432 EXPLORATION GEOPHYSICS 



This nosing begins after the decrease in gradient just referred to and 

 continues on a strike of about S. 45° E. 



As might be expected, the third anomaly coincides almost exactly with 

 the Kettleman south dome. Also, as was true in the first instance, the 

 flattening in the gravity gradient between the second and the third gravity 

 features is in the structural saddle between the middle and south domes. 



Continuing still further along the structural trend, we find that con- 

 ditions change beyond the end of the third gravity anomaly. The one to 

 one correlation of gravity and structural highs which carried through three 

 townships no longer holds. There is a zone of decreased gravity gradient 

 in T. 26 S. R. 20 E. which farther to the southeast (along the axis) 

 develops into a very pronounced and elongated gravity minimum. This 

 rather narrow gravity depression extends into T. 27 S. R. 21 E. and fits 

 extremely well with the Lost Hills anticline. 



The close agreement between these gravity expressions and subsurface 

 geologic features (or oil fields) is even more strikingly illustrated by refer- 

 ence to Figure 262, which shows the residual gravity anomalies in the area 

 surveyed at a 0.50 milligal contour spacing. To obtain them, the reduced 

 gravity station values of Figure 260 are freed from the effect of the 

 regional gravity gradient. This gradient (as shown by Figure 260) is a 

 decrease in gravity from the northwest to the southeast along the entire 

 trend. The amount of the regional correction to be applied at each station 

 is determined by a special analysis of the gravity data. 



The positions of the 4 residual gravity anomalies (as presented in 

 Figure 262) fit quite closely the 3 Kettleman domes and the Lost Hills 

 anticline. In the cases of the middle and south domes the residual anomalies 

 show considerable gravity closure, which reflects like structural closure in 

 these two features. The residual gravity contours bring out certain details 

 not readily apparent from regionally uncorrected results of Figure 260. 

 For example, the subsurface structural divides between the domes, which 

 cause a flattening in the gradient, are shown as true saddles by the residual 

 gravity contours. 



No doubt there is a geological reason to explain the sudden change 

 from a gravity maximum representing a structural high at the south dome 

 to a gravity minimum representing a structural high at the Lost Hills. 

 This transition takes place over a distance of but a few miles, and it cannot 

 logically be a mere perversity of nature to plague geophysicists. An exam- 

 ination of the stratigraphy of the locality yields a reasonable answer. 



The geologic section at the Lost Hills shows a considerable thickness of 

 the (Miocene) Reef Ridge shales, which are punky and diatomaceous and 

 from samples show an average density of 0.9. They are about 900 feet 

 thick over the northern part of the Lost Hills structure, and their thickness 

 increases somewhat to the south. The steep flanks of this anticline cause a 

 large thickness of these relatively light shales to be present vertically below 



