Nitrogen and sulphur compounds are present in all crude oils and, in some instances, may represent the major portion of the crude. While many sulphur compounds in crude oils have been identified, or the structure of the sulphur-containing group determined, relatively little is known concerning the structures of the nitrogen-containing compounds. Organic nitrogen is present in abundance in plant and animal detritus accumulating in aquatic sediments. From current knowledge of the structures of nitrogen compounds formed by plants and animals and conditions for post-depositional degradation, predictions can be made concerning the type of nitrogen compounds likely to be present in crude oils. It is improbable, however, that the quantity and variety of sulphur compounds present in many crude oils could have been derived entirely from organo-sulphur compounds contained in the living source material. While derivatives of sulphur compounds synthesized by plants and animals may he present in crude oils, a larger portion of the sulphur compounds appearing in the crude must be formed after incorporation of the organic source material into the bottom sediments. Mechanisms proposed for the geochemical synthesis comprise reactions of plant--or animal-produced unsaturates and oxygen-nitrogen heterocyclics with sulphur or hydrogen sulphide formed by bacterial reduction of sulphate ion.

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