HU, \\liiil\ laliT uduld li;i\(' lo be cased oil; well N' will ha\c missed 

 stratum AA Imt will lia\c a sjiood iirodiKtion from stratum liH; well 

 /. will havi' struck uas. i hi haps \ieldin<,' a spray of oil and us the <^as 

 becomes' exhausted the nil will lisc in the stratum and the gas well 

 will become an oil well. Well^ 1) and A will experience fortunes sim- 

 ilar to those of wells K and (' on the opposite side of the fold. 



SV\(I.I\.\L STRUCTURK 



When the formation contains no water the oil will sink down the 

 sloping; sides of adjacent anticlines and collect in the syncline or 

 basin-like depression between them; this condition is rlepicted in Fig. 

 2, in this instance the outcrop of the oil bearing stratum at CC, 

 would probably show a dry oil sand. It is to be observed that in Fig. 

 2 the formation is broken and faulted near the axis of the fold; this 

 structure would very likely occasion oil springs at points marked DI). 

 Synclinal deposits of petroleum are of rare occurrence for the oil 

 measures generally contain water. 



DOME STRUCTURE— CLOSELY RELATED TO ANTICLINE. 



( "losely allied to the anticline is. the dome structure. An anticline 

 i.- a fold in the rocks along a line from which the strata dip in oppo- 

 site directions; when this line of folding is so short that its length 

 only equals, or but little exceeds the breadth of the fold it is spoken 

 of as a dome. Figs. 3 and 4 are contour sketches showing the anti- 

 cline and the dome structures. The dip of the strata is indicated by 

 the direction of the arrows. It will be noted that in Fig. 3, the cen- 

 ter line or axis x^A is much longer than the breadth of the fold; in 

 Fig. 4, axis AA but little exceeds the breadth of the structure. In the 

 dome structure the strata dip away quite or almost from a common 

 center. Many folds may be correctly described as elongated domes. 

 Dome like structures are often formed by folds crossing one another. 



TERRACE STRUCTURE. 



Next of kin to the anticline and dome is the terrace formation, 

 Fig. 5. This is a fold in which the strata are all inclined in one direc- 

 tion. It is occasioned by a sudden increase in the angle of the dip. 

 As shown in Fig. 5, a deposit of oil and gas may accumulate in such 

 a structure, the oil and gas being driven upward by water through 

 the sandy strata until their progress is arrested by the flattened water 

 soaked sand at the top of the terrace. There may be a slight reversal 

 of the dip at the cro\vn of the fold. 



FAULTED STRUCTURES. 



Any impediment which stops the progress of water, gas and oil 

 through inclined porous strata may occasion remunerative accumula- 

 tions of oil and gas. In some instances faults are impediments which 

 produce oil pools. In Fig. 6, a gravity fault is represented as having 

 produced such a result. On the down throw side of the fault the con- 

 tinuation of stratum AB had dropped to point D leaving the broken 

 end of stratum AB abutting a stratum of impervious shale. The 



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