The Chemical Relationships Between Crude Oils and Their Source Rocks1
There is a need for a simple chemical or physical index for relating a crude oil to its source rock—those ancient sediments wherein the petroleum or its precursor was originally deposited. Such an index would be valuable for confirming, or disproving, possible relationships suggested by the lithology or stratigraphy of the formations. The ideal index would be based on quick and simple tests, so that a large number of samples could be easily evaluated.
Consequently, an exploratory study of the organic content of ancient sediments has been carried out. The geochemical approach used in this investigation was directed toward finding a relationship between the crude oil and the organic material extracted from its presumed source rock. The study has included 1. investigation of methods for extracting sufficient organic material from the sediments for subsequent study, and 2. comparative examinations of the source-rock extracts and related oils by physical and chemical means.
Solvent extraction of large batches of source rocks, by refluxing with a benzenemethanolacetone mixture, was chosen as the best method for isolating organic material without chemical alteration. The organic-carbon content of the sediments studied ranged from 0.20 to 5.5 weight per cent. Only fractional percentages of this organic matter could be extracted, the quantities ranging from 3 to 13 per cent of the total. The isolated extracts were dark brown or black, ranging in consistency from oil-like liquids to soft tar-like solids and hard brittle solids.
The results of numerous physical and chemical analyses on pairs of extracts and crude oils are summarized as follows
1. No unique property has yet been found to link all of the crude oil-source rock pairs examined. Possibly, although not probably, some of the geological pairings may be in error.
2. No agreement was found in physical properties among the oil-extract pairs. The source-rock extracts all contained much more oxygenated material than the corresponding oils, which affected such properties as density, refractive index, etc., when the totals, or when corresponding distillation fractions were examined. Even when chromatography was employed, no well-defined trends of physical-property relationships were found.
3. The most significant chemical correlation data appeared to be the similarity of aromatic-type distributions in corresponding molecular weight fractions of crude oil and extract. Four out of eight pairs examined displayed this similarity in two or more corresponding fractions. This procedure, however, is time consuming, requiring molecular distillation and chromatography followed by ultraviolet analysis. An empirical method for comparison of the infrared spectra correctly predicted the aromatic distribution relationships in six out of eight cases. The latter procedure, though not foolproof, might be useful as a screening procedure. Similarity of other hydrocarbon types was not found for the four matching pairs.