OIL AND GAS 



485 



that saturated paraffins and aromatic hydrocarbons 

 are released from kerogen when heated and are 

 removable in solution in hot oil-field brine. Field 

 observations of the stages in the progressive dia- 

 genesis and metamorphism of coals and particulate 

 coaly matter contained in sediments buried to dif- 

 ferent depths, and from vi'hich hydrocarbons have 

 been expelled, provide one means for gauging that 

 potentially fluid hydrocarbons are released during 

 natural thermal diagenetic transformations of or- 

 ganic matter (White, 1915; Teichmuller, 1958; 

 Brooks, 1970) through a wide range of depths, from 

 probably less than 5,000 feet to almost certainly 

 more than 15,000 feet, depending upon local geo- 

 thermal gradients and duration of maturation con- 

 ditions. Such conclusions have been reached inde- 

 pendently by comparing presumably related reser- 

 voir and source rock hydrocarbon compositions 

 (Baker, E. G., 1962) coupled with observations of 

 depth-dependent variations in ratios of hydrocarbon 

 carbon to total noncarbonate organic carbon (Phi- 

 lippi, 1957 and 1965). They are in excellent quanti- 

 tative agreement also with some results of studies 

 of depth-dependent variations in gross composition 

 of dispersed organic and bituminous matter in non- 

 reservoir strata (Larskaya and Zhabrev, 1964; 

 Neruchev, 1964). Together with interpretations of 

 systematic relations observed among the distinctive 

 carbon isotopic compositions of natural organic sub- 

 stances (including natural gases, petroleum gases, 

 specific petroleum fractions, whole petroleums, solid 

 hydrocarbons, asphaltenes and tars, kerogen, coals, 

 and unaltered biogenic materials), these results sug- 

 gest that progressive low-temperature decomposi- 

 tion or thermal cracking of protopetroleum, com- 

 posed importantly of high molecular weight com- 

 pounds derived in substantial part from plant lipids, 

 accounts for all the chemical relationships thus far 

 observed (Silverman and Epstein, 1958; Silverman, 

 1964; Frank and Sackett, 1968; Vinogradov and 

 Galimov, 1970; among others). Further research 

 on the kerogen fraction of fine-grained interstitial 

 organic matter of sedimentary rocks is badly needed 

 to help clarify in very young and very old sedi- 

 mentary rocks both the shallow and deep limits of 

 depth (temperature) at which diagenesis makes 

 parents of organic precursors of fluid hydrocarbons. 

 Such research would probably be facilitated by ap- 

 plying electron spin resonance and infrared spec- 

 troscopic techniques in the search for additional 

 reliable compositional data, especially C/H ratios 

 (Marchand and others, 1969), that the coal studies 

 have suggested may be important temperature in- 

 dices. More research is also needed on the observ- 



able effects of later thermal alteration or destruction 

 of petroleum which accumulated during an earlier 

 episode of hydrocarbon release and migration in a 

 reservoir at shallow or moderate depth. Sufficient 

 evidence is available now to say that even mature 

 petroleum rich in paraffinic compounds of relatively 

 low molecular weight undergoes further thermal 

 cracking if subjected to sufficiently high tempera- 

 tures (Evans and others, 1971, p. 151-153) and 

 would break down ultimately to highly fugacious 

 methane on the one hand and solid graphitic carbon 

 on the other. These are also the ultimate products 

 of thermal degradation of the insoluble organic resi- 

 dues that remain in nonreservoir strata, or for that 

 matter of coal. 



The metalloporphyrins of many crude oils are not 

 only evidence of a biological origin for the petro- 

 leums, but they also set a limit on the temperature 

 at which the transformation from sedimentary or- 

 ganic detritus to petroleum constituents occurs. 

 Green porphyrin pigments, decay derivatives of 

 chlorophyll, are commonly present in fine-grained 

 marine and lake sediments. Through a series of 

 hydrogenation and decarboxylation steps, sediment 

 porphyrins may be diagenetically altered to metal- 

 loporphyrins containing vanadium or nickel such 

 as occur in petroleum. The thermal stability of such 

 compounds is such that they do not survive at tem- 

 peratures above 200°C (Welte, 1965, p. 2257), and 

 they thus set an upper limit on the temperature of 

 the transformation process responsible for their 

 presence in a petroleum. Unfortunately, there is no 

 certainty that all constituents of a crude oil origi- 

 nated at the same time or place or at the same 

 temperature. It would be a matter of some inter- 

 est, however, to attempt to determine the depth of 

 disappearance of metalloporphyrins from crude oils 

 of widely differing depths (but confined to particu- 

 lar stratigraphic horizons), as a means of deter- 

 mining whether or not the various crude oils repre- 

 sent fractions of one or more phases of petroleum 

 genesis. 



PETROLEUM MIGRATION AND ENTRAPMENT 



Once formed, fiuid hydrocarbons must be expelled 

 from the capillary pores of the fine-grained organic- 

 rich source rocks before migration, entrapment, and 

 accumulation can occur in shallower permeable res- 

 ervoir rocks. The expulsion mechanism is not well 

 understood, although it probably is complex. Ex- 

 pulsion may occur simply because of differential 

 capillary pressure exerted because of two-phase 

 fluid saturation of adjacent rocks of differing capil- 

 lary sizes (Washburne, 1915, especially p. 832-834; 



