486 



UNITED STATES MINERAL RESOURCES 



Hubbert, 1953, p. 1979; Gussow, 1954, p. 821). 

 Alternatively, hydrocarbons may dissolve in hot 

 aqueous pore waters under pressure, either directly 

 (Uspenskii, 1962, p. 1161; McAuliffe, 1966; Mein- 

 chein, 1959) or with the solubilizing assistance of 

 organic "soaps" (Baker, D. R., 1962). In either case, 

 porosity reduction because of gravitational com- 

 paction through continuing subsidence and loading 

 (sedimentation) tends to express the pore waters 

 and their dissolved or emulsified hydrocarbons from 

 the finer grained and less permeable source rocks 

 into coarser grained and more permeable (and por- 

 ous) carrier and reservoir beds (Athy, 1930; Hed- 

 berg, 1936; Uspenskii, 1962, p. 1163; PhiHpp and 

 others, 1963; Magara, 1969). Thermal recrystalli- 

 zation and dehydration (diagenesis or eometa- 

 morphism of clays or other hydrous mineral phases) 

 in source rocks depressed to depths of thousands of 

 meters provide further opportunities for flushing of 

 dissolved or suspended hydrocarbons (Burst, 1969 ; 

 Magara, 1969) into reservoir rocks or carrier beds. 

 The underground temperature gradient in sediments 

 and sedimentary rocks favors the "primary migra- 

 tion" of oil from source beds by stimulating dehy- 

 dration reactions in the clay minerals which tend 

 to decrease mineral adsorption capabilities in deeper 

 levels (Snarskiy, 1962), by providing a temperature- 

 dependent adsorption gradient favoring movement 

 of hydrocarbons from regions of higher to regions 

 of lower temperature (Watts, 1963), by stimulating 

 upward diffusion along the temperature gradient 

 (Watts, 1963; Witherspoon and Saraf, 1964), and 

 by establishing local inter-pore or inter-bed pressure 

 gradients because of the large volumetric expansion 

 of hydrocarbons, as compared to water or brine, 

 subjected to a temperature increase (McCulloh, 

 1967). 



In a subsiding depositional basin, a number of 

 processes all work hand in hand. These include 

 physical depression of petroleum source rocks, aug- 

 mentation of rock temperature, chemical transfor- 

 mation of organic precursors to hydrocarbon con- 

 stituents of petroleum and natural gas, loading and 

 gravitational rock compaction with attendant reduc- 

 tion of pore space, and the establishment of tem- 

 perature and pressure gradients along which ex- 

 pressed pore water and entrained or diff'used 

 mobile hydrocarbons tend to migrate toward the 

 surface. The greater the depth, the higher the tem- 

 perature and the simpler (molecularly) the mobile 

 (liquid or gaseous) hydrocarbon phases that are in 

 thermal equilibrium with the environment. The 

 greater the depth, the higher also the confining 

 pressure, and the greater the tendency of thermally 



energized (and potentially greatly expandable) mo- 

 bile hydrocarbon phases to escape to lower pressure 

 regions. No wonder that for many years courts of 

 law in the United States classed oil and gas as 

 "fugacious minerals" or "minerals ferae naturae" — 

 wild beast minerals (Hedberg, 1971, p. 15). 



"Primary migration" apparently can begin when 

 suitable source rocks have been depressed to a depth 

 of 500-600 meters or more (Philipp and others, 

 1963; Welte, 1965, p. 2263), although considerable 

 evidence suggests that about 1 kilometer may be a 

 more general upper limit (Welte, 1972, p. 122). 

 During basin subsidence, an oil source rock can 

 undergo progressive geochemical development dur- 

 ing which an evolutionary series of hydrocarbons 

 are generated under gradually higher temperature 

 conditions. "Primary migration" of these hydrocar- 

 bons proceeds at least as long as gravitational com- 

 paction continues to reduce source rock porosity and 

 diagenetic alteration of clays continues to release 

 unbound water for expulsion. "Primary migration" 

 merges into "secondary migration," in which hydro- 

 carbons expelled from fine-grained source rocks 

 begin to move more freely through coarser-grained 

 carrier beds (or fractures) having physical charac- 

 teristics of reservoir rocks. If permeable avenues 

 exist to permit the expressed fluids egress directly to 

 the surface, the hydrocarbons escape more-or-less di- 

 rectly. If, on the other hand, the carrier beds pro- 

 vide no direct or unimpeded avenue of escape to 

 the atmosphere, the ultimate dumping ground for 

 all mobilized hydrocarbons, trapping occurs in re- 

 gions of minimum pressure along potential paths 

 toward the surface, and petroleum or natural gas 

 accumulations are formed. Throughout the migra- 

 tion process, no matter what the mechanism, the 

 tendency for the migrating fluids is to attain the 

 most stable state (generally the lowest level of po- 

 tential energy) possible. This tendency is consistent 

 with the expulsion of fluid hydrocarbons from fine- 

 grained source rocks and with their migration 

 across strata or through carrier beds or fractures 

 toward the surface from deep regions of higher 

 temperature and greater confining pressure. The 

 tendency is also consistent with aggregation of hy- 

 drocarbons into accumulations in local regions of 

 lower pressure where they displace all but films of 

 the water in the rock pores; with long-continued 

 slow leakage of hydrocarbons (selectively the low- 

 molecular weight compounds) from large accumu- 

 lations; and with the well-known propensity for oil 

 and gas wells to "blow out" or "go wild," releasing 

 their "minerals ferae naturae" abruptly (and with 

 potential destructiveness) to the atmosphere. 



