32 EXPLORATION GEOPHYSICS 



The results of actual drilling have shown that the reflection seismic 

 method has mapped oil structures with required accuracy to depths of 

 approximately 15,000 feet, and current data indicate that reliable records 

 are oftentimes obtained below that depth. 



In common with the other methods, the reflection technique has inherent 

 limitations which hinder or prevent its application in certain areas, f These 

 limitations are especially pronounced: (1) in extensively faulted areas, (2) 

 in steeply dipping areas, (3) in areas which are covered by thick layers of 

 material having irregular velocities, such as usually encountered where 

 there exist several hundred feet of alluvium, glacial till, deep weathering, 

 etc., (4) in areas in which the reflecting properties of the subsurface for- 

 mations are so mediocre that difficulty is encountered in recognizing corre- 

 sponding reflections from the same formation throughout the area. 



Choice of Methods for Particular Geologic Problems. — The general 

 factors to be considered in selecting the proper geophysical methods for 

 exploration have already been discussed briefly. Perhaps the most im- 

 portant factors to be considered in particular cases are the geological and 

 terrain conditions of the area. Commercial accumulations of petroleum 

 are always associated with extensive areas of sedimentary rocks, and with 

 very few exceptions the oil-bearing rock is of sedimentary origin — most 

 commonly porous sand, sandstone, limestone, or fractured shale. Accumu- 

 lation of petroleum is governed by structural, stratigraphic, and lithologic 

 conditions. The essential requirements are: (1) petroleum "source beds," 

 (2) a porous and permeable reservoir rock such as sandstone to contain the 

 oil and gas, (3) an impervious overlying bed or cap rock, such as shale, 

 to prevent upward escape of the oil and gas, and (4) a favorable structural 

 or stratigraphic trap for the accumulation of oil and gas. The selection of 

 regions favorable for the first of these conditions is the responsibility of 

 petroleum geology. In the search for conditions fulfilling the last require- 

 ment, we are dependent upon petroleum geophysics. The presence or 

 absence of the second and the third requirements can be determined only 

 by actual drilling. 



The search for certain types of geologic features as favorable sites for 

 new oil fields is governed entirely by knowledge of the manner of occur- 

 rence of oil in existing fields throughout the world, and an understanding 

 of the geologic factors which control these occurrences. The general con- 

 ditions affecting oil accumulations have become well established since the 

 anticlinal theory! first achieved prominence some 80 years ago. Traps 

 which cause accumulation of important oil deposits have been discussed by 

 DeGolyer.§ "Geologically, the trap may be of structural, stratigraphic or 

 chemical origin. If of structural origin, it may be the result of folding, of 



t Report on symposium on "Seismograph Prospecting for Oil," A.I.M.E., Geophvsical Prospect- 

 ing, Tech. Pub. No. 1059, April, 1939. 



t For additional information see W. H. Emmons, The Geology of Petroleum (McGraw-Hill, 1931). 



§ E. DeGolyer, "The Development of the Art of Prospecting," The Guild of Brackctt Lectures 

 (Princeton Univ. Press). 



