OIL AND GAS 



483 



the expense of the percentage of Paleozoic oil and 

 gas because of the volumetric predominance of 

 Cenozoic and Mesozoic marine strata beneath all 

 U.S. continental shelves (Weeks, 1971, p. 99 ; Gilluly 

 and others, 1970, especially p. 368). The fivefold 

 larger average volume of geologically very young 

 pools than pools in most Mesozoic and Paleozoic 

 rocks (Hopkins, 1950, fig. 4) is of importance in 

 relation to this fact. 



Not only the relative amounts, but also to some 

 degree the chemical and physical characteristics of 

 crude oils depend systematically upon the geologic 

 age of the rocks in vi^hich they occur. Statistical 

 comparisons of crude oil properties from U.S. fields 

 (and a fevi^ Venezuelan and Canadian fields) over 

 a vs^ide range of geologic ages and depths (Hopkins, 

 1950, fig. 1; McNab and others, 1952; Biederman, 

 1965) show beyond doubt that petroleums of low 

 density occur with greater frequency in Paleozoic 

 rocks and that those of high density make up the 

 greatest proportion of crudes in Tertiary rocks. 

 Furthermore, the older crude oils show a clear ten- 

 dency toward a higher percentage content of low 

 molecular weight fractions and a low content of 

 oxygen- and nitrogen-containing materials. The 

 young crude oils contain appreciable amounts of 

 oxygen- and nitrogen-containing compounds, more 

 like the organic precursors from which they evolved. 

 Evidently, a progressive time-dependent evolution 

 or maturation occurs from dense crude oils rich in 

 large cyclic compounds, forming early and tending 

 to occur in young sedimentary rocks, to less dense 

 crude oils composed of more paraffinic compounds 

 of lower molecular weights and more characteristic 

 of older rocks. Perhaps this is why 50 percent of 

 the hydrocarbons reservoired in U.S. "giant" fields 

 of Paleozoic age are gas (Halbouty and others, 

 1970, p. 532). However, hydrocarbon composition 

 is influenced strongly not only by the length of time 

 it has had to mature, but also by the depth (tem- 

 perature) to which it has been subjected, and to 

 some extent by compositional characteristics of the 

 source materials (Martin and others, 1963; Dean 

 and Whitehead, 1964; Hedberg, 1968) ; thus, simpli- 

 fied interpretations of compositional data are gen- 

 erally risky and equivocal. 



The influence of depth of occurrence on hydro- 

 carbon composition has long been recognized 

 (White, 1915; Barton, 1934; Hopkins, 1950; McNab 

 and others, 1952; Biederman, 1965; Landes, 1967). 

 Hudson (1963, p. 133) emphatically underscored the 

 empirical fact that commercial quantities of crude 

 oil do not occur in the United States at depths 

 greater than about 20,000 feet and are unlikely to 



occur deeper than about 16,000 feet. Phase rela- 

 tions among the compounds of petroleum and natu- 

 ral gas indicate that only the simple hydrocarbon 

 compounds in a gaseous phase can exist at tempera- 

 tures and pressures prevalent at depths of about 

 20,000 feet or more (Halbouty and others, 1970, p. 

 533). At shallower depths (and lower temperatures) 

 than those beyond which hydrocarbons do not per- 

 sist as liquid petroleums, other depth-dependent 

 variations in petroleum composition occur. Specific 

 gravity of deep crude oil is statistically lower than 

 that of shallow oil of the same age, and the deeper 

 crude oil contains higher proportions of lower 

 molecular weight compounds (McNab and others, 

 1952, p. 2558-2560). Thus, it appears clear that the 

 higher temperatures imposed by great depth of 

 burial affect petroleum maturation and hydrocarbon 

 alteration in much the same way as do extended 

 periods of time. This in turn poses several questions 

 of fundamental and practical importance, such as: 

 Through what depth range are earth temperatures 

 appropriate to cause thermal disintegration of the 

 disseminated organic matter in sedimentary rocks 

 to form liquid and gaseous hydrocarbon compounds? 

 How do such thermal disintegration processes affect 

 the residual organic matter (both the kerogen and 

 the solvent-soluble fraction) left behind dispersed 

 through the rock? By what mechanisms do newly- 

 formed fluid hydrocarbons migrate from minute 

 interstitial capillary openings in fine-grained rocks 

 into units permeable enough to serve as reservoir 

 rocks ? What is the fate of relatively immature dense 

 petroleum, containing minor proportions of low 

 molecular weight compounds, if subjected to ele- 

 vated temperatures attendant upon subsidence and 

 deep burial of an initially shallow accumulation? 

 A very large amount of work has been done in the 

 search for answers to these questions and still only 

 partial answers exist. Let us defer further consid- 

 eration of the questions until more of the pertinent 

 facts have been summarized. 



Just as petroleum quality (composition) is dis- 

 tinctively dependent upon reservoir depth so, for 

 discovered hydrocarbons, is petroleum quantity. In 

 his pioneer treatment of the exceptional importance 

 of exceptionally large hydrocarbon accumulations, 

 Heald (1950, p. 19-20) explicitly recognized that 

 most petroleum and gas occurs in reservoirs at 

 depths shallower than 10,000 feet. He stated, "If 

 history is a dependable guide, the existence of pro- 

 ducible oil or gas at a depth of not more than 6,500 

 feet, is almost a prerequisite if a major field is to 

 be opened." On the basis of worldwide production 

 statistics available through 1960, Radchenko (1965, 



