478 



UNITED STATES MINERAL RESOURCES 



nomically marginal operation of "stripper wells," of 

 "tight gas reservoirs," of secondary recovery 

 operations ? 



The ansvv'ers to these questions, and myriads like 

 them, are of the utmost concern or importance today 

 to business leaders, industrialists, economists, poli- 

 ticians, international strategists, and military plan- 

 ners. To an important extent, the answers stem 

 from geological, geophysical, and geostatistical re- 

 search and investigations. Whether such questions 

 are answered correctly or incorrectly, the life of 

 every citizen living in the United States in the 

 year 2000 will be affected profoundly by the actions 

 that are being taken now on the basis of the answers 

 available. The very long lead times required to dis- 

 cover and develop wholly new petroleum and natural 

 gas reserves requires that planning be done now to 

 fill needs anticipated or projected for the last decade 

 of the 20th century (Hartley, 1969; Harvey, 1970; 

 Moody, 1970). Even longer lead times are needed 

 to plan, finance, and conduct research into new and 

 improved methods of seeking the hard-to-find de- 

 posits of hydrocarbons that will be needed by future 

 generations. 



GEOLOGIC SETTING 



Petroleum ("rock oil"), and natural gas ("earth 

 gas") are mixtures or solutions of raw chemicals. 

 They are natural fluids composed predominantly of 

 compounds of hydrogen and carbon. The simplest 

 of these compounds is methane, the principal con- 

 stitutent of natural gases. More complex hydrocar- 

 bon compounds of higher molecular weights that 

 have been identified as constituents of petroleum 

 number in the many hundreds, but about half of the 

 weight of average crude oil consists of light hydro- 

 carbon compounds containing from three to 14 

 carbon atoms per molecule. Many semi-independent 

 lines of evidence indicate that these natural hydro- 

 carbon chemicals are of ultimately biological origin. 

 The most compelling evidence is the overwhelmingly 

 predominant tendency for both the fluid hydrocar- 

 bons and associated organic matter to occur in 

 sedimentary rocks, especially in those of marine and 

 lacustrine origin that were deposited rapidly in 

 basins or depositional centers. Posthumous organic 

 detritus interred in subaqueous sediment is the 

 starting material from which these scarce and valu- 

 able fugitive raw chemicals were derived. 



Hydrocarbon compounds are virtually ubiquitous 

 in differing amounts at and near the surface of the 

 earth and in the rocks and pore fluids beneath the 

 surface. Small quantities occur in living protoplasm 

 (from a few parts per million on a dry weight basis 



in most higher organized living matter to a few 

 thousand parts per million in some plankton and 

 microorganisms), mostly of compounds having 15 

 carbon atoms or more per molecule. Even in the 

 atmosphere, trace amounts occur near biological 

 sources, in coal mines, at oil and gas seepages, at 

 some volcanogenic vents, and in association with 

 dust particles. Small quantities make up part of the 

 few milligrams of dissolved organic substances per 

 liter of average sea water, and minute amounts 

 occur in some meteorites and in a few places even 

 within the crystals of coarse-grained igneous rocks. 

 But by far the greatest proportion of the hydro- 

 carbons in the earth occurs in sediments and sedi- 

 mentary rocks. 



Organic carbon and hydrocarbons are distributed 

 extremely unevenly in the earth's sedimentary 

 mantle. In Holocene sediments, they are generally 

 most abundant in marine silt and clay deposited 

 rapidly in basins or depressions on or at the edges 

 of continental shelves. They decline in abundance 

 as grain size increases and are least abundant in 

 coarse-grained sediments. They are present even in 

 nonmarine deposits, but at generally much lower 

 levels, and are abundantly present there only in 

 lacustrine deposits, especially those that are 

 hypersaline. 



The average content of organic carbon in Holo- 

 cene marine clastic sediments of continental deriva- 

 tion was long ago recognized to be about 1.4 per- 

 cent (Trask, 1937, p. 149 ; Trask, assisted by others, 

 1932, p. 249). Subsequent investigations confirm 

 this as a rough approximation but also suggest that 

 the complicating effects of basin size, continental 

 shelf width, basin morphology, and hydrodynamic 

 and hydrobiological features render any "average" 

 figure of little use (compare for example, Emery, 

 1960, tables 12 and 27, and Bordovskiy, 1965, p. 

 37-44). Hydrocarbons are associated with the or- 

 ganic matter of Holocene marine sediments in the 

 lipid (or "bitumen") fraction. According to Trask, 

 assisted by others (1932), the mean "bitumen" con- 

 tent of such sediments is 0.06 percent, and the data 

 of many authors show that the ether-extractable 

 content of individual samples, although quite vari- 

 able, is directly proportional to total organic car- 

 bon, therefore being generally highest in quickly 

 buried pelitic sediments. These early findings of 

 Trask have also been supported by subsequent in- 

 vestigations (Smith, 1956; Bordovskiy, 1965, p. 57- 

 68) but with the addition of such important details 

 as the facts that such "bitumens" consist of about 

 65 percent asphaltenes and about 10 percent oils 

 (0.005 to 0.015 percent of dry sediment) and that 



