461 



Figure 2. — Diagrammatic sketch showing typical oil and gas types of traps. (A) 

 Anticlinal trap showing gas accumulating in the crest of the structure with 

 oil and water occurring successively lower on the flanks of the anticline ; this 

 diagram also shows the gradational relationship usually present at the oil 

 and gas contact and at the oil and water contact. (B) Simple fault trap 

 showing gas and oil arrested in its migration up-dip by the presence of a fault 

 which has brought impervious shale opposite the porous and pervious sand- 

 stone which now forms the reservoir. (C) Stratigra phic traps where migration 

 of oil and gas is arrested in the left-hand pool by impervious shale uncon- 

 formably overlaying the porous reservoir sand, and in the right-hand pool by 

 porous sandstone gradually changing up-dip to an impervious shale facies. 

 (D) Oil and gas accumulating against impervious crystalline rock ; the crystal- 

 line rock may be an intrusive such as a volcanic plug or it may be an erosional 

 remnant around which later sediments have been deposited. 



[Figure adapted from "Oil, Gas and Helium in Arizona : Its Occurrence and Potential," 

 (Arizona Oil and Gas Conservation Commission, c. 1966).] 



