680 R. W. PACK OIL FIELDS OF THE PACIFIC COAST 



Cretaceous and the youngest in the upper Miocene. The sands yielding 

 oil occur either intercalated with these shales, or immediately above or 

 immediately helow them, and in general the best sands are those that 

 overlie the shales, especially if they rest unconformably upon the shale. 



There has been a great deal of speculation regarding the origin of oil in 

 different parts of the world, and even now there is by no means a general 

 agreement as to just the manner in w^hich it has come into being or from 

 what materials it has been formed; probably no rule will fit all cases; 

 but regarding the California fields the conclusion is unavoidable that the 

 oil has originated in the diatomaceous shales, and, as these shales differ 

 from the other shales in the general region only in the fact that they are 

 composed largely of the remains of minute organisms, it likewise seems 

 certain that it is from these organisms that the oil has been produced. 

 Were this not the case, and were the oil formed in some other place, some 

 explanation must be offered for the peculiar tendency of the oil to collect 

 in the sandy beds in and about these organic shales, and yet to leave no 

 trace in any of the sandy beds that occur in the thousands of feet of sedi- 

 mentary rocks that overlie or underlie the shales, when to all appearances 

 these sandy beds offer equally as attractive a reservoir for the collection 

 of oil as do the beds in which the oil is now found. 



Effect of geologic Structure on Accumulation of Oil 



Second in importance only to the materials from which the oil is 

 formed is the geologic structure, for the attitude of the beds determines 

 the area beneath which oil has accumulated. The structure of the south- 

 ern coast ranges of California is complicated, and the beds are both 

 sharply flexed and considerably fractured, yet, despite that fact, oil has 

 accumulated in many places and in many different types of structure. 

 However, when one considers the more productive fields, one is imme- 

 diately impressed with the fact that almost all these fields are located 

 along anticlines — not necessarily close along the axes of these folds, but 

 in areas where the folding has been distinctly upward and where the 

 anticlinal fold may be said to be the dominant feature. 



It is hardly pertinent here to enter into any discussion of the anticlinal 

 theory or any of the other theories regarding the accumulation of oil, yet 

 whatever have been the forces that caused the accumulation of oil in Cali- 

 fornia, or however they have worked, it is apparent that they resulted in 

 most cases in the collection of the oil in the anticlines rather than in any 

 other place. The occurrence of oil in monoclines may be regarded as but 



