METHODS OF LOCATING WELLS the examination of the geological conditions in the field, coupled with all available data, we may, by a systematic study, make the location for a prospect or an off-set well intelli gently. In a "wild-cat" territory the initial test should be made on the highest point of the best structure found, preferably a dome or level top anticline. The horizon sought for is determined by the examination of the columnar section made from observing the various outcrops and studying their suitability as oil and gas reservoirs. Alternating shales and sandstones are considered as an ideal section, especially if the shales predominate. Any information as to the convergence of the strata should be allowed for, as well as for the shifting of the anticlinal axis of the pro ducing horizon to one side of the axis of the surface structure, caused by the asymmetry of the anticline. It will be noticed that where the angles of dip on each side of the axis are different, there will be a shifting of the axis of all underlying strata and in the direction of the lesser dip. (Fig. t6.) The distance of the shift or heave may be determined if the approximate depth of the sand is known by plotting the structure to scale, both the surface and the sub-surface structures, and the distance may thus be scaled off. Otherwise trigonometric factors have to be employed.
If the initial well proves to be a successful oil well, the next location, to be placed in a favorable direction, should be along the strike of the rocks, so that the sand may be found at a cor respondingly same level as in the original well. Should the initial well prove to be a gas well, and the object of the search being oil, the location should be down the dip from the gas well.
The general trend of an oil pool is to be expected in the direc tion of the strike of the rocks, which will be changed only by lensing, and under such conditions the direction may be varied; such pools are known as streak pools. The direction of the major or long axes of other pools in the vicinity may throw some light on the pos3ible trend, as the conditions of neighboring pools may be similar. It is advisable to know whether a pool is found in a
well known sand, or in a patch of sand that has been struck unexpectedly. Such sands are known as stray sands, and may prove tc be of small extent, and may give rise to narrow pools, which are generally referred to as "sucker-rod" pools. Although this cannot be laid down as an iron clad rule, still stray sands need careful watching and their characteristics noted.
In a new territory where no chance is offered for the study of neighboring pools, the probable direction of the pools may be in the direction parallel to the inferred shore-line, at the time of the deposition of the "sand"; as the coarser particles were deposited near and parallel to the shore-line. Only by a geo logical study can the direction of such a shore line be determined; and its direction may be expected at right angles to the deepest water, which would be indicated by the thickening of the strata, changing from coarser to finer sediments and to limestones, which will be increasing in purity towards deeper water.* The general persistence and pressure after a field has been developed for some length of time is claimed to indicate the presence of oil or gas in near-by untested territory. This condi tion is indicated by the oil man's statement "that the well is being fed from some place the way the pressure stays up." Analysis of the gases as well as the water encountered in drilling are of importance. Dry gas generally indicates that it is far removed from the oil, or in some cases (in shallow sands) that no oil is present in the reservoir. The presence of gas without oil in a thick water-bearing formation greatly lessens the chances of oil being found in that formation; this will hold good if the structure is lower than the water table in that for mation in a certain synclinal basin. Oil may be expected at or above the water table, along the homoclinal dip. The big pools of Corning, Ohio, and Scio, Ohio, are so situated.