Bulletin . er in further operations. The results mentioned show that in the important matter of cor-rectly finding suitable formation in thin strata to shut off water,rotary failures are undoubtedly due to discrepancies in measuringdepths in rotary holes with drill pipe, or to inaccurate logging of thedepths of formation changes, or to the impossibility of bailing thehole, during drilling, to identify oil or water bearing strata. Theforegoing comparison of rotary with cable-tool drilling has beengiven in considerable detail because shutting off water is one of themost important steps in drilli
Bulletin . er in further operations. The results mentioned show that in the important matter of cor-rectly finding suitable formation in thin strata to shut off water,rotary failures are undoubtedly due to discrepancies in measuringdepths in rotary holes with drill pipe, or to inaccurate logging of thedepths of formation changes, or to the impossibility of bailing thehole, during drilling, to identify oil or water bearing strata. Theforegoing comparison of rotary with cable-tool drilling has beengiven in considerable detail because shutting off water is one of themost important steps in drilling an oil well. Imbroee, A. Underground conditions in oil fields : Bull, i!>n, Bureau f Mines,1920, p. 67. CLASSIFICATION OF WELL-DRILLING METHODS. 69 Figure 5 shows stratigraphically equal parts of graphic logs oftwo wells drilled in a California field, one with cable and the otherwith rotary tools, and illustrates some of the comparisons made .8 • o — J3 O CO l_| 129 f_j m »:«:?» i. o 8 E « 6 o I 0) U i 1 §-35 8 g o CD a o o 6> <?\^ cap o o o qcoq <a a <o o /OejOU -Hqeo aiqeO 8 On XT:- ;: O B fa above. The rocks penetrated are alternating beds of sand, shale,gravel, conglomerate, and thin layers of silicified sandstone. The shale and bowlder bed, probably conglomerate, at depths of 2,187 ! 70 PROSPECTING AND TESTING FOR OIL AND GAS. to 2,225 feet, in the cable-tool log, A, is a formational marker and isimportant for correlation. It would be impossible to correlate thisstratum, or any other stratum, with formations shown in the rotarylog, B, as in a log the artificial can not be distinguished from thereal bowlders. This comparison is not exceptional. Plate II (p. 22) shows the use of a marker for correlating strata inseveral wells. The red rock marker correlated by the line A-Abecomes a guide for estimating probable depths of strata to and in-cluding the top of oil sands correlated by the line B-B. Obviously,dril
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