292 



Sediments 



Mexico). Favoring production is the fact 

 that between the times of secretion of lipids 

 by phytoplankton and deposition of organic 

 matter in the topmost layers of sediments, 

 aromatic fractions and longer chain paraffin 

 fractions formed. The latter may occur by 

 hydrogenation of lipid molecules or par- 

 tial regeneration of proteins. Production of 

 methane and other gaseous hydrocarbons 

 also occurs after deposition. Aromatics and 

 gases evidently are products of diagenesis in 

 the sediments and may owe their origin to 

 breakdown of complex organic molecules by 

 radiation developed within the organic mat- 

 ter or associated mineral grains (Whitehead, 

 1950-1951; Burton and Sullivan, 1951). The 

 slight odd-even carbon atom preference of 

 phytoplankton lipids is accentuated in the 

 sediments but lost in petroleum as though 

 survival had been followed by diagenetic and 

 post-diagenetic changes. Liquid hydrocar- 

 bons that develop only deep in the sediment 

 must be products of diagenesis, and the proc- 

 ess continues for a far greater time than has 

 so far been reached at the bottoms of basin 

 cores. Chlorophyll of phytoplankton sur- 

 vives in the sediments and petroleum as 

 porphyrins, with the particular type changed 

 to pheophytin a, pheophorbide a, and others 

 in the sediments and to more complex ones 

 in petroleum. Thus porphyrins, as well as 

 the types of hydrocarbons, indicate survival 

 in forms modified by diagenesis. It appears 

 evident then that neither survival alone nor 

 production alone is sufficient to explain sat- 

 isfactorily the changes that have been ob- 

 served between phytoplankton, sediments, 

 and petroleum. Both survival and produc- 

 tion occur together in ways that are not yet 

 fully understood throughout the period that 

 organic debris sinks through the water col- 

 umn and is buried to depths of several meters 

 in the basin sediments, and they continue 

 even at greater depth (or time) where (or 

 when) their relative roles become further 

 confused by changes produced by migration. 

 When hquid hydrocarbons are present in 

 the sediments and the interstitial water of 

 the sediments becomes largely squeezed out 

 by compaction, it is reasonable to expect that 

 the waters will carry with them some of the 



liquid hydrocarbons. During their migra- 

 tion the liquid hydrocarbons must become 

 modified by at least the effects of natural 

 chromatography of adsorption on the clays 

 through which they filter. This migration 

 very much intensifies the difficulty of learn- 

 ing about the origin of petroleum — both 

 through the changes that occur en route and 

 through the uncertainty of identifying the 

 sites of the source and of the stages of change 

 in the migrated hydrocarbons. Problems of 

 establishing the causes and sequences of 

 changes in coal are relatively simple because 

 the coal does not migrate (Fig. 232). Quite 

 possibly the lipid fraction of organic matter 

 supplies the bulk of the petroleum compo- 

 nents, and so the organic materials remain- 

 ing in the source rocks are not a true meas- 

 ure of the source materials of petroleums. 

 Besides hydrocarbons, petroleum contains 

 many sulfur, nitrogen, and oxygen com- 

 pounds and metals. Study of these mate- 

 rials and of their origins and sources may 

 provide new information about petroleum 

 genesis as outlined by Stevens (1956). It 

 appears probable that much more will be 

 learned about petroleum genesis in the fu- 

 ture by such chemical studies than by geo- 

 logical ones, although evidence from either 

 disciphne must be reasonable in terms of the 

 other one. 



Many of the changes in organic matter 

 leading to petroleum occur at depths greater 

 than can be reached by cores of basin sedi- 

 ments, so it is of interest to examine the 

 Pliocene strata that fill Los Angeles Basin. 

 These Pliocene and Miocene marine sedi- 

 ments approach a thickness of 4 km and ac- 

 cording to Edwards (1951) have a volume of 

 about 8450 cu km. The basin itself has an 

 area of about 3750 sq km, some of which has 

 been folded, leaving a 2500 sq km area which 

 is flat at present (Table 5). The sediments 

 consist of shales interbedded with sands de- 

 posited by turbidity currents (Slosson, 1958; 

 Conrey, 1959), particularly for the Pliocene 

 part of the section. The sands are coarsest 

 and thickest in the northeastern part of the 

 basin, closely resembling the nature and dis- 

 tribution of sands in the adjacent but not yet 

 filled Santa Monica and San Pedro Basins 



