in a 50 mile (80 km) wide zone running through western West Virginia. The Appalachian Basin, created as a result of upwarping of the original Appalachian Mountains, is deepest directly to the west of this axis. As a result, the thickest sediments, and therefore the thickest accumulations of potential source and reservoir rock, are found in northwestern West Virginia and the neighboring regions of Pennsylvania and Ohio. The reservoirs in this region are Paleozoic in age, with the most important producing zones coming from the Devonian-aged Oriskany Sandstone and Mississippian-aged.Greenbrier Limestone (also known as "Big Lime"). Trapping is a result of mostly anticlines, although some stratigraphic traps are produced as a result of lenticular sand bodies. The Oriskany Sand is a continuation of the same oil and gas sands that can extend from New York into Virginia. The Greenbrier ("Big Lime") is everywhere present below ground in West Virginia, extending into neighboring Kentucky, and is in some places more than 260 feet (80 meters) thick. This great thickness allows the trapping of large quantities of oil and gas. Not all reservoirs are as extensive. An example of production from more isolated reservoirs is the petroleum production from the Mississippian-age Fort Payne Formation in eastern Tennessee. These reservoirs occur in carbonate mounds (called bioherms) that were formed in shallow seas. The mounds were then buried by mud and the whole complex turned to limestone and shale millions of years later. If oil migrates into a limestone mound, the shale cap forms a stratigraphic trap.
The highly fractured sandstones in this area often yield porosity where there would otherwise be none. A sandstone is known as "tight" if it is very well cemented, decreasing the effective porosity. However, this same sand can become an effective reservoir if it becomes fractured. This has happened all along the spine of the Appalachians and in many places to the west as a result of tectonic forces responsible for mountain building. It is also possible for petroleum engineers to fracture rocks themselves, artificially increasing porosity and permeability. The result is the transformation of "tight" sands into prolific oil and gas producers.
The future of exploration in the Appalachian Basin appears to be in deeper units, such as the Ordovician Trenton-Black River Formation. The Trenton-Black River is a carbonate rock unit which is both an oil and gas producer in states north of the Southeast Region even into Ontario, Canada. In 1999, a highly successful discovery well struck natural gas in Roane County, West Virginia to a depth greater than 10,000 feet (3,050 meters), proving this rock formation to be productive southward in the Appalachian Basin. More Trenton-Black River wells will be drilled, with similar success predicted. Rocks as old as the Cambrian-age Rome sand may also play a significant role in the continued development of the region.
The Paleontological Research Institution