Locating test wells

The subsurface structural relationships have been deciphered and a decision has been reached that conditions seem sufficiently favourable to justify the drilling of a test well, it will next be the task of the exploration geologist to select the most promising position for the well. In locating test wells the geologist will be primarily influenced by structural conditions.

Structure being the primary consideration, the geologist will strive to locate the test well as to secure the maximum advantages that the structure affords. In locating test wells on dome and anticlinal structure, the prospector should aim at penetrating the petroliferous stratum at its structural crest. Here nature’s forces are concentrated the gas pressure is greatest, and the possibilities of high and long-continued production are at their best. A well drilled in any other location, if unproductive, would still leave in doubt the presence and absence of oil in the formation.

During the drilling of a test well there are occurences that may be significant in indicating the proximity or presence of petroleum. Knowing and observing these occurances the geologist can advise those in charge of drilling operations concerning the prospects for further drilling. Even when the surface signs are unmistakable, the exidence to be obtained from the log of the well during the drilling stage will be of great value in checking previous estimates and also in the subsequent development of the field through the drilling of additional wells.

Oil-saturated sand or even traces of oil in the pulverized material from the well are, of course, direct evidence and usually justify a production test of the stratum from which they come. Flows of hydrocarbon gases from a drilling well are always favourable evidence, especially if they contain gasoline vapours or if they have a petroleum odour. Many sedimentary formations produce march gas/methane/which, if dry/ i.e. without hydrocarbons of higher molecular weight, is often formed in the absence of petroleum, and therefore it is not necessarily indicative of the presence of oil.

Aside from the more direct indications of petroleum mentioned above, it should be pointed out that mere alternation of stratified porous and impervious beds in horizons known to be petroliferous is in itself a favourable indication.