Energy in the Coastal Plain

Overview

Oil and natural gas resources are abundant in the Coastal Plain and are produced from reservoir rocks on- and offshore in Texas and Louisiana, as well as from Pennsylvanian sandstone in southwestern Arkansas and southeastern Oklahoma. Other fossil fuel resources include lignite and bituminous coal from Texas and Louisiana. The Coastal Plain also supports a variety of alternative energy sources, including wind, hydroelectricity, biomass, and nuclear power generation.

Oil and gas

Salt domes

As Pangaea broke apart during the Jurassic, the Gulf of Mexico opened and began to take shape. In its early stages, the Gulf experienced periods of restricted marine circulation. During these periods, salt was deposited through evaporation of seawater. This Jurassic salt occurs in flat layers now known as the Louann Salt Formation.

Today, the Louann Salt is covered by over 6000 meters (20,000 feet) of sedimentary rock. Salt has moved upward through the overlying rock layers through time, forming hundreds of salt domes. As these salt domes push through the overlying rock, they can form oil traps, or gaps in the rock where oil and gas can accumulate. Oil and gas can also accumulate under and along salt structures. Salt domes have created prolific oil reservoirs both on- and offshore in the Gulf Coast region.

Conventional oil and gas

Since the late Jurassic, the Gulf of Mexico has been accumulating thick sediment deposits, which have been supplemented since that time by sediments eroded from the Mississippi River watershed in central North America. The Coastal Plain along southern Louisiana and Texas was submerged under high sea levels for much of the Late Cretaceous and Paleogene periods. Because of this, thick layers of limestones, shales, and sandstones occur there. Many Late Cretaceous and early Paleogene shales became source rocks for oil, significant quantities of which have migrated into more porous sandstone and limestone, ultimately pooling in reservoirs. Impermeable sedimentary deposits such as gypsum, anhydrite, limestone, and dolomite serve as cap rock that trap the oil and gas.

The underlying Jurassic salt structures that occur in abundance along the Gulf of Mexico explain the geographic distribution of many oil and gas reservoirs from eastern Texas to western Alabama. Impermeable rocks pushed up by salt domes became caprock where oil could be trapped.

Gushers, an icon of oil exploration during the late 19th and early 20th centuries, occurred when highly pressurized reservoirs were breached by simple drilling techniques. Oil or gas would travel up the borehole at a tremendous speed, pushing the drill bit out and spewing out into the air. One of the most famous oil gushers, Lucas Gusher at Spindletop oil field in Beaumont, Texas, is credited with starting the Texas Oil Boom in 1901 that stimulated the growth of the oil industry. At its peak, Lucas Gusher ejected 100,000 barrels of oil per day before it slowed enough to be capped off. Today, southeastern Texas and southern Louisiana are together known as the East Texas Oil Field, which is the second largest oil field in the US outside of Alaska.

Offshore oil and gas

Today, oil and gas production associated with Coastal Plain sediments has moved mostly offshore, into the Gulf of Mexico. Offshore reservoirs exist where sandy sediments accumulated. Substantial quantities of sediment were eroded from the midcontinent into the Gulf of Mexico through the ancestral Red, Mississippi, and Sabine river systems. These sediments accumulated on the continental shelf and also along deeper submarine fans on the continental slope and even the abyssal plain. Much of this offshore oil and gas formed and trapped in similar ways to that onshore. Many of the offshore reservoirs are thick Paleogene and especially Miocene deposits, with Cretaceous- and Eocene-aged source rocks.

Drilling in the deepest parts of the Gulf is extremely challenging due to weather, the harsh environment, and water pressure at depth. This became publicly apparent during the Deepwater Horizon oil spill in 2010, when a seafloor gusher discharged 4.9 million barrels of oil over a period of 87 days before it was capped.

Shale gas (hydraulic fracturing)

The late Jurassic Haynesville Shale is a source rock that became a major target for hydraulic fracturing in the mid-2000s. The Haynesville Shale is stratigraphically above and slightly younger than the Louann Salt Formation. It was deposited later in the Jurassic as the Gulf of Mexico continued to expand. At this time, the continental shelf was an oxygen-poor environment with restricted circulation. Black, organic-rich shales are commonly laid down in such low-oxygen environments.

The unit lies greater than 3000 meters (9800 feet) under the surface in large parts of southwestern Arkansas, northwest Louisiana, and east Texas, and is about 70–100 meters (230–330 feet) thick. The formation varies geographically and includes shale, sandstone, and limestone. The shale has large amounts of natural gas trapped in its pores.

The Late Cretaceous Eagle Ford Shale in south Texas is another extremely productive onshore area for the extraction of both gas and oil through hydraulic fracturing. The Eagle Ford Shale was deposited along the edge of the continental shelf in quiet, slightly oxygen-poor conditions. Drilling began in 2008, and within a few years the area was one of the largest oil and gas producers in the U.S. The Eagle Ford Shale is about 20–100 meters (70–230 feet) thick. It is drilled across a wide spectrum of depths, from about 1300 to over 4000 meters (4300–13,100+ feet) deep, accounting for the variety of hydrocarbons extracted.

Oil and gas refining, storage, and distribution

Refining and distribution

In the southern part of the coastal plain, refineries are king. Texas and Louisiana are the two largest producers and refiners of crude oil in the nation, and Louisiana has one of the largest shipping ports for oil in the U.S. Natural gas also drives the Coastal Plain. Harvesting of natural gas and the infrastructure to move this energy resource throughout the region are extensive, and natural gas is one of the most consumed energies in the Coastal Plain region. Louisiana has six deep-draft ports—including the Louisiana Offshore Oil Port (LOOP)—that transfer large quantities of oil and other commercial goods for shipping around the country and the world.

Storage

Due to their inherent impermeability, salt domes are often solution-mined to create caverns that have been used to store petroleum, gas, and even chemical waste. In solution mining, water is pumped underground to dissolve the salt, creating a cavity.

Salt domes in the Coastal Plain are also used to store large quantities of oil and gas. The United States Strategic Petroleum Reserve is one such storage operation, currently holding 619 million barrels of oil in caverns across four salt domes in Texas and Louisiana. Other salt dome storage facilities include Clovelly Dome, which is used to store crude petroleum before it is shipped to refi neries, and the Regas Terminal, which stores 127 million cubic meters (4.5 billion cubic feet) of natural gas in a cavern larger than the Eiffel Tower.

Coal

Texas produces more lignite than any other state due to the presence of extensive Eocene-age deposits along the Gulf Coastal Plain. These deposits were formed by flowering plants and conifers living in marshy environments, brackish lagoons, and between streams near the Eocene coastline.

Lignite deposits also extend across northern Louisiana and eastern Arkansas, east into Mississippi and Alabama, and north into the Mississippi Embayment. The coal deposits are relatively young and have not been deeply buried, thus the coal has not been subjected to the sort of pressures and temperatures that yield higher-grade coal.

Renewable energy

Although the vast majority of energy in the Coastal Plain is derived from fossil fuels, there is also a variety of alternative and renewable energy production in the region. The Guadalupe River supports a handful of small hydropower plants, and a few wind farms are scattered along the southern Gulf Coast, in areas of favorable wind conditions. The largest of these, Gulf Wind, is a 283 MW project located in Kenedy County, Texas.

The Coastal Plain is also rich in biomass resources―organic materials burned to generate energy―with many areas generating up to 200,000 tons of biomass material every year from forestry, urban waste, and agriculture. A cluster of biomass power plants can be found near the city of Houston, producing 29 MW of power, enough to provide around 10% of Houston’s annual electricity demand. 

Nuclear power

The Coastal Plain also supports a few nuclear power plants. The South Texas Nuclear Project Electric Generating Station is a 2560 MW nuclear power station, located along the Colorado River about 140 kilometers (90 miles) from Houston. There are two nuclear facilities in Louisiana, the Waterford and River Bend reactors, both located along the Mississippi River.