A geological liquid consisting of hydrocarbons and relatively small amounts of sulfur, nitrogen, and oxygen in the form of derivatives of hydrocarbons. Crude is formed under pressure from residual plant matter accumulated in a swamp over a period of time. It may come from the earth with varying quantities of water, gas, and other organic matter. Some crudes are very volatile and others are very viscous liquids. Crude oil may contain such impurities as sulfur and hydrocarbons. These are compounds containing a variety of combinations of carbon with hydrogen. There are a very large number of these compounds and they form the basis of a number of petroleum products. These compounds may exist as gases or liquids or solids. A number of hydrocarbons are also contained in bituminous coal, oil shale and tar sands. Crude is generally classified into three basic types: paraffin base crude oils contain a high degree of paraffin wax and little or no asphalt. Besides wax, they also yield large amounts of high-grade lubricating oil. Asphalt base crudes contain large proportions of asphaltic matter, and mixed base crudes contain quantities of both paraffin wax and asphalt. This is why crude oils do not always look alike. Some are almost colorless or pitch black. Others can be amber, brown or green. They may flow like water or creep like molasses. Some crudes, containing relatively large amounts of sulfur and other mineral impurities, are called "sour" crudes. Others, having a fairly low sulfur content, are called "sweet" crudes.