Miscellaneous Subsurface Methods 6iJ9 



recorded on electrodes two and three with respect to the No. 1 curve. The 

 last three columns give the amount and direction of the dip computed 

 from the previous data. 



Selection of Levels 



Widespread experience indicates that the accuracy of a dipmeter sur- 

 vey depends principally upon a careful selection of the zones in a well 

 over which the measurements are made. It is obvious, too, that numerous 

 dip determinations made at relatively close intervals in a well will more 

 clearly define structural and stratigraphic conditions than a few randomly 

 spaced levels. 



After it has been decided at what depth positions dip determinations 

 are needed, a zone or level from 25 to 50 feet in length is chosen nearby 

 for recording the three curves. Each level of such length thus provides 

 a number of bedding surfaces on which a dip determination is made; if 

 the dip measurement is made on but one contact surface, a freak dip di- 

 rection due to minor bedding irregularities might be considered as rep- 

 resenting the true formational dip. 



It has been found that the most satisfactory zones for a dipmeter 

 level are those consisting of relatively thin beds, 2 or 3 to 10 feet thick, 

 having sharp contacts with adjacent formations. Such zones for a re- 

 sistivity-dipmeter level are thin limestones or resistive sandstones inter- 

 bedded with shale, for an SP-dipmeter level thin sands or sandstones in- 

 terbedded in shale. 



Thin shale or sandy shale beds, on the other hand, within thick sand 

 sections and thin shale beds or minor resistivity variations within a massive 

 limestone frequently give erratic dip determinations. Sometimes, too, 

 dipmeter measurements at the contact between thick sandstones or lime- 

 stones and the adjacent shales show abnormal results, although reliable 

 values frequently have been obtained at such contacts. The two composite 

 logs in figure 329 illustrate the selection of dipmeter levels. 



Application 



Dipmeter surveys provide data assisting in the solution of many 

 structural and stratigraphic problems encountered in exploratory and 

 pool development wells. The correct location of offset wells after one has 

 been drilled is a common problem. If the initial well is a wildcat and a 

 dry hole, it is necessary to know, first, whether the sediments are flat or 

 whether there is some evidence of structure, and, second, what the direc- 

 tion and amount of dip are. Similarly a thin oil reservoir may be found 

 overlain by unwanted gas or underlain by water. It is obvious that in the 

 first case additional wells should be downdip, whereas in the second other 

 wells should be updip. 



Well 2 in figure 330, drilled in the Gulf Coast, had noncommercial oil 

 shows in the top of the Frio at 6,222 feet. A dipmeter survey indicated 



