294 



Sediments 



at depth, originally had a higher content. 

 Emery and Rittenberg (1952) attempted to 

 estimate the original organic content on the 

 basis of the carbonate-nitrogen ratio, un- 

 aware that much of the carbonate in near- 

 shore basins had been reworked from older 

 sediments and thus was not deposited from 

 the same plankton source as the nitrogen. A 

 better method is to assume that the original 

 content of organic matter in the surface sedi- 

 ments was the same as the average for the 

 present Santa Monica and San Pedro Basins, 

 about 6. 1 per cent. This value is close to the 

 5 per cent estimated by Emery and Ritten- 

 berg. At a depth of about 3 meters the con- 

 tent of organic matter would typically be 

 reduced one-third to about 4 per cent. A 

 further loss to 2.6 per cent at great depth is 

 not unreasonable, especially in view of the 

 fact that the 2.6 per cent total organic mat- 

 ter is based on analyses of nitrogen, which 

 is lost faster than is organic carbon in the 

 sediments. 



Approximate figures for supply and depo- 

 sition of total organic matter (Table 29) can 

 be worked out for the area of Los Angeles 

 Basin, 3750 sq km, assuming that conditions 

 during the past were the same as those of the 

 present Santa Monica and San Pedro Basins. 

 Rates of deposition of porphyrins and hydro- 

 carbons in Los Angeles Basin were taken in 

 the same ratios to total organic matter as 

 they have in Santa Monica Basin (Table 14). 

 Up to 1952 the Los Angeles Basin oil fields 

 had produced 3.8 billion barrels of petroleum 

 and had 1.1 billion barrels of proved re- 

 serves, giving a total ultimate recovery of 4.9 

 billion barrels (Emery and Rittenberg, 1952). 

 About two-thirds, or 3.3 billion barrels, is 



from Lower Pliocene strata. If the latter 

 had a duration of 5 million years, the annual 

 production of exploitable petroleum would 

 have been 660 barrels, or 100 tons. Annual 

 amounts of hydrocarbons and porphyrins 

 required for this weight of petroleum are 

 based on reasonable figures for composition 

 of petroleum (Orr, Emery, and Grady, 

 1958). 



Although no great weight can be given 

 the estimates of Table 29, they do show 

 trends which probably are reliable. Only 

 about 6.4 per cent of the organic matter that 

 is produced by phytoplankton was de- 

 posited in the bottom sediments of Los An- 

 geles Basin. Somewhat less than half of 

 this, 2.8 per cent of production, still re- 

 mains in the shales, and only 0.005 per cent 

 is likely to be exploited as petroleum. Thus, 

 one barrel of petroleum required the growth 

 of organic matter equivalent to 19,000 bar- 

 rels. The hydrocarbons, of which petroleum 

 for the most part consists (about 80 per 

 cent), are more efficiently preserved, so that 

 1.3 per cent of the original production by 

 phytoplankton is recovered as petroleum. 

 Of course, we must recognize that this is 

 only a net figure and that some of the origi- 

 nal hydrocarbon molecules were lost and 

 replaced by others probably broken out of 

 more complex organic materials by bacte- 

 rial and abiological processes. Quite evi- 

 dently, chlorophyll is produced by phyto- 

 plankton in far greater quantity than its 

 derivatives are needed to satisfy the require- 

 ments of petroleum produced in Los Angeles 

 Basin. At present the annual production of 

 petroleum in Los Angeles Basin is about 

 100 million barrels per year, a rate roughly 



Table 29 



Production and Deposition of Organic Materials in Los Angeles Basin, tons per year 



