Crude-Oil Correlations and Their Role in Exploration

Abstract

Sufficient parameters for most petroleum correlations are provided by correlation indexes, which measure the relative amounts of paraffins, naphthenes, and aromatics in each distillation fraction, and by the sulfur and nitrogen contents of the oils. Such data are available from the United States Bureau of Mines for several thousand crude oils, and they provide a chemical profile of the entire petroleum. Correlation indexes are similar to computed ratios of hydrocarbons derived from more detailed analytical methods. There are geological examples and scientific reasons which are not in agreement with Barton's concept of the maturation of petroleums from aromatic to paraffinic oils with increasing age and burial depth, although A.P.I. gravities were found to increase and the sulfur content to decrease with depth in accordance with Barton's hypothesis. Mild subsurface thermal cracking can explain the sulfur and nitrogen decrease, because the residuum undergoes conversion mostly to gases with only small amounts of light liquids being formed. Mild cracking could cause the A.P.I. gravity to increase while the correlation index character remains almost unchanged. However, the character of extensively cracked deep-seated petroleums may be changed greatly. Geological examples exist which are inconsistent with the hypothesis that ligno-humic compounds from terrestrial sources are the principal suppliers of aromatics to petroleums. Lacking any reasonably large quantities of both naturally occurring aromatics and naphthenes, the writer concludes that these cyclic compounds are formed by conversion from other materials, which most likely are straight-carbon-chain compounds of lipids. The dehydrogenase activity of anaerobic bacteria appears to be the most important mechanism for the creation of naphthenes, aromatics, and asphaltenes from lipids, judging from geological examples. Anaerobic bacterial activity in source sediments appears to be related to the degree of aeration, and it is reflected mineralogically. Examples show that paraffinic crudes are related to oxidized source sediments, aromatic-asphaltic crudes are related to euxinic source sediments, and naphthenic crudes are intermediate in character. Nitrogen compounds in crudes are believed to have been derived solely from nitrogenous source materials, which most likely are proteins and amino acids. High nitrogen contents in crudes have been related empirically to high phosphate content in source rocks. This is consistent with Emery's observations in Recent sediments that nitrogen regeneration parallels phosphate regeneration. Nitrogen analyses provide an independent correlation parameter that is not related directly to the oxidation-reduction state of the source environment. Inspissation of crudes through leakage or surface seepage can explain the alteration of some shallow accumulations. Paraffins appear to have been selectively removed from these petroleums as the gases were expelled, which is in accord with the principle of homology. Thus the correlation indexes increase as the A.P.I. gravity decreases. Components of the residuum appear to be relatively unaffected, with the result that ratios between components such as sulfur and nitrogen remain essentially unchanged. Anaerobic bacteria do not appear to have caused any significant changes in petroleum after they have been expelled from source rocks, except where the oils have come in direct contact with large deposits of sulfate minerals. The effects of selective filtration, selective adsorption, and water washing on the correlation indexes of certain crudes were found to be small. Therefore, except for shallow, inspissated oils and deep, thermally altered oils and distillates, the correlation indexes of petroleums appear to be relatively free from post-diagenetic change. Several fields in Wyoming, Utah, and Colorado provide illustrations of the use of oil correlations to identify parent source rocks and to detect cross-formational migration. Correlations make it possible to determine whether certain formations have their own indigenous oil source, and they permit the recognition of major sources of oil. Correlations lead to the conclusion that the oils in Permo-Pennsylvanian Minnelusa reservoirs in eastern Wyoming were derived from Permian Phosphoria source rocks in western Wyoming, thus requiring regional pre-Laramide, or Mesozoic, migration to have occurred. Exceptionally low gas saturations in the original Phosphoria-source petroleums preclude the application of Gussow's principle of differential entrapment in this case. This concept of Mesozoic migration of Phosphoria-source oils through the Tensleep to the Minnelusa in eastern Wyoming may assist in tracing the migration of petroleum to favored areas or favored traps.