ACCUMULATION AND MIGRATION OF THE OIL 681 



special cases of the occurrence along anticlines, for these monoclines are 

 essentially flanks of anticlines on which the Tertiary beds that contain 

 the oil are truncated and outcrop. Oil contained in these truncated beds 

 has, therefore, in the absence of an imperyious capping stratum, free 

 access to the surface, and theoretically may be forced out on the surface 

 and lost by evaporation. Practically, however, no such free avenue of 

 escape is available, for the California oils are easily oxidized to form a 

 heavy viscous material which quite effectually clogs the pores of the bed 

 (which acts as a reservoir) near its outcrop, and thus forms of it a tight 

 container for the oil. The oxidation of the oil and its consequent tarrifi- 

 cation is accomplished chiefly through the interaction of the oil and sul- 

 phate waters, which are the common surface waters in the California 

 flelds, and the subsequent addition of sulphur compounds to the oil. 



Migration of the Oil 



The migration of th^ oil that is now found in the productive fields has 

 been extensive, and the accumulations of it have been drawn not only 

 from the diatomaceous shale that lies within the geographic limits of the 

 productive field, but also by lateral migration from shale lying far beyond 

 those limits. In the valley fields, for example, the structures that contain 

 the oil are located about the borders of the San Joaquin Yalley — a great 

 structural trough — in which it is evident there are great masses of diato- 

 maceous shale buried beneatli the younger sedimentary rocks. These 

 shales in the center of the trough have, it is believed, contributed largely 

 to the accumulations of oil now occurring about the edge of the valley. 



It has frequently been urged that extensive lateral migration of heavy 

 viscous oil, such as that found in many of the California fields, through 

 fine-grained sedimentary rocks is inconceivable. But upon closer analysis 

 it does not appear to be quite so impossible after all ; for one must remem- 

 ber that the viscosity of these heavy oils varies tremendously with small 

 changes of temperature, and that at a temperature of about 200° Fahren- 

 heit they are hardly more viscous than the lightest Pennsylvania oils ; also 

 the petroleum now found in the California fields probably had a somewhat 

 higher temperature when it moved into the position it now occupies than 

 it has at present, and although this temperature may never have been so 

 high as 200° Fahrenheit, still it was high enough to lessen greatly the 

 viscosity. Finally, the petroleum has almost certainly been in contact 

 more or less of the time with sulphate waters, and has, therefore, been 

 liable to rei-tain changes, chiefly the formation of sulphur compounds 



