How Salt Domes Were Created
What is a Salt Dome?
A salt dome is a geologic formation caused by a phenomenon known as diapirism, in which lighter materials force their way up through denser ones. In the case of a salt dome, a column of salt has risen toward the surface because it has a lower density than the rocks above it. When buried to depths greater than a few thousand feet (under pressure and heat), salt has the ability to deform and flow like a high-viscosity oil ascending in slow motion through a thick layer of water above, much like a lava lamp. As these huge pillars of salt rise, multiple deep seated hydrocarbon producing sandstone fairways are pushed relatively close to the surface at a depth of approximately 2500’ to 10,000’. Many salt domes contain pockets of oil and natural gas, which become trapped as the salt rises. These deposits have nowhere to go until they are drilled, and they can be quite significant.
Why Do Salt Domes Form?
The formation of a salt dome takes centuries. It starts with the formation of an isolated marine inlet, which slowly evaporates, concentrating the salts. Geologists believe that these inlets must be flooded and evaporated several times to reach the concentration of salt needed to create a salt dome. Once a large deposit of salts is created, sediments are deposited over the salt as the century’s progress, but the salt will continue drifting to the top because it is less dense than the sediments around it. As a result, the salt creates a distinctive bulge, and it appears to be boring its way through the surrounding rock when viewed in cross-section.
Economic Importance of Salt Domes
Salt domes are very important to the petroleum industry. As a salt dome grows, the cap rock above it is arched upwards. This cap rock can serve as an oil or natural gas reservoir. As a dome grows and moves up through the Earth, it punches through and bends rock along the way. The rocks that it penetrates are arched upwards along the sides of the dome. This upward arch allows oil and natural gas to migrate toward the salt dome where it can accumulate in a structural trap. Oil can also come to rest right up against the salt, which makes salt an effective trap rock mechanism. The added economic benefit of this upward arching action when a salt dome rises is that it produces over-pressurized zones atypical at new shallow depths. These new shallow sands yield increased hydrocarbon flow rates and superior reserve recovery unique only to salt domes. The economic benefits are further heightened by the reduced drilling cost due to their shallow nature, creating a perfect storm of opportunity. A single salt dome can have many associated reservoirs at a variety of depths and locations around the dome. For these reasons, salt domes are considered to be the most prolific oil producing structures in the world.
The First Salt Dome Oil Discovery
Salt domes were almost unknown until an exploratory oil well was drilled on Spindletop Hill near Beaumont, Texas in 1900 and completed in 1901. At a depth of about 1000 feet, the well penetrated a pressurized oil reservoir that blew the drilling tools out of the well and showered the surrounding land with crude oil until the well could be brought under control. The initial production from the well was over 100,000 barrels of crude oil per day - a greater yield than any previous well had ever produced. The Spindletop discovery ignited a drilling spree on similar structures across the Gulf Coast area. New discoveries motivated geologists to learn about the structures below that held such vast amounts of oil.
Why Salt Domes Prevail In The Houston Salt Basin
Over 400 salt dome features have been identified in East Texas. They originate from the Louann Salt, a subsurface rock unit that is laterally persistent throughout the area. As North and South America separated to form the Gulf, this salt stratum became the mother bed foundation of all sedimentary layers to follow. Throughout millennia the area surrounding Houston bay has always been a delta sitting atop two intersecting tectonic plates. The delta runoff provided the necessary sediment loading to bury the salt and established a complex overlying network of interlaced source rock and reservoir rock. The ongoing tectonic forces triggered faults, enabling the salts upward movement, while simultaneously allowing hydrocarbon migration. This combination created a perfect storm for salt dome development, and the ensuing oilfields in the area. These salt domes are the focus of Magna Resources exploration and development.
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