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Word Count: 280
There are three essentials in the creation of a crude oil field First a source rock whose geologic history allowed the formation of crude oil This usually is a fine-grained shale rich in organic matter Second migration of the oil from the source rock to a reservoir rock usually a sandstone or limestone thats thick and porous enough to hold a sizable accumulation of oil A reservoir rock thats only a few feet thick may be commercially producible if its at a relatively shallow depth and near other fields However to warrant the cost of producing in more challenging regions the Arctic North Slope for example the reservoir may have to be several hundred feet thick Third entrapment The earth is constantly creating irregular geologic structures through both sudden and gradual movements - earthquakes volcanic eruptions and erosion caused by wind and water Uplifted rock for example can result in domelike structures or arched folds called anticlines These often serve as receptacles for hydrocarbons The probability of discovering oil is greatest when such structures are formed near a source rock In addition an overlying impermeable rock must be present to seal the migrating oil in the structure The oldest oil-bearing rocks date back more than 600 million years the youngest about 1 million However most oil fields have been found in rocks between 10 million and 270 million years old Subsurface temperature which increases with depth is a critical factor in the creation of oil Petroleum hydrocarbons rarely are formed at temperatures less than 150 degrees Fahrenheit and generally are carbonized and destroyed at temperatures greater than 500 degrees Most hydrocarbons are found at
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