The application of this principle should be remembered when development work is being carried on in districts where folding obtains and where prospecting wells are being sunk to the deeper portions of known productive measures. In such cases, water sands containing traces of oil and gas may be encountered at the depth at which oil was to be expected and the futility of further prospecting in the immediate neighborhood becomes thereby demonstrated.
When strata have been disturbed and dislocated so that they are no longer completely continuous, they are said to be faulted. The plane of fracture, known as the fault plane, is rarely vertical but will incline, thus leaving one side above the other. Normal faults (Fig. 18a) are those in which the upper side, or hanging wall, has fallen to a relatively lower position than the foot-wall ; thrust faults (Fig. 18b) are those in which the reverse is the case and the hanging wall has been thrust forward and pushed upward against the sloping fault plane surface of the foot-wall ; these are more common than the former. Folding seldom exists without the presence of faults, varying in size from fractures of a few inches to displacements of thousands of feet. Their influence on the accumulation of petroleum is discovered in the field only with great difficulty in many localities, and seems to follow no set rule.
A popular misconception seems to be that faults are inimical to structure associated with the presence of oil and that where faults may be observed, the prospects of finding oil are remote. While it is quite true that where the country is much 'broken up,' that is to say, faulted to an extreme degree, the conditions are not favorable and the discovery of oil in a well drilled in such a , locality may prove the presence of petroleum for only a small surrounding area, yet it must be remembered that folding and faulting are the results of the same kinds of earth movements, and the two are usually associated.
The fractures or open spaces formed at the summits of anti clinal folds by faulting have in many cases, no doubt, provided space for the accumulation of vast quantities of oil. In other cases they have disturbed the measures to such an extent that they have lost such petroleum as may at one time have been contained therein. Such irregularities also tend to increase greatly the mechanical difficulty of drilling. Several well known examples exist where definite fault planes have been the sources of immense production. In such cases, as in the Ventura field, the direction of the fault plane when once ascertained determines the situation of the wells, which extend across the country narrow straight line. A frequent cause of monoclinal structure is the
faulting that occurs at the time folding is going on, because the strata lack the necessary flexibility to lend themselves to bending into the anticlinal form and become broken.
In the brief review that has been given of the development of structural forms, it should not he imagined that folds have the beautiful symmetry usually ascribed to them in sketches, or that they are always easily deciphered in the field. They usually have one side steeper than the other, the side having the greater dip being in the direction from which the pressure was applied that caused the folding. It will be seen (Fig. 19) that under such a condition a marked difference obtains as far as the petroleum development is concerned and that the gently sloping side will offer room for more wells at shallow depths than does the more steeply inclined flank of the anticline.
Folds may turn under and back again as shown in Fig. 20, in which case they are known as overturns ; they may, and usually do bend, and when the forces that have brought about the deforma tion of the strata have been applied from several different direc tions at different times the resulting structure and shapes may become exceedingly involved.
Frequently they will tend to flatten or broaden out in the direction of their strike. Or they may retain their folded structure but will dip as an entirety in the direction of the strike, in which case they are said to plunge. Either of the latter two examples may bring about the dome structure in which the measures dip away in all directions from some central point. Both from a theoretical standpoint, and from the results of actual developments of oil fields, the dome structure is seen to be the most favorable for the accumu lation of bodies of petroleum. When the oil measures are overlain by an impervious stratum, namely, the cap rock, that prevents further upward migration of the oil and gas, the conditions are ideal for their gathering towards the summit of the measures, and this type is found in some of the most famous and productive districts. Perfect domes, however, are rare and they are more often found with one axis longer than the other, with the axes bent, and frequently with no symmetry whatever as far as the relation of the dips to the axes is concerned.