Is There Oil in Your Backyard?
This region of the United States is not only the most prolific in oil production, but one of the great sources of oil in the world. Since the discovery of oil at Spindletop, near Beaumont, Texas, the world of oil has never been the same (learn more about it). According to the US Energy Information Administration, two of the top five states in terms of proved petroleum reserves (as of the year 2001) are Texas (> 4.9 billion barrels) and Oklahoma (556 million barrels), with Louisiana ranking 7th. This doesn't even take into account offshore oil, which is estimated at greater than 4.9 billion barrels in the Gulf of Mexico. Taking into account the Gulf of Mexico oil (including federally-owned land) Texas has about 25% of the US's proved reserves (tops in the nation), and Louisiana shoots to fourth (behind Texas, Alaska and California) at 14% (source, EIA, Office of Oil and Gas, 2001). Offshore Louisiana has about 2.9 billion barrels of proved oil reserves, which is the most offshore anywhere in the US. To make these statistics simple, the 6 states we are calling the south-central region of the US contains about 44% of the US's oil which has yet to be taken from the ground (9.6 billion barrels of the more than 22 billion barrels).
The main area of exploration these days in the South Central Region is offshore, in the deep waters of the Gulf of Mexico. It was once thought that oil could not be trapped in deep water, since reservoir rocks such as limestones and sandstones were not thought to occur in these deep waters. Within the last 25 years, this has proven to be false - sandstones can indeed occur in deep water. The next major obstacle was the ability to drill in deep offshore waters. Until 1947, no oil well had been drilled out of sight of land, and even then, drill rigs were not capable of drilling in water depths of much more than 25 feet (7-8 meters). With today's technology, wells are routinely drilled in water deeper than 1,000 feet (~300 meters), with the deepest wells being drilled at depths over 7,500 feet (~2,300 meters). These advances, both in our understanding of the geology of the Gulf of Mexico, as well as in the technology to drill in extreme conditions, has breathed new life into oil exploration and production in the Gulf of Mexico. 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. As the technology of offshore drilling continues to improve, the trend for looking for oil in deeper and deeper waters of the Gulf of Mexico will continue into the foreseeable future.
During the past 75 million years, the Gulf Coast has been progressively pushed further southward as sediments have piled up along the shore, moved here by the Mississippi River and other smaller streams. Since the Gulf of Mexico has been a depositional basin for so long, there has been no shortage of mostly sandstones, siltstones, and shales that have been deposited. Even today, potential reservoir rock continues to be deposited by the Mississippi River as it transports sand and mud into the ocean. As the river shifts back and forth (also known as "avulsing"), it deposits sand in some places, while other areas see clay deposition. These (and other) factors have created the sedimentary rocks we see today along the Gulf Coast, both above ground, and in the subsurface.
While the last 75 million years have been important in shaping
the geology of the Gulf Coast, the petroleum story here starts much earlier,
in the Jurassic Period 200 million years ago. At this time, Africa, North
America and Europe were still joined together, forming the supercontinent
of Pangea. As Pangea split apart, the Gulf of Mexico began to form. Jurassic
shales from this time period are the predominant source rock for oil in
the Gulf Coastal area.
The Paleontological Research Institution