How the States Rate
Proved Reserves - South Central US
as of 2001
Source:US Energy Information Administration's US
Crude Oil, Natural Gas, and Natural Gas Liquids Reserves, November 2001
While the days of the "gusher" is a thing of the past (which is a good thing - gushers are extremely dangerous and damaging to the environment), plenty of oil and natural gas production is still occurring in onshore areas of Texas, Louisiana, Oklahoma, Kansas, and Arkansas.
Just onshore, in south eastern Texas and southern Louisiana,
the flowage of salt domes has been the predominant mechanism for creating
traps for oil. Salt of Jurassic age occurs here. When it is put under
immense pressure by overlying rocks, this salt, which is less dense
than the rocks surrounding it, will begin to flow upward. As it does
so, it displaces, folds, and faults the rocks around it. In this way,
traps can be created.
Nearly all of the salt domes in along the Gulf Coast have a disk-like cap-rock, composed of minerals such as gypsum, anhydrite, limestone and dolomite, over part or all of their surface. This cap-rock generally forms when ground water interacts with the surface of the salt. In addition, calcite and dolomite dissolve when they come in contact with ground water. This can create large cave-like holes in the cap-rock, which may then be filled with oil that has migrated up from below. When this zone is penetrated by the drill, oil comes out of the ground fast and furious. This scenario lead to the formation of the great Spindletop oil accumulation, discovered in 1901, as well as Jennings Field in southwestern Louisiana, discovered just 9 months later.
The geology of Oklahoma is extremely complex, but thanks to the extensive exploration for oil and natural gas during the last century, it is fairly well understood. The first important strike was near Bartlesville in 1897, and Oklahoma has been a leading producer of oil in the United States ever since. Nearly every one of the state's 77 counties has produced or continues to produce oil. The major depositional basins responsible for the majority of oil in the state are the Anadarko Basin, located in the western and central portion of the state, and the Arkoma Basin, located between the Ozark and Ouachita mountains, on the eastern side of the state.
Reservoir rocks in Oklahoma range in age from Early Cretaceous to Cambrian. Both sandstones and carbonate rocks are oil producers in the state. What makes Oklahoma so productive is the many reservoir rocks that can be found in one drill well, due to the incredible thicknesses of sedimentary rocks in the many depositional basins. There are more reservoirs of Pennsylvanian age than any other in Oklahoma, and production from these rocks is substantial These rocks have a thickness of many thousands of feet, and are mostly sandstone and shale.
Geologically, Kansas has perhaps the simplest story of the states located in the region. The state lies within the Western Interior Coal Basin, bounded to the north by the Wisconsin Shield, to the east by the Ozark Uplift, and on the south by the Ouachita and Arbuckle-Amarillo uplifts. Kansas is bountifully supplied with source beds to furnish oil and gas, ample reservoir rocks, and seal rocks to retain the hydrocarbons, and plenty of traps to capture the deposits into commercial pools.
The great Hugoton gas field in extreme southwestern Kansas was discovered in the 1920's. Even today, this area, known as the Hugoton embayment, is the leading area for gas production in the state. The most abundant form of trap in Kansas is structural, with the anticline playing the most pivotal role. However, varying permeabilities of the reservoir rocks often play a role in the distribution of the gas or oil across the crests of these anticlines. Another widespread method of hydrocarbon trap is truncation due to erosion. Especially important is the angular unconformity beneath the Pennsylvanian rocks, which assists in the trapping of oil and gas on both the Nehama (northeastern Kansas) and the Central Kansas uplifts.
More information will soon follow, detailing the many other incredibly petroleum-rich basins in the South Central Region of the United States.
The Paleontological Research Institution