Why can oil be found in the Gulf anyway?

The ongoing environmental drama in the Gulf of Mexico provides an opportunity to briefly consider the geology and geography of oil production in the region. The ill-fated Deepwater Horizon platform was one of about 4,000 such platforms in the Gulf, which collectively produce over 1.5 million barrels of oil per day. This amount represents approximately 30% of domestic oil production in the United States. But why can oil be found in the Gulf anyway? The answer lies in the structural and sedimentary geology of the area.

The Gulf is rich in oil due to a complex history that involves marine transgressions (rising sea levels), regressions (falling sea levels), and delta formation during its evolution. In the context of sea level history, geologists know that numerous periods of rising and falling sea levels have occurred in the Gulf’s history, with each lasting for millions of years. These fluctuations in water level were perhaps several hundred meters in height. During low sea stands, plants and animals occupied the exposed surfaces. The organic residue from this biota was then buried by marine sedimentary deposits during the next transgression. This cycle of surface exposure, accumulation of organic material, and subsequent burial during a marine transgression occurred numerous times, resulting in a thick sedimentary sequence of limestone, sandstone, and shale. The organic deposits were subsequently converted to oil and natural gas through intense pressure and heat caused by the weight of the overlying rocks.

The evolution of the Mississippi Delta has also contributed to the formation of hydrocarbon reservoirs in the Gulf. At this location, clouds of microscopic organisms develop that feed on the dissolved load carried into the Gulf by the Mississippi River. As these organisms die, they settle on the ocean floor, forming a mix of sediment and organic ooze. Over time, these layers are buried progressively deeper by additional deltaic and marine deposits. As a result, the organic remains are also converted to oil and natural gas because they are cooked in the manner previously described.

The numerous oil wells in the Gulf are tapping into vast reservoirs of petroleum that have formed over millions of years. Some reservoirs consist of pockets where oil collected after it was forced upward through overlying layers of sandstone after it formed. At some level, this upwardly moving oil encountered a shale layer, which trapped it from rising further. As more oil moved upward from underlying strata, the volume within the trapped reservoir increased. In many other places, oil is found in relation to salt domes associated with the Louann Salt. This wide-spread evaporite formed during an extensive period of aridity and low-sea stand in the mid-to-late Jurassic Period, between 175- and 145-million years ago. This salt bed is less dense than the sediments above it and has thus been forced upward by pressure in many places. Over time, oil has flowed upward along the edge of these features until it encounters some form of trapping layer that causes it to collect in mineable quantities.

So, as we view this unfolding environmental disaster, it is worth considering the time and events required to develop reservoir-quality hydrocarbons in the Gulf of Mexico. It is also important to note that a primary reason the spill has yet to be contained is because the damaged well is about one mile below the ocean surface where access is especially difficult. This remote location reflects the fact that most of the easily accessible oil on land or near the coast has already been extracted, forcing oil companies to search in progressively deeper waters. As a result, the limits of technology are stretched, especially when problems arise. For the American consumer, these geological and geographical relationships mean that the coming years probably mean higher prices at the pump as our search for oil becomes more complicated. In the meantime, the Gulf of Mexico is bearing the environmental and societal costs of our growing need for oil.

By Alan Arbogast, Professor of Geography, Michigan State University.  Author of, Discovering Physical Geography, John Wiley & Sons, Inc.

Associated web sites:

http://oceanexplorer.noaa.gov/explorations/06mexico/background/oil/oil.html

http://www.priweb.org/ed/pgws/backyard/sections/southcentral/southcentral2.html