SUGGESTIONS OF A NEW METHOD OF MAKING UNDERGROUND OBSERVATIONS

In the north‐central part of the State of Texas, where wells are being drilled to the depth of 4,000 feet and more in order to penetrate the Bend formation (Pennsylvanian), which is the chief oil‐bearing horizon in that part of the State, it has been found that the structure of the Bend and that of the underlying Ellenburger limestone (Ordovician) is quite generally the same. The plotting of underground structure by drillers’ logs or by observations on cuttings from the Bend is very difficult, for the reason that there are small variations in the rocks of the Bend, not only vertically but also horizontally, and that these small variations lie near the limit between shale and limestone. Whether a certain part of the formation is reported as shale or as limestone will in large part depend upon the judgment and experience of the driller, or other observer. It has been exceedingly difficult to trace a definite horizon in the Bend with any considerable certainty. Exposures of the Bend indicate also that there are horizontal changes in this formation. For those who use drillers’ logs in the making of structural maps, it has been found expedient to use the contact of the Bend and the underlying Ellenburger as the key horizon. Contours are drawn on the upper surface of the Ellenburger.