484 



UNITED STATES MINERAL RESOURCES 



p. 213 of 1968 translation) calculated the following 

 percentages of petroleum production volume accord- 

 ing to depth ranges for all fields. 



Production 



Depth (percent of 



(meters) (feet) total volume) 



<500 <1,640 6.9 



500-1,000 1,640-3,280 32.3 



1,000-1,500 3,280-4,920 26.1 



>1,500 >4,920 34.7 



Virtually two-thirds of all oil produced up to 1960 

 came from reservoirs shallower than 1,500 meters. 

 For the 45 North American oil fields recognized as 

 containing ultimate recoverable reserves of 500 

 million barrels or more (Moody and others, 1970, p. 

 11 and fig. 11) and comprising "35 percent of the 

 continent's total ultimate recovery," the average 

 reservoir depth peaks sharply at about 4,000 feet 

 (averaged on the basis of volume of ultimately 

 recoverable reserves), and almost no reservoirs lie 

 below 14,000 feet. These and like statistics, based 

 on a great deal of exploratory drilling and produc- 

 tion, lead one to conclude that most, if not all, com- 

 mercially producible crude oil reservoirs occur at 

 depths shallower than 20,000 feet and that more 

 than one-half of the ultimate petroleum production 

 of the world will come from depths shallower than 

 10,000 feet (and possibly from depths shallower 

 than 5,000 feet) . 



The reasons for such findings obviously are of 

 the greatest possible practical importance, and they 

 are to be found above all in the fact that total 

 porosities of clastic sedimentary rocks, including 

 reservoir sandstones, diminish regularly with in- 

 creasing depth because of gravitational consolida- 

 tion following burial (Athy, 1930; Hedberg, 1936; 

 Dallmus, 1958 ; Maxwell, 1964 ; Atwater and Miller, 

 1965; McCulloh, 1967). "This decrease in porosity 

 ... is the most important single factor controlling 

 the amount of oil or gas in place per unit volume of 

 sandstone reservoir rock" in the Tertiary strata of 

 the United States Gulf Coast region (Atwater and 

 Miller, 1965). Lithology, geologic age, and varia- 

 tions in geothermal regime are some of the compli- 

 cating factors, but notwithstanding these, porosi- 

 ties of most clastic rocks approach limiting values 

 close to 5 percent at depths of 10,000 to 20,000 

 feet, whereas average reservoir porosities in excess 

 of 30 percent are possible (and commonplace in 

 young sedimentary rocks) at depths shallower than 

 4,000 feet (McCulloh, 1967, fig. 4). Shallow depths, 

 high sandstone porosities, large reservoir volumes, 

 and large hydrocarbon reserves (and production) all 

 go together. Porosities of reservoir rocks of other 

 lithologies follow more complicated rules, and it 



would be of considerable interest to determine 

 whether or not petroleum from other than sandstone 

 reservoir rocks predominates in the fraction pro- 

 duced from depths greater than 10,000 feet. 



The data have been reviewed which show that 

 petroleum and natural gas are distributed extremely 

 unevenly through the earth's sedimentary mantle. 

 Most of the commercially producible volumes of 

 hydrocarbons that have been discovered to date 

 occur in Tertiary and Mesozoic marine strata ; more 

 than half occurs in Mesozoic and Tertiary reser- 

 voirs of the Persian Gulf region. Between one-half 

 and two-thirds of the petroleum occurs in reservoirs 

 less than 10,000 feet deep, and essentially all com- 

 mercial oil production is from depths shallower than 

 20,000 feet. Crude petroleum composition depends 

 statistically upon depth and geologic age and only 

 to a minor extent on original depositional environ- 

 ment. Shallow or young oil tends to be dense and 

 chemically complex and exhibits strong similarities 

 to altered biogenic precursors found in young non- 

 lithified sediments. Deep or old oil tends to be light, 

 chemically simple, and has a high content of low 

 molecular weight paraffinic compounds. Reservoirs 

 at depths of 20,000 feet and more contain gas only, 

 although there is a gradual downward transition 

 from deep accumulations of crude oil rich in dis- 

 solved "gas" and gasoline fractions, through reser- 

 voirs containing natural gas liquids (NGL), to 

 natural gas reservoirs in which only minor quanti- 

 ties of higher molecular weight compounds occur. 

 If meaningful prognostic predictions are to be made 

 of where and in what amounts producible accumu- 

 lations of petroleum and natural gas may occur, one 

 must recognize not only that the distribution of 

 fluid hydrocarbons in sedimentary rocks and basins 

 is extremely uneven (although subject to certain 

 broad general patterns), but a substantial under- 

 standing of the underlying physical and chemical 

 reasons for their uneven distribution is needed. We 

 must have an understanding of the nature of the 

 processes by which organic detritus interred in 

 sediment is transformed to protopetroleum and 

 thence to fluid hydrocarbons capable of migration, 

 and of the chemical and physical processes by which 

 such mobilized hydrocarbons are expelled from 

 source rocks and trapped during their migrations 

 to form commercial accumulations. 



FLUID HYDROCARBONS FROM 

 SEDIMENTARY ORGANIC DETRITUS 



Laboratory pyrolysis experiments (Hunt, 1962; 

 Abelson, 1964) and extraction experiments (Welte, 

 1966) performed on kerogen-bearing rock indicate 



