640 Subsurface Geologic Methods 



tions also are of much greater apparent thickness. This abrupt reversal 

 from west in wells 1 and 2 to east in well 3 represents a sharp asymmetric 

 fold or drag near a normal fault of large displacement. 



An interesting sidelight is the abnormally thick shale section BC in 

 well 2. This thickening represents duplication of section by a thrust fault 

 of small displacement, a phenomenon not uncommon in the area. 



Drag folding adjacent to a normal fault dipping in a direction op- 

 posite to formation dip may result in sharp dip reversals as noted above. 

 On the other hand, drag distortion near a normal fault dipping in the 

 same general direction as the bedding will cause only steepening of the 

 dip. Figure 337 illustrates the latter condition. 



The dipmeter survey in well 1 showed a 6° to 8° SE. to ESE. dip in 

 the upper section of the well, with a sharp increase to 15° ESE. at 

 about 6,900 feet. The correlation of well 1 with well 2, the latter a short 

 distance southeast of well 1, gave a small SE. dip above 6,000 feet in 

 agreement with the dipmeter results. Below this depth the section in well 

 2 shortened, indicating that the well was cut by a northwest-dipping normal 

 fault. The correlation between the two wells, shown in intervals A, B, 

 and C, was uniform then to a depth of 6,900 feet, where the section in 

 well 1 shortened. The correlation of zones E and F between the wells is 

 obvious with zone D in well 2 cut out of well 1. It is apparent that a 

 southeast-dipping normal fault cuts well 1 at about 6,900 feet in the 

 zone where the dipmeter level was recorded. The fault is indicated by 

 the steepening in dip due to drag distortion and is also verified by cor- 

 relation. 



Sometimes marked dip reversals are observed in a single well as 

 shown in figure 338. The dipmeter survey in well 1 shows dips of about 

 35° NE. above 3,000 feet; below, the dips are of the same magnitude or 

 greater to the west and southwest. The westerly dips in the lower part 

 of the well are normal for the area, as proved by dip surveys in other 

 wells; wells to the east, however, have steep east dips. This dip reversal 

 locates an anticlinal fold whose axial plane is possibly just east of the 

 well. A normal fault with large displacement dipping east, too, is known 

 to lie just east of the well. The east dip in the upper part of the well rep- 

 resents possibly an effect of the folding accentuated by drag near the fault. 



It is interesting to note that bed C, by correlation with a type section 

 shown in well 2, appears to be a reptition of C. This situation again indi- 

 cates a small thrust fault between C and C. 



Dipmeter results, in conjunction with seismic and geologic data, 

 have been utilized to determine a thrust fault of much greater displace- 

 ment. The electric log in figure 339 has a normal succession of beds to 

 approximately 5,800 feet. Formations below this depth are identified as 

 young beds definitely out of place for a normal stratigraphic sequence. 

 Bed A at about 7,200 feet is identified as a repetition of the same bed at 

 about 3,700 feet. This fact indicates thrust faulting. Other information too 



