Ch. 31] SHORE-LINE TRENDS 573 



USES OF SUBSURFACE DATA 



All the methods of obtaining subsurface data, discussed above, were 

 developed in an effort to obtain the most precise information possible 

 on the rock formations penetrated in the drilling of a well. The prin- 

 cipal use of such data is to establish the lithologic sequence and then 

 correlate the different formations between wells and between pools. 

 This is followed by the construction of structure maps on key beds 

 above, below, or at the top of a producing formation. Atwill (1942, 

 p. 156) has illustrated a typical example of a structure section 

 which is reproduced in Fig. 6. This profile was drawn with a sea level 

 datum; thus, the three formations illustrated all dip to the east. The 

 Gatchell sand (lowest formation shown) thickens abruptly and rises 

 in the stratigraphic section in a seaward (east) direction from the 

 ancient shore line. This fact, together with the occurrence of very 

 carbonaceous to coaly material in the silt beds below the top of the 

 Green sand updip from the Gatchell, is indicative of an offshore bar 

 origin for the latter formation. It may be noted that the oil-water 

 and gas-oil contacts of this reservoir sand are essentially horizontal 

 and parallel. 



SHORE-LINE TRENDS 



It is essential to establish the principal shore-line trends at the outset 

 of a subsurface stratigraphic study. This can best be accomplished 

 by trial correlations of several well logs, whether they be of the 

 electric, radioactivity, sample, or drillers' type, or combinations of all 

 four types. The prime consideration is to select what appears to be a 

 genetic sequence of strata devoid of unconformities. Such a sequence 

 usually has a nearly uniform thickness and similar lithology in the 

 direction of shore-line trend. A trial profile laid out normal to this 

 trend usually thickens in a basinward direction. Three or four repre- 

 sentative well logs are usually sufficient for the purpose of establishing 

 principal direction of the shore line. Once a probable shore-line direc- 

 tion has been ascertained, it can be readily traced by the use of ad- 

 ditional logs selected from wells drilled in the direction of this line. 

 Changes in the trend of a shore line usually can be recognized by a 

 change in thickness of the stratigraphic interval being correlated. An 

 increase in thickness usually means that the trend of the shore line is 

 curved in a landward direction and, conversely, a decrease in thickness 

 of interval means that the shore line is curving in a seaward direction, 

 and a more seaward well log must be selected. Unconformities any- 



