The accumulation of oil requires a favorable source, a reservoir, good seal-rock quality, and suitably timed thermal history and structuring. The accumulated oil, especially its light fractions, may be subsequently removed by hydrologically controlled processes such as water washing, biodegradation, and tilting of the oil-water contact. These processes are dependent on the climate. In regions that have become increasingly cold or dry during late Cenozoic time, low rainfall, low ground-water flow rates, and low input of nutrients and microorganisms have protected the oil; in warm or temperate rainy climates, high flow rates and high input of nutrients and microorganisms have led to partial or total removal of oil. Thus, most of the rich (>500 000 barrels/day) oil provinces on land are in cold or dry regions, where water is recharged in highlands that receive little rain (<500 mm/yr), such as Texas, Oklahoma, Wyoming, Alaska's North Slope, California, Algeria, Libya, Egypt, the Middle East, the Volga-Ural basin, and western Siberia. Where upland recharge areas are warm or temperate and rainy, as in the eastern United States, western Europe, sub-Saharan Africa, Brazil, India, and most of China, rich oil provinces on land (outside young deltas) are rare, and biodegradation is widespread.

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