|a hydrocarbon mixture, heavier than water, distilled from coal tar|
crude oils below 20 API gravity are usually considered to be heavy. The lighter conventional crudes are often waterflooded to enhance recovery. The injection of water into the reservoir helps to maintain reservoir pressure and displace the oil toward the production wells. In general, waterflooding is most effective with light crude oils of API gravity 25 and higher and becomes progressively less effective with oils below 25 API. With crudes of 20 and lower, waterfloods are essentially ineffective and thermal recovery becomes necessary. Very few thermal projects are successful in recovering oil of less than 10 API gravity. Heavy crude oils have enough mobility that, given time, they will be producible through a well bore in response to thermal recovery methods. Tar sands contain immobile bitumen that will not flow into a well bore even under thermal stimulation. The recovery of these resources requires mining.
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