Fossil Fuel Resources of the Southeastern U.S.: a brief review
Fossil fuels include coal, oil and natural gas; the Southeast produces all three. These fossil fuels are clearly important to our economy and standard of living, providing the fuel we need for heating, cooling, cooking, driving and industry. Mining and production are a major part of the rich economic and historical development of the Southeast. Most fossil fuel discoveries in the Southeast are found in two distinct geologic provinces – the Appalachian, Illinois, and Black Warrior Basins of the Inland Basins region, and the Gulf Coastal Plain. Three hundred million years ago, however, huge amounts of mud, sand, and organic material settled at the bottom of a shallow sea that was once located there. Vegetation accumulated at the swampy shoreline of the inland sea. It is the burial of substantial accumulations of organic material for millions of years that gives us coal, oil, and natural gas today. Let’s look at the processes by which each of these fossil fuels form.
Coal is formed from the burial and alteration of peat. Peat is formed from the organic remains of plants in swamps, mostly roots and leaves. The abundance of plant material in swamps, bogs and marshy areas, combined with stagnant conditions, makes these environments ideal for the formation of coal. As sediment is flushed into the swamp by water, plant material is buried. Bacterial decay of large quantities of plant material uses up available oxygen, causing aerobic decay rates to drop. Not all swamps have peat accumulations and not all peats become coal.
For thick peats to become coal they must be buried. As organic material gets buried more and more deeply, pressure on it builds from overlying sediments, squeezing and compressing the peat. The pressure of burial, temperature of burial, and length of time the peat remains buried act to slowly alter the peat. The alternation process is called coalification. Coalification results from compaction, and from chemical and physical alteration of the peat during burial. The amount of coalification is classified as coal rank. The different ranks of coal in increasing order are lignite, bituminous, and anthracite (Figure 8.1). In general, higher rank coals have less moisture and more carbon than lower rank coals. Also, higher rank coals produce more energy per unit volume (measured in BTU’s) than lower rank coals.
Figure 8.1: Stages of Coal Formation: Piles of peat are buried. As pressure builds from more and more overlying sediment, peat is compressed into lignite. As pressure continues to increase (along with temperature), bituminous and anthracite coal forms. Figures by J. Houghton.
Oil and Gas
Coal, oil and gas are all composed of organic matter. The differences in the kinds of organic matter determine which type of fossil fuel is formed. Coal tends to be formed from land plants, accumulating in swampy areas. Oil, on the other hand, is made primarily of phytoplankton, bacteria and plant material (algae) from the ocean. Coal remains solid because of the nature of the land plant material, whereas the marine organic material transforms under high heat into oil and natural gas. Natural gas, primarily made of methane, forms either alone or in association with coal and oil, when high temperatures transform solid organic material to a gas.
Unlike coal, which forms and stays in one place, oil and gas form in one place but may migrate to another. Organic material from marine plants and animals becomes buried under increasing amounts of sediment that squeeze and heat up the organic material over time. The sediment containing the organic material eventually become sedimentary rock, commonly shale. The oil and gas generally do not stay in the rock in which they originally formed (the “source” rock) because they tend to migrate upward through cracks and pores within permeable rocks toward the surface where there is less pressure (Figure 8.2). If the oil and gas reach the surface, they evaporate into the atmosphere or are broken down chemically by microorganisms.
Figure 8.2: Unsorted rock has low porosity (pore space shown in black). Sorted rock (bottom) has high porosity.
However, if somehow trapped below the surface, oil and gas pool within the rock. An impermeable layer, such as shale or “tight” sand, is what halts the oil and gas migration to the surface. A sandstone is known as “tight” if it is very well cemented, the pore spaces between sand grains are closed that would otherwise be open, and the effective porosity is decreased. This same sand, however, can become an effective reservoir if it becomes fractured, opening up pathways for movement of the oil and gas. The Appalachian Mountains have been intensely fractured, as have many places to the west, as a result of tectonic activity during Paleozoic mountain building. Petroleum engineers can induce rock fractures, artificially increasing porosity and permeability. The result can be the transformation of “tight” sands into prolific oil and gas producers.
Fossil fuels are used for more than just heating and generating electricity. They also have many industrial and chemical uses including the production of plastics, synthetic rubber, detergent, wax, fertilizer, cosmetics, medicine, food additives, paint and dyes. Additionally, bituminous coal is important in the steel industry for making coke (a carbon-rich solid needed to make steel).
Trapping Oil & Gas
Beneath the Earth’s surface, oil will ooze through rocks if there is enough porosity and permeability, but the oil will not accumulate into large quantities unless something traps it in a particular place. There are a variety of geologic traps, including structural and stratigraphic traps, and combinations of these two.
Structural traps may be folds or faults that impede the flow of oil. Oil that finds its way into a reservoir rock that has been bent into an A-shape fold (anticline) will flow to the crest of the fold, and get stuck (provided, of course, that there is an impermeable layer, such as shale, above the arch to seal the oil in place.) Fault traps also form when an impermeable layer above and a fault zone (generally impermeable because the friction along a fault plane creates poorly sorted, broken rock) impede the flow of oil. Salt domes are also a type of structural trap.
A stratigraphic trap accumulates oil due to changes of rock character rather than faulting or folding of the rock. The term “stratigraphy” basically means “the study of rocks and their variation.” Sedimentary rock layers may grade into different types of sedimentary rocks, sometimes over short distances even within a single layer. Thus, oil might be found in a lens of porous sandstone (that was once a sandbar or stream channel) that is surrounded by impermeable shale (that was once mud adjacent to and burying the sandbar or stream channel).
Figure 8.3: Examples of structural traps. Figures by J. Houghton.