PETROLEUM RESOURCES AND INDUSTRIES 

 which are developed on the monocline or plunge 

 obliquely off it int9 the adjacent valley. In some 

 instances the synclines associated with these anti- 

 clines are also productive. In one case a dome is 

 developed on the flank of a monocline. The Coal- 

 inga anticline, the largest of the subsidiary struc- 

 tures, is 65 miles in length. The areas of concentra- 

 tion in the coastal fields are usually determined by 

 well-defined anticlines, or nodes or domes on long 

 and sometimes sinuous folds. 



Practically all of the wells secure their oil from 

 porous marine sedimentary sandstone. In rare 

 instances, notably in the Santa Maria district, a 

 portion of the oil comes from cracks and inter- 

 stices in fractured, hard, flinty shales. The oil 

 reservoirs are usually capped by hard blue or brown 

 shale or clay. Occasionally, as in the McKittrick 

 and Los Angeles fields, impervious beds, brought 

 into position by horizontal or oblique faulting, have 

 acted as an efficient barrier to the escape of the 

 oil, while in others the oil sands are sealed by the 

 asphaltic contents near the outcrop. 



The oil sands in the California fields are usually 

 associated with water sands, known as "top" or 

 "bottom" in accordance with their occurrence above 

 or below the oil sands. "Edge" water also occurs 

 in most 9f the fields and its entry into a well marks 

 the termination of the productivity of the well, as it 

 occurs in the same bed with the oil and follows it 

 up as the oil is removed from the stratum. The 

 problem of casing out the water from the oil sands 

 is one of the most serious confronting the operators 

 of the State, and great damage has been done, and is 

 now being done, by water which, through careless- 

 ness or ignorance, has been permitted to flow into 

 the productive sands, thus lowering the quality of 

 the oil and retarding or completely stopping their 

 production. Voluntary remedial measures are being 

 practiced throughout all of the fields and, with care, 

 much damage can be averted in the future. 



TECHNOLOGIC METHODS. The standard churn 

 drill or cable method and the rotary method of 

 drilling are both utilized in the California fields. 

 The former is by far the more common and is in 

 use in those fields in which the wells are shallow 

 or the formation hard. The standard method is also 

 more desirable for drilling test or "wild-cat" holes, 

 as the logs obtained by this method are more reli- 

 able than those obtained by the rotary. The rotary 

 is used in the moderately deep wells, 2000 to 3500 

 feet, when the formation is largely soft. A com- 

 bination of the two methods is often advantageous, 



81 



