Stratigraphic, Structural, and Correlation Considerations 57 



5. Domal structures caused by igneous intrusions 

 V. Unconformities 

 VI. Lenticular sands (on structure) 

 VII. Crevices and cavities irrespective of other structure 



1. In limestones and dolomites 



2. In shales 



3. In igneous rocks 

 VIII. Structures due to faulting 



1. On upthrowTi and downthrown sides 



2. Overthrusts 



3. Fault blocks 



Wilson °^ in 1934 proposed a very logical and well-organized classi- 

 fication based on local deformation of strata, variation of rock porosity, 

 and combinations of the two. His classification is as follows: 



I. Closed reservoirs 



A. Reservoirs closed by local deformation of strata 



1. Reservoirs closed by folding 



a. Reservoirs in closed anticlines and domes 



b. Reservoirs in closed synclines and basins 



2. Reservoirs developed by offsetting of strata by faulting of 

 homoclinal structure 



3. Reservoirs defined by combinations of folding and faulting 



4. Reservoirs formed through disturbance of strata by intrusions 



a. Intrusions of salt 



b. Intrusions of igneous rock 



5. Reservoirs developed in fault and joint fissures and in crushed 

 zones 



B. Reservoirs closed because of varying porosity of rock. No de- 

 formation of strata required other than regional tilting 



1. Reservoirs in sandstone caused by lensing of sandstone or by 

 varying porosity in sandstone 



2. Lensing porous zones in limestones and dolomites 



3. Lensing porous zones in igneous and metamorphic rocks 



4. Reservoirs in truncated and sealed strata 



a. Closed by overlap of relatively impervious rock 



b. Closed by seal of viscous hydrocarbons 



C. Reservoirs closed by combination of folding and varying porosity 



D. Reservoirs closed by combination of faulting and varying porosity 

 II. Open reservoirs 



None of economic importance. 



Sanders ^^ proposed a broad subdivision of trap types: (1) structural 

 traps, (2) Stratigraphic traps, and (3) combinations of structural-strati- 

 graphic traps. Heroy ^^ recognized four groups of oil and gas traps: (1) 

 depositional traps, (2) diagenetic traps, (3) deformational traps and (4) 

 combination traps. 



In 1945 Wilhelm ^^ in a very worthy discussion, subdivided petroleum 



^'Wilson, W. B., Proposed Classification of Oil and Gas Reservoirs, Problems of Pet. Geol. (Tulsa, 

 Am. Assoc. Petroleum Geologists, pp. 433-445, 1934). 



^ Sanders, C. W., Stratigraphic Type Oil Fields and Proposed New Classification of Reservoir Traps, 

 Am. Assoc. Petroleum Geologists Bull., vol. 27, pp. 539-550, 1943. 



"8 Heroy, W. B., Petroleum Geology, Geology, 1888-1938, Fiftieth Anniversary Volume, Geol. Soc. 

 America Bull., pp. 534-539, 1941. 



°' Wilhelm, O., Classification of Petroleum Reservoirs, Am. Assoc. Petroleum Geologists Bull., vol. 

 29, pp. 1537-1579, 1945. 



