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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

　　Petroleum has a lower density than the water that occupies pores, voids, and cracks in the source rock and the overlying rock and sediment layers. This density difference forces the generated petroleum to migrate upwards by buoyancy until sealed reservoirs in the proper configurations serve as traps that concentrate and collect the petroleum. Some of the generated natural gas may not migrate out and away from its source rock, but instead remains within microscopic pores and dissolved in the organic matter of its source rock. This retained natural gas has proven to be an economically significant resource that is referred to as shale gas. The Barnett Shale in the Fort Worth basin of Texas is a good example of this type of accumulation.

　　In some basins, petroleum may not encounter a trap and continue migrating upward into the overlying water or atmosphere as petroleum seeps. Crude oil that migrates to or near the surface of a basin will lose a considerable amount of its hydrocarbons to evaporation, water washing, and microbial degradation leaving a residual tar enriched in large complex hydrocarbons and asphaltenes (Figure 5). Tar deposits range in size from small local seeps like the La Brea tar pits of California to regionally extensive occurrences as observed in the Athabasca tar sands of Alberta.

　　Figure 4: Continued burial of sediment and

rock layers in subsiding basin.

托福阅读材料推荐:地球化学研究

　　Figure 5: Deeper burial of rock layers in

　　subsiding basin.

　　Burial of the source rock may continue to depths greater than 20,000 ft. (6100 m) in some sedimentary basins. At these depths, temperatures in greater than 350ºF (177ºC) and pressures greater than 15,000 psi (103 MPa) transform the remaining organic matter into more natural gas and a residual carbon referred to as char. Oil trapped in reservoirs that are sometimes buried to these depths also decomposes to natural gas and char. The char, which is also called pyrobitumen, remains in the original reservoir while the generated natural gas may migrate upward to shallower traps within the overlying rock layers of the basin. The Gulf Coast basin that extends into the offshore of Louisiana and the Anadarko basin of the US mid-continent are good examples of these deep basins.

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