molten, or recently crystallized (in a geological sense), rock heats groundwater that carries heat toward the surface where it can be harnessed to generate electricity. Few locations can claim the coincidence of both hot rocks and groundwater; consequently, geothermal resources have been developed only locally. Research in geothermal energy includes the use of magma as a direct energy source and the extraction of energy from hot dry rock by injecting water and recovering it to extract the heat.

Earth scientists will continue to address the nation's energy needs through the discovery of energy resources, their assessment, and their economic development in an environmentally sound manner. Fulfilling these responsibilities calls for an adequate scientific understanding of the environment in which the resource occurs. An appreciation of the past—how that environment and the resource have evolved over geological time—complements the anticipation of the future—how resource development will affect the environment.

Growth of the worlds population and rising per capita energy use will lead to increased energy demands in the next century. Despite concerns about the environmental effects of increasing carbon dioxide levels, the world simply cannot withdraw from its dependence on fossil fuels in less than several decades. Substitution, the development of new technologies, and conservation can reduce this reliance.

The development of solar, tidal, and wind energy requires continued study, as does increased use of geothermal sources. The continued use of nuclear energy is dependent on the efficient and responsible handling of radioactive wastes. Future energy resources may include forms that seem exotic today: hydrogen fuels and nuclear fusion are subjects of ongoing research. Regardless of the mix of eventual sources, society has become more conscious of the problems of integrated development, consumption, and waste disposal—the activities constituting the use of any energy or mineral resource.

Petroleum Resources

Significant accumulations of oil and gas are geological anomalies, and they constitute only a minute fraction of the total material within the sedimentary basins where they are found. These two factors create the scientific and technical complexity of the geoscientist's role in petroleum recovery and of the industry itself. Even in relatively well-understood sedimentary basins, such as the Gulf of Mexico, the success rate for exploration drilling is only about one in six. In the true frontier areas that risk might be increased by a factor of 10 or more. The cost of a single exploration well can be in the tens of millions of dollars. Appropriately, great effort and expense are invested in geoscientific research that increases the probability of successful strikes and effective extraction.

The historical development of petroleum resources has reached a critical point. In the conterminous United States, additional discoveries are sought within the framework of developed resources. Recent increases in reserves have resulted predominantly from enhanced recovery methods in known fields and from in-fill drilling between successful strikes. Some frontier contributions can be expected from the deep-water Gulf of Mexico and from Alaska. Worldwide, however, new and undeveloped resources are sought. The two searches are complementary, and they present a remarkable opportunity for geoscientists. Researchers can apply a full spectrum of approaches and technologies to the search for, and the exploitation of, petroleum deposits. These range from the use of traditional interpretations of oil genesis to the application of innovative quantitative and genetic models of producing reservoirs. Vast quantities of new data from both satellite imagery and seismic surveys offer unmatched opportunities for gaining insights that were unimaginable only a few decades ago. These new methods are being applied all over the world but especially in the Middle East.

According to the most recent estimates, the countries of the Middle East overlie more than one-half of the world's undeveloped petroleum resources. This accumulation came about as part of the cyclical process involving the opening and closing of the ocean basins, which is the consequence of plate tectonics.

About 100-million-years ago the area that is now Arabia was located along the north coast of the supercontinent of Gondwana, forming an Atlantic-type continental margin at a tropical latitude (Figure 4.13). Anoxic episodes, characterized by the absence of free oxygen, encouraged accumulation of organic debris in sediments deposited at water depths of a few hundred meters along the margin of the continent. In shallower water close to sea level, abundant limestone reefs grew; with oscillating sea levels, evaporite salt deposits precipitated among the limestones. Uplift to the west, in what is now Africa, resulted in erosion that episodically carried sand into the offshore area. These related events provided the carbon-rich source rocks in which petroleum would

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