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托福阅读材料推荐:地球化学研究

2016-07-18 15:41:06来源：网络

　　As these organic-rich sediment layers are buried by deposition of overlying sediments in the subsiding basin, the sediments are compressed and eventually lithified into rocks referred to as black shale, bituminous limestone, or coal. Methane producing microorganisms referred to as methanogens may thrive under certain favorable conditions within the organic-rich sediment layer during its early burial. These microorganisms consume portions of the organic matter as a food source and generate methane as a byproduct. This methane, which is typically the main hydrocarbon in natural gas, has a distinct neutron deficiency in its carbon nuclei (i.e., carbon isotopes), which allows microbial natural gas (a.k.a., biogenic gas) to be readily distinguished from methane generated by thermal processes (a.k.a., thermogenic gas) later in a basin's subsidence history. The microbial methane may remain in the organic-rich layer or it may bubble up into the overlying sediment layers and escape into the ocean waters or atmosphere. If impermeable sediment layers, called seals, hinder the upward migration of microbial gas, the gas may collect in underlying porous sediments, called reservoirs (Figure 3).

托福阅读材料推荐:地球化学研究

　　basin.

　　Economically significant accumulations of microbial natural gas have been estimated to account for 20 percent of the world’s produced natural gas. Microbial methane may remain trapped in the organic-rich sediment layer through out its lithificaton and contribute to economic accumulations referred to as coal-bed methane and shale gas.

　　Burial of the organic-rich rock layer may continue in some subsiding basins to depths of 6,000 to 18,000 feet (1830 to 5490 m).At these depths, the organic-rich rock layer is exposed to temperatures of 150 to 350 ºF (66 to 177 ºC) for a few million to tens of millions of years. The organic matter within the organic-rich rock layer begins to cook during this period of heating and portions of it thermally decompose into crude oil and natural gas (i.e., thermogenic gas) (Figure 4).

　　This overall process of cooking petroleum out of an organic-rich rock layer involves the appropriate combination of temperature and time and is referred to as thermal maturation. If the original source of the organic matter is mostly higher plants (e.g., trees, shrubs, and grasses), natural gas will be the dominant petroleum generated with lesser amounts of crude oil generation. If the original source of the organic matter is plankton (e.g., algae, copepods, and bacteria), crude oil will be the dominant petroleum generated with lesser amounts of natural gas generation. Organic-rich rock layers that have undergone this process of petroleum generation are considered to be thermally mature and referred to as source rocks.

　　Organic-rich rocks that have not been thermally matured are referred to as being thermally immature. These immature organic-rich rocks may be referred to as oil shale if artificial heating at high temperatures (~1000ºF/~538ºC) in surface or near-surface reactors (a.k.a., retorts) yield economic quantities of oil. Oil shale retorting occurred in Scotland between 1860 and 1960 and is currently active in Estonia and Brazil.

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