Analyses of 313 crude oils from Cambrian to Cretaceous formations were studied to correlate the geologic occurrence of these oils with such characteristics as composition by hydrocarbon type (aromatics, naphthenes, and paraffins), content of gasoline and gas oil (determined by distillation and refractometric methods), distillate yields and residuum, sulphur and nitrogen contents, and cloud points.

Five general categories, based on likenesses that may indicate a similar history, include most of the oils, but smaller groupings are also discussed.

Table AVERAGE VALUES FOR CATEGORIES

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Category I (Ellenburger and Simpson oils). The Simpson shale is considered as a likely source.

Category II (a few Ellenburger and Simpson oils, Fusselman, Devonian, Mississippian, Pennsylvanian, Wolfcamp, a few Yeso oils). Probable sources are dark basinal shales of Woodford, Mississippian, Pennsylvanian, and Wolfcamp age, commonly associated with unconformities.

Category III (Yeso and San Andres oils). These occur commonly now on the Northwest and Eastern shelves where sulphate content is high.

Category IV (Spraberry, Delaware Mountain, some Wolfcamp, and Yeso oils). These are relatively unaltered oils, associated with or derived from basinal shales.

Category V (San Andres, Grayburg, Queen, Seven Rivers, Yates, Rustler, Castile, Cretaceous oils). These oils appear altered; suggested reasons: reaction with sulphur, fresh-water leaching of volatile aromatics, microbial oxidation of wax.

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