托福阅读材料推荐:地球化学研究

　　Further burial to temperatures and pressures in excess of 600ºF (316ºC) and 60,000 psi (414 MPa), respectively, represent metamorphic conditions in which the residual char converts to graphite with the emission of molecular hydrogen gas. The resulting metamorphic rocks are graphitic slate, schist or marble. Thermodynamic considerations indicate that water remaining in these rocks should react with the graphite to form either methane or carbon dioxide depending on the amount of molecular hydrogen present. Currently, the deepest wells in sedimentary basins do not exceed 32,000 ft (9760 m). Therefore, the significance of natural gas generation under these extreme conditions remains uncertain.

　　Sedimentary basins vary considerably in size, shape, and depth all over the Earth’s crust (Figure 6).

　　Figure 6: General outline of major sedimentary basins.

　　A large number of variables and different combinations of these variables determine whether a sedimentary basin contains microbial methane, natural gas, crude oil, tars, or no petroleum. Not all basins have organic-rich sediment layers deposited during their subsidence history. As a result, these basins will contain no appreciable quantities of petroleum regardless of how deep the basin subsides. Other basins that do have an organic-rich rock layer may not have been buried to sufficient depths to generate natural gas or crude oil through thermal maturation, but may contain microbial methane accumulations. An organic-rich rock layer in some basins may thermally mature to generate mostly natural gas because of the dominance of higher plant debris contributing to its organic matter. Conversely, an organic-rich rock layer in other basins may thermally mature to generate mostly crude oil because of the dominance of lower plant debris contributing to its organic matter. More than one organic-rich rock layer may be deposited in the burial history of some basins with all, one, or none subsiding deep enough to thermally mature to generate petroleum. In other basins that have an organic-rich rock layer and sufficient burial to generate petroleum, the lack or scarcity of seals and reservoirs to collect generated petroleum may result in natural gas losses to the atmosphere or large degraded oil and tar deposits at or near the basin surface.

　　Research on these variables is critical to understanding the occurrences of known petroleum accumulations from which predictions can be made as to where undiscovered petroleum still resides within the Earth's crust. Research depends heavily on data collected from rock outcrops around and subsurface drilling in sedimentary basins. This geological data is essential to understanding of the development of sediment and rock layers (i.e., stratigraphy) within a basin and the history of their subsidence and trap development (i.e., tectonics). However, the vastness of sedimentary basins, limited well data, and migration of petroleum away from its source also requires research to 1) establish fingerprinting methods to determine genetic correlations among different petroleum types and their source and 2) conduct laboratory experiments to simulate petroleum generation and alteration to predict types, amounts, and extent of petroleum generated under varying subsurface conditions. Collectively, this understanding of genetically related petroleum, source rock identification, levels of thermal maturation, migration distances, and degrees of near-surface degradation allows construction of computer models of petroleum generation, migration, and accumulation through time within an evolving sedimentary basin. The USGS Energy Resources Team addresses these research issues under the Petroleum Processes Research Project.

　　*Figures are modified after those in Public Issues in Earth Sciences, USGS Circular 1115 entitled “The Future of Energy Gases” By P.J. McCabe, D.L. Gautier, M.D. Lewan, and C. Turner (1993).