54 



T. 0. Bosivorth — Outlines of Oilfield Geology. 



situated for receiving and storing the oil, for this second formation 

 probably contains the necessary sandstones, and also all the uncon- 

 formable strata are likely to be more or less arnaceous where they 

 abut against the plane of unconformity, thus facilitating the transfer 

 of the oil from the rocks bt4ow (Fig. 9). Cases of this kind occur 

 in California in the Coalinga Oilfield, where oil is obtained from 

 sandstones in the Miocene which lies unconformably over the Eocene; 

 also in lloumania, where the Meotic formation transgresses over the 

 Saliferous. 



Oil-generating Strata. 

 Fig. 9. Oil accumulation in porous strata unconformable on oil-generating 



strata. 



9. Strata tipping unconformally from an old Shore-line. — This is 

 supposed to be the case in the Maikop Field, the oil having 

 accumulated in more or less lenticular sands abutting against a shore- 

 line of Cretaceous rocks. Such conditions are favourable to accumulation, 

 for the porous beds are gently inclined and are sealed up at their upper 

 ends (Fig. 10). 



Cretaceous. 

 Fig. 10. Oil accumulation in strata tipping unconformably from an old 



shore-line. 



10. Exceptional Circumstance. — Occasionally large quantities of oil 

 are obtained from fissures, fault-planes, planes of unconformity, 

 cavities, etc. 



B. Oil Accumulations depending on Denudation. — Outcropping 

 petroliferous strata have been referred to by practical men as 

 'monoclines' in contradistinction to anticlines, which at one time 

 were thought essential for the accumulation of oil. These ' monoclines ' 

 are often really limbs of denuded anticlines, and the term would be 



