OIL AND GAS 



487 



Entrapment of fluid hydrocarbons occurs wher- 

 ever their tendency to escape to the atmosphere is 

 interrupted or significantly impeded by any imper- 

 vious or relatively impermeable barrier having a 

 geometry suitable to hydrocarbon aggregation and 

 accumulation, rather than to diversion. Both static 

 (lithologic or mineralogic) and dynamic (hydrody- 

 namic) barriers are effective, and an almost infinite 

 variety of reservoir "traps" (McCullough, 1934) 

 occur, the classical example of which, the anticlinal 

 dome, consists of an upfolded (inverted cup-shaped) 

 stratum of a porous permeable reservoir rock capped 

 by a layer of shale, anhydrite, or other rock rela- 

 tively impermeable to penetration by or transmis- 

 sion of large hydrocarbon molecules. Petroleum or 

 natural gas constituents migrating gravitationally 

 up-dip along natural escape routes through the per- 

 meable stratum are diverted toward the locus of 

 minimum fluid pressure at the dome crest where they 

 tend to be trapped and aggregate to form an anti- 

 clinal "pool" or reservoir. No attempt is made here to 

 describe the myriads of other kinds of "traps" that 

 have been recognized (Levorsen, 1954, p. 138-282). 

 Any region of porous and permeable rock is a 

 "trap" if it is connected more or less directly with 

 pathways along which hydrocarbon fluids migrate 

 (or once migrated), either in aqueous solution or 

 as discrete intergranular filaments, droplets, glob- 

 ules, or slugs, and if it is enveloped or sealed up-dip 

 (downstream from a potential viewpoint) by a 

 barrier that prohibits hydrocarbon escape. When a 

 petroleum or natural gas accumulation is opened by 

 drilling, the fugacious hydrocarbon contents are per- 

 mitted to resume their journey to the atmosphere. 

 There they are ultimately oxidized, one way or 

 another, to carbon dioxide and water, thereby bring- 

 ing to completion the cycle that began (in some 

 instances hundreds of millions of years earlier) 

 with photosynthetic fixation of carbon in the or- 

 ganic compounds of living plants. The ultimate fate 

 of most hydrocarbon accumulations that are not 

 opened by drilling is to very slowly leak their con- 

 tents to the surface (hence the historical impor- 

 tance to petroleum exploration of surface seepages, 

 and the continuing efforts to understand surface 

 geochemical anomalies and to develop practical geo- 

 chemical prospecting methods (Pirson, 1970, p. 

 208-239; Armstrong and Heemstra, 1972)). Other 

 accumulations are opened by erosion at the surface 

 following tectonic uplift, oxidized gradually through 

 the actions of oxygenated waters circulating from 

 the surface, or degraded by bacterial decay (World 

 Oil, 1972). Doubtless the relative scarcity of geo- 

 logically ancient petroleums is partly due to their 



fugitive nature in relation to the abundant oppor- 

 tunities for escape through geologic time. Possibly 

 the fivefold to fifteenfold greater average volume of 

 Pliocene than older reservoirs in the United States 

 (Hopkins, 1950, fig. 4) is thus explainable. 



Figure 59 is an attempt to portray diagrammatic- 

 ally, but semiquantitatively, a few of the more con- 

 spicuous or diagnostic steps in the evolution of fluid 

 hydrocarbons (such as comprise petroleum and 

 natural gas) from organic debris interred in sedi- 

 ment and subjected to progressively higher tem- 

 peratures upon burial in a subsiding basin. 



PETROLEUM AND NATURAL GAS RESOURCES 



Oil and gas fields are effective hydrocarbon traps 

 that have been discovered through human resource- 

 fulness. In general, those traps that have been dis- 

 covered are those that are conspicuous (from the 

 viewpoint of the host of exploration techniques that 

 have been invented and employed), are of very large 

 size, or are located in regions that have undergone 

 thorough prospecting. Remaining to be discovered 

 are innumerable small reservoirs, large reservoirs 

 that are inconspicuous (when subjected to the ex- 

 ploration techniques available), and traps of all 

 sizes in regions that have not yet been prospected 

 thoroughly or at all. 



Who can say what the undiscovered resources of 

 petroleum and natural gas are of the world or the 

 United States? What are the incentives for seeking 

 still more sophisticated and effective methods of 

 exploring for undiscovered traps? In considering 

 these questions one must keep in mind that not all 

 subsurface hydrocarbon accumulations are worth 

 finding. For every accumulation that can be found 

 and produced economically, there are doubtless many 

 accumulations of such small size that the volumes 

 of hydrocarbons producible are insufficient to repay 

 the costs of finding (including as they do the very 

 great costs of drilling and completing the well, the 

 high costs of exploration, and of all other efforts 

 associated with such a technologic exercise). Under 

 these circumstances it is obvious that small shifts 

 in the supply/demand ratio for petroleum and 

 natural gas bring about substantial shifts in num- 

 bers of accumulations and volumes of hydrocarbons 

 from the paramarginal to the recoverable category. 

 Similar effects are brought about by incremental 

 improvements in exploration or exploitation tech- 

 nology. One must also keep in mind in considering 

 these questions that the purposeful discovery of 

 fluid hydrocarbons is a human endeavor, and one 

 in which people have not been long engaged. Very 

 large areas of the earth, especially portions of the 



