OIL AND GAS 



481 



duced profitably to the surface. The results of such 

 endeavors can be regarded in a sense as a rock- 

 hydrocarbon assessment of sorts; not of the total 

 hydrocarbon content of sedimentary rocks of all 

 types, but a sampling of that fraction of the total 

 organic matter which was mobilized suflSciently at 

 one time or another to have been expelled from its 

 source stratum and enabled to migrate through per- 

 meable pathways toward regions of lowered pres- 

 sure (ultimately the atmosphere), and to be trapped 

 along the way to accumulate more-or-less tempo- 

 rarily in a producible and findable reservoir. 



The industrial search for commercial hydrocar- 

 bons has not been a random search, except locally 

 to a minor degree. Therefore, the results of the 

 search cannot be treated in all regards as data 

 gathered randomly or according to some consciously 

 designed sampling scheme. For example, rocks at 

 great depth have been prospected much less thor- 

 oughly than rocks at shallower depths because of 

 economic considerations. (Shallow reservoirs have 

 been easier to find and cheaper to exploit, and shal- 

 low discoveries provided sufficient production to 

 serve the needs of the United States until recently.) 

 Similarly, reservoirs associated with structural 

 traps, especially anticlinal traps, have been subject 

 to a more diligent search than a variety of other 

 more subtly-trapped accumulations because explora- 

 tion techniques thus far available are best suited to 

 structural mapping. In attempting to use the sta- 

 tistical data resulting from the search for petroleum 

 and natural gas, limitations (or sampling bias) such 

 as these should be kept in mind. 



Limitations of another sort should be explicitly 

 recognized. To a very large extent, the most reliably 

 useful data available from drilling for hydrocarbons 

 are the records of the amounts and kinds of fluids 

 produced. These have been measured. Less reliable 

 and much more diflScult to interpret are estimates 

 of the producible "proved reserves," or of the 

 amounts of crude oil or natural gas "initially in 

 place" in a reservoir or field. Such estimates depend 

 not only upon interpretations of a variety of diffi- 

 cult geologic and engineering measurements, but 

 also in part upon assessments of variable economic 

 factors. "Cumulative production" plus "proved re- 

 serves" constitute that fraction that is known or 

 judged to be economically and physically extract- 

 able of the hydrocarbons "initially in place" in a 

 reservoir. Statistical data from the United States 

 indicate that currently an average of 30 percent of 

 the "oil in place" and 80 percent of the "gas in 

 place" are economically extractable (Moore, 1970a, 

 p. 133; Elkins, 1971). Keeping these limitations in 



mind, let us examine the data available from drill- 

 ing and production. 



Cumulative production of crude oil through 1971 

 was 264 billion barrels for the world and 97 billion 

 barrels from the United States. Proved reserves for 

 the world and the United States are 682 and 38 ^ 

 billion barrels, respectively (see fig. 61). World and 

 U.S. cumulative production for natural gas are 620 

 and 415 trillion cubic feet, respectively, and proved 

 reserves are 1,725 and 279 trillion cubic feet, re- 

 spectively. When dissected and examined in detail, 

 these figures reveal that the known commercially 

 producible hydrocarbons in the world and the United 

 States are distributed extremely unevenly, but ac- 

 cording to the following general and important 

 regularities. 



Of overriding importance is the fact that a very 

 large percentage of the total oil (and gas) occurs 

 in a very small percentage of all known accumula- 

 tions. More than 85 percent of the world's hydro- 

 carbon production plus reserves occur in less than 

 5 percent (238 fields) of all producing accumula- 

 tions. Even more remarkable, 65 percent of the 

 hydrocarbons (petroleum and gas) occursin slightly 

 over 1 percent of all fields — the 55 "supergiants" 

 (a billion barrels or a trillion cubic feet or more; 

 Halbouty and others, 1970; Klemme, 1971), and an 

 astounding 15 percent occurs in only two immense 

 accumulations in the Middle East region (Ghawar 

 field in Saudi Arabia and Burgan field in Kuwait). 

 In smaller areas similar conclusions hold. For ex- 

 ample, Halbouty (1968) showed that of the 22,898 

 oil and gas fields in the United States as of January 

 1, 1968, only 308 (1.33 percent) will yield about 60 

 percent of the presently estimated ultimately pro- 

 ducible hydrocarbons. The importance of this ten- 

 dency was first clearly stated by Heald (1950, p. 

 15). "If we are concerned with our future, we will 

 study the record of major discoveries, the distribu- 

 tion of major fields, the geological conditions under 

 which they exist, and will strive to recognize the 

 areas wherein major discoveries are now unlikely 

 or impossible as well as those in which major fields 

 may be." 



Almost equal in importance to the tendency for 

 a few accumulations to contain most of the pro- 

 ducible hydrocarbons is the tendency for those hy- 

 drocarbons to occur with greatest frequency and 

 abundance in geologically young rocks. Halbouty 

 and others (1970) provide the most recent data that 

 show how the world's known oil and gas are dis- 



^ Estimate by American Petroleum Institute. S. 

 Geological Survey, estimates U.S. proved 

 (oral commun., Feb. 9, 1973). 



. Schweinfurth, U.S. 

 at 45 billion barrels 



