It is suggested that explanations of the composition of petroleums and the method of their formation must be consistent with the limiting physical conditions shown by a study of producing fields and geological conditions. Of these conditions, relatively low temperatures, usually not exceeding 140°F., are most significant. There is a complete gap in our knowledge, particularly about chemical changes, between the contemporary sediments and the organic matter contained in them, and the heavy asphaltic and naphthenic petroleums which probably are petroleums in an early transition stage. Both geological and chemical evidence is in agreement and consistent with a relatively very low-temperature history for all petroleums.

The conclusions of D. C. Barton from a study of Gulf Coast oils published in 1934 that heavy asphaltic naphthenic oils, containing little or no gasoline, change with age and depth to lighter oils containing more light constituents and less heavy residue, are extremely important in view of this low-temperature history. The data on which Barton based his conclusions have been re-examined to include oils produced from newer fields and the trend of the changes noted by Barton has been confirmed and is discussed in a later paper. The variations in composition, however, for oils of the same age and depth are considerable. The conditions bringing about such changes are considered to be primarily the catalytic effects of the minerals in the formations with which the crude oil has been in cont ct, together with the relatively low temperatures for varying geologic ages. These conditions would require considerable variation from a simple linear function of age and depth since there is a wide variation in the catalytic effect of different minerals in producing chemical changes. Differences in the temperature gradients or bottom-hole temperatures also probably account for some of the variations in composition of oils of the same age and similar depth in different fields. Active surface minerals include most clays and sands containing clay, but such catalytic action has been noted with minerals other than clay. It is believed that the purer limestones and dolomites have little or no such catalytic action and that oils undergo substantially no change after migrating into such reserv ir rocks. This is believed to be the explanation of many cases in which heavy oils containing little or no light constituents are found in limestone reservoirs, although of relatively great age and at depths greater than overlying formations containing younger but lighter oils.

The chemical complexity of petroleums is also best explained by the catalytic effect of active-surface minerals. The absence of olefins and the presence of aromatics is also indicative of such catalytic action. As measured by the polymerization of unsaturated hydrocarbons, such catalytic activity of active-surface minerals is not at all limited to clays of the type of fuller's earth, but is common, although variable, in many sedimentary rocks, including sandstone and shales.