Persian Gulf. Each of the four developmental categories, while overlapping in time and location within a basin, has been studied separately, and in the final assessment all are combined into an integrated approach for exploration.

Organic material is oxidized at the surface by processes such as bacterial decay. Under some conditions the bacterial decay is incomplete, and burial of the remains segregates carbon-rich material within sedimentary rocks. It is this sequestered organic matter that, upon further burial, is capable of generating fossil fuels. Coal consists of solid plant matter preserved where it was originally buried, while oil and gas are fluids formed from buried organic matter that subsequently migrates from its source area.

Researchers try to establish exactly how distinct fluids are generated and what kinds of oil and gas relate to what source materials. This knowledge can help direct the search for new oil deposits. For example, although 50 years ago it became obvious that lake beds, such as the Green River Formation of Utah, may contain significant quantities of oil and gas source materials, it is only in the past decade that lake beds worldwide have been emphasized in oil and gas exploration.

Sedimentary basins are depressions on the surface where loose particles accumulate and eventually turn into solid rock. These depressions result from the effects of thermal convection on the lithosphere and from the application of loads generated by a variety of processes, including mountain building and rifting. The process of building a mountain belt puts a heavy load at the surface and forms depressions on either side; rifting thins the lithosphere and forms a depression where dense material, usually consigned to a deeper position, is brought closer to the surface. Lighter or buoyant loads are locally associated with rifts in volcanic areas where hot or light rock is abundant near the surface. These buoyant loads, which correspond to elevations on the crust, are distributed sporadically along the lengths of active rift systems like that in East Africa. An environment in which basins are particularly well developed is the rifted Atlantic-type margins of continents where juxtaposition of dense oceanic lithosphere against less dense continent generates an elevation difference of several kilometers. This is the environment in which most active sedimentation on Earth is concentrated, much of it building the world's great deltas.

The thickness of material accumulating in sedimentary basins may become greatly amplified in comparison with the depth of the initial depression. For every unit of sedimentary load deposited at the surface, a corresponding mass of material, with a somewhat higher density, flows away in the underlying asthenosphere. In this way further subsidence is induced; the fill in sedimentary basins may become as thick as 15 km.

Petroleum source rocks are accumulations that are rich in organic matter. Most are shales, but siltstones, limestones, evaporites, algal mats, and even coals can be important. The essential characteristic likely to create petroleum resources is rapid accumulation of organic material—fast enough to exceed the oxidation of carbon due to bacterial decay. Typical environments are primarily aquatic—such high biological activity takes place in lakes, swamps, lagoons, bays, and estuaries. These anoxic, or nonoxidizing, conditions have developed episodically in deep ocean basins—at times and places of unusually high nutrient flux and limited bottom-water circulation. Restricted ocean basins—those just beginning to open (such as the Red Sea), or nearing closure (like the modern Mediterranean), or left as remnants of former oceans (such as the Black Sea)—are strong candidates for anoxia. Through analogy, this recognition guides oil exploration to sites of ancient anoxic conditions. Climate exerts a strong control over the processes of nutrient flux and anoxia, so studying the distribution of organic-rich rocks contributes to an understanding of ancient climatic processes—and vice versa.

Buried organic material changes within sedimentary basins. Temperature and the duration of burial control complex modifications, called maturation, producing a diverse group of organic compounds. The most important compounds, for the purpose of forming fossil fuel resources, are the kerogens. Different kinds of plant matter yield different kerogens that, in turn, are capable of producing different proportions of oil and gas. The kinetics of the maturation process vary with the complex organic compounds of the specific kerogen involved, but generally oil is produced from less mature kerogens than gas. Some kerogens are specifically oil prone and some specifically gas prone from the beginning. According to their maturity, source rocks may be incapable of producing any fluid hydrocarbons at shallow depths, capable of producing oil at intermediate depths, and capable of producing gas at greater depths. At very great depths kerogens become overmature, having released all their mobile hydrocarbons, and eventually become hydrogen-free graphite.

In oil and gas exploration the optical properties of vitrinite, a dark glassy material occurring in most



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