Region 3: The Coastal Plain

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, hydro, biomass, and nuclear power generation.

Conventional Oil and Gas

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 which salt was deposited through evaporation in flat layers now known as the Louann Salt Formation. These salt deposits, which now underlie much of the Coastal Plain, contribute to the character and size of oil fields under eastern Texas and Louisiana (Figure 7.12).

Figure 7.12: The Louann Salt Formation extends under the surface of the Coastal Plain to the blue line. The “blobs” are known salt structures where the salt layer has been deformed.

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 Texas and Louisiana was submerged under high sea levels for much of the late Cretaceous and Paleogene, and it is now the site of thick layers of limestone, shale, and sandstone. Many late Cretaceous and early Paleogene shales became source rocks for oil, significant quantities of which have migrated stratigraphically into the sandstone and porous limestone, ultimately pooling in reservoirs trapped under a variety of impermeable sedimentary deposits such as gypsum, anhydrite, limestone, and dolomite. 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. Oil accumulated throughout the Coastal Plain in large part because of the region’s underlying Jurassic salt layers. Salt structures occur in abundance along the Gulf of Mexico (see Figure 7.12), which explains the geographic distribution of oil and gas reservoirs―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 (Figure 7.13), is credited with starting the Texas Oil Boom in 1901 that stimulated the growth of the oil industry (Figure 7.14). At its peak, Lucas Gusher ejected 100,000 barrels of oil per day before it slowed enough to be capped off. Although iconic, gushers were extremely dangerous and wasteful; as well as spewing thousands of barrels of oil onto the landscape, they were responsible for the destruction of life and equipment. The advent of specialized blowout prevention valves in the 1920s enabled workers to prevent gushers and to regain control of blown wells. Today, this equipment is standard in both on- and offshore oil mining.

Figure 7.13: The Lucas Gusher at Spindletop in 1901.

Figure 7.14: Oil derricks line a street, once known as “the world’s richest acre,” in downtown Kilgore, Texas. At the height of the Texas Oil Boom, more than 1100 oil wells operated within city limits.

Salt Domes

Rock salt (the mineral halite) is solid and impermeable, but when it is under very high pressure it can flow like a thick liquid. When a layer of salt is buried under thousands of feet of overlying sediment, it will start to deform. Because it is less dense than the rocks above it, it flows upward toward areas of lower pressure, forming geological structures named for their shapes (e.g., domes, canopies, tables, and lenses). Salt domes are extremely common geologic features along the Gulf Coast, and their origin lies in the Jurassic, when salt was deposited through evaporation in flat layers now known as the Louann Salt Formation. Today, this salt layer is covered by over 6000 meters (20,000 feet) of sedimentary rock, through which the salt has moved upward with time, forming hundreds of salt domes.

As salt structures grow, they in turn influence the topography of the surrounding landscape, creating zones of uplift, surrounding areas of subsidence, fractures, and faults. When salt flows upward, it deforms the surrounding strata, creating gaps in which oil and gas may pool and be trapped. Oil and gas also accumulate under and along the salt structures. Salt domes have led to some of the most prolific oil reservoirs in the Gulf Coast, both on- and offshore. In addition, due to their inherent impermeability, the salt domes themselves are often solution-mined (by pumping water underground to dissolve the salt) to create caverns that have been used to store petroleum, gas, and even chemical waste.

Offshore Oil and Gas

Today, oil and gas production associated with Coastal Plain sediments has moved mostly offshore, into the Gulf of Mexico. Much of this oil and gas formed and was trapped in similar ways to that onshore, with salt structures leading to offshore traps (see Figure 7.12). Substantial amounts of sediment were eroded from the midcontinent into the Gulf of Mexico by way of river drainage systems, including the ancestral Red, Mississippi, and Sabine rivers. Offshore reservoirs now exist where sandy sediments accumulated, not just on the continental shelf, but also in deeper submarine fans along the continental slope and even abyssal plain. Many of the offshore reservoirs are found in thick Paleocene to Miocene deposits, with Cretaceous and Eocene-age source rocks. Drilling in the deepest parts of the Gulf is extremely challenging due in part to weather, harsh environments, and water pressure at depth. This became publicly apparent during the Deepwater Horizon oil spill event in 2010, when a seafloor gusher discharged 4.9 million barrels of oil over a period of 87 days before it was capped (Figure 7.15).

Figure 7.15: The Deepwater Horizon oil slick on May 24, 2010, four days after the blowout. The image was taken by NASA’s Terra satellite.

Shale Gas

Like the Barnett Shale in the Central Lowland, the late Jurassic Haynesville Shale is a source rock that became a major target for hydraulic fracturing in the mid-2000s. 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, but it is the shale that retains large amounts of natural gas trapped in its pores.

The Haynesville Shale, stratigraphically above and slightly younger than the Louann Salt Formation, was deposited during the Jurassic as the Gulf of Mexico continued to expand. At this time, the continental shelf was an oxygen- poor environment with restricted circulation, and black organic-rich shales are commonly laid down in these low-oxygen environments.

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 US. The Eagle Ford is about 20 - 100 meters (70 - 230 feet) thick, and 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.

Coal

Texas produces more lignite than any other state due to the presence of extensive Eocene-age deposits along the Gulf Coastal Plain, formed from flowering plants living in marshy environments, brackish lagoons, and between streams near the Eocene coastline. These deposits also extend across northern Louisiana and eastern Arkansas, east into Mississippi and Alabama, and north into the Mississippi Embayment (see Figure 7.4). 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.

Fossil Fuel 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 US. Natural gas also drives the Coastal Plain; harvest of natural gas and the infrastructure to move this energy resource throughout the region are extensive, and this allows for natural gas to be one of the most consumed energies in this region. Louisiana has six deep-draft ports—including the LOOP (the Louisiana Offshore Oil Port)—that transfer large quantities of oil and other commercial goods for shipping around the country, and the world.

Salt domes in the Coastal Plain are also used to store large quantities of oil and gas. Storage caverns are created by injecting the salt with water to dissolve a cavity within the salt structure―a process called solution mining (Figure 7.16). 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 refineries, 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.

Figure 7.16: Solution mining is used to create a storage cavern inside a salt dome.

Alternative 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 (see Figure 7.19). 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.

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.