22 



T. 0. Bo^icoyth — OatUiies of Oilfield Geology. 



be so it would still be difficult to discriminate between the fold and 

 the intrusion as to cause and effect). 



Cretaceous. Volcanic 



Neck. 

 Fig. 4. Oil accumulation due to igneous doming. 



In Mexico there are numerous volcanic necks which have burst 

 through Tertiary strata, raising them and producing a dome-like 

 structure around each core of igneous rock, and thus forming very 

 favourable conditions for the concentration of oil in the sands (Fig. 4). 



5. Inclined Strata intersected hy Dykes. — In the same area dykes 

 intersect the strata and sometimes act as dams to the oil ascending 

 an inclined sandstone bed (Fig. 5). 



Fig. 5. Oil accumulation in a sand where dammed by dykes. 

 6. Fault-sealed Oil-roch. — Often oil ascending along an inclined 

 porous bed is prevented from escaping at the surface by a fault whicli 

 has displaced the outcropping portion and substituted impervious 

 clays (Fig. 6). 



Fig. 6. Oil accumulation in porous beds dammed by a fault. 

 Sometimes a whole row of wells has been placed alongside such 

 a fault. A notable example is the Western Coalinga Oilfield, Los 

 Angeles, California, but the case is quite common in many fields in 

 combination with other structures, and many a big well may owe 

 its success to the sealing effect of a small fault. 



