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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

　　本文是TPO 43 阅读背景资料！The origin of petroleum以及El Niño, Southern Oscillation。托福阅读材料推荐:地球化学研究是很好的托福阅读背景知识补充阅读材料。希望对大家托福备考有所帮助。更多精彩尽请关注新东方在线托福网!

Geochemistry Research

ORGANIC ORIGINS OF PETROLEUM

　　Petroleum is a naturally occurring substance consisting of organic compounds in the form of gas, liquid, or semisolid. Organic compounds are carbon molecules that are bound to hydrogen (e.g., hydrocarbons) and to a lesser extent sulfur, oxygen, or nitrogen. The simplest of these compounds is methane with one carbon atom bound to four hydrogen atoms (Figure 1). Asphaltenes are the most complex with more than 136 carbon atoms bound to more than 167 hydrogen atoms, 3 nitrogen atoms, 2 oxygen atoms, and 2 sulfur atoms (Figure 1). Petroleum gas is referred to as natural gas, which should not be confused with the abbreviated term used to describe the refined fuel "gasoline". Natural gas consists predominantly of simple hydrocarbons with only one to five carbon atoms (i.e., methane to pentane, respectively, Figure 1). Liquid petroleum is referred to as crude oil and consists of a wide range of more complex hydrocarbons and minor quantities of asphaltenes (Figure 1). Semisolid petroleum is tar, which is dominated by larger complex hydrocarbons and asphaltenes (Figure 1).

托福阅读材料推荐:地球化学研究

　　Figure 1. Some examples of organic compounds in petroleum, from the simplest (methane) to the most complex (asphaltene).

　　Petroleum formation takes place in sedimentary basins, which are areas where the Earth's crust subsides and sediments accumulate within the resulting depression. As the sedimentary basin continues to subside, sediment accumulations continue to fill the depression. This results in a thickening sequence of sediment layers in which the lower sediment layers eventually solidify into sedimentary rocks as they experience greater pressures and temperatures with burial depth. The sediment layers that accumulate vary in character because the sources and depositional settings of the sediments change through geologic time as the sedimentary basin subsides and fills. It is critical to petroleum formation that at some time during the accumulation of sediments at least one of the sediment layers contains the remains of deceased plants or microorganisms. Throughout geologic time, the world oceans have expanded and receded over the Earth’s land surfaces and contributed sediment layers to subsiding sedimentary basins. Development of stagnant water conditions in some of the expanded oceans caused the bottom waters to be depleted in oxygen (anoxic), which allowed portions of decaying plankton (e.g., algae, copepods, bacteria, and archaea) that originally lived in the upper oxygen-bearing (oxic) waters to be preserved as a sediment layer enriched in organic matter (Figure 2). Swamps and marshes may also develop marginal to oceans overlying subsiding basins. In these depositional settings, sediment layers enriched in decaying land plants (e.g., trees, shrubs, and grasses) may occur.

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