Alabama Landscapes
Energy
Oil and Natural Gas
Natural gas and oil (also called petroleum, which means "rock oil") are hydrocarbons (fuels composed essentially of carbon and hydrogen). Carbon and hydrogen are bonded into molecules of different number of atoms and thus different sizes. The main physical difference is that oil, composed of larger CH molecules, is liquid or solid at room temperatures, wheres natural gas with smaller molecules is not (Table O1).
Table O1. Characteristics of oil and natural gas
| Name | Number of carbons in molecule | Boiling Point (degrees F) |
| gases | 1-4 | < 93 |
| light crude | 6-18 | 93-343 |
| heavy crude | >18 | >343 |
Oil and most natural gas are the decayed and biochemically-altered remains of marine organisms that have accumulated on the ocean floor, covered with sediment and buried. Because they are mobile, natural gas and oil can migrate through openings in rocks (called pores) towards the surface of the earth. Only if the molecules strike a barrier or "trap" (a rock that will not allow them to migrate: Figure O1) will they accumulate and fill all available spaces in the host rock. Such an accumulation is often called a "pool." Some common traps are shown below:
Figure O1. Two common oil and gas traps in Alabama. Black: impermeable rocks that act as traps
Alabama's oil and gas fields occur in two distinct areas in the Coastal Plain (Figure O2).
Figure O2 Major oil (green) and gas fields (blue) in the Coastal Plain.
Oil is extracted from Jurassic strata at depths of more than 10,000in the Citronelle Field (green on Figure O2). In this field, oil is trapped in anticlines associated with salt intrusion. Gas is extracted from Jurassic strata at depths of greater than 2,000 in the southern Mobile Bay and offshore field (Figure O3). Here, gas is in stratigraphic traps
Figure O3. Gas well, Mobile Bay. (Image © Mike Neilson)
Coal Bed Methane
Natural gas also forms during the transformation of plant material to coal. The gas formed is methane (CH4 : each molecule consists of 1 carbon and 4 hydrogen atoms) and accumulates in and surrounding the beds of coal. For that reason it is called coalbed methane. Large volumes of methane have been extracted from the Warrioir coal field in the Cumberland Plateau (Figure O4).
Figure O4 Coal-bed methane fields in the Cumberland Plateau. (Alabama Geological Survey)
Production
Although the first oil well in Alabama was drilled in 1865 near Huntsville, large scale extraction did not begin until 1940s. Since then, Alabama's production has changed to the point that in 2003 the state was ranked 16th in the nation in petroleum production and 10th in the nation in natural gas production.
Coalbed methane was first extracted in 1980 from the Pleasant Grove Field (Figure O4). In 2002 Alabama produced 11% of all coalbed methane in the United States
Oil and traditional natural gas production has decreased in the last decade, while coalbed methane production has leveled off (Figure O5).
Figure O5. Oil, natural gas and coalbed methane production 1992-2002. (Alabama Oil and Gas Board)
Estimates suggest that Alabama has about 53 million barrels of oil and 2,295 billion cubic feet of natural gas and about 1,9000 bcf of coalbed methane in the ground at the end of 2004 Click here to see what that means for the future.
END