After the structural outlines of beds near the surface have been determined, all possible information should be used in projecting these structures downward to the oil-producing horizons. Where a number of wells have been previously drilled in the vicinity, examination of their records may indicate certain lateral variations in the thickness of the beds between the horizon which has been mapped and the producing horizon. The effect of such lateral variations may be either to accentuate the surface structure, or to cause it to disappear entirely and thus to indicate lack of favorable trapping conditions. The possibility of several oil-producing beds, at different depths—a not uncommon condition in many fields—should also be kept in mind.
As already indicated, anticlines are not always essential to make the necessary trapping conditions. In the Beaumont field of Texas, for instance, it has been shown that irregular primary deposition of sediments differing in porosity both vertically and horizontally allowed the oil to migrate upward irregularly along the porous beds and parts of beds, and to be trapped between the more impervious portions of the beds.
Further questions to be considered in the exploration of an area are the content of organic matter in the sediments which may have served as a source of oil, the presence of impervious cap-rocks or of variations in porosity sufficient to retain the oil, the thickness of sediments and the extent to which they have undergone differential stresses, the amount of erosion and the possibilities that oil, if formed, has escaped from the eroded edges of porous strata, and, where carbonaceous beds are present, their degree of carbonization, and many other similar matters.
Each field in fact has its own "habit," determined by the interaction of several geologic factors. This habit may be learned empirically. Geologists have often gone wrong in applying to a new district certain principles determined elsewhere, without sufficient consideration of the complexity and relative importance of the sundry geologic factors which in the aggregate determine the local habit of oil occurrence.
Geographically associated fields characterized by similarity of oil occurrence, age, and origin, are known as petroliferous provinces. The factors entering into the classification of fields are so numerous that more precise definition of a petroliferous province is hardly yet agreed upon.
The part played by the economic geologist in oil exploration and development is a large one for the obvious reasons given above. Probably no other single division of economic geology now employs so large a number of geologists. Practically no large oil company, or large piece of oil exploration and development, is now handled without geologic advice. Quoting from Arnold:[29]
It ought to be as obvious that exploration with the drill should be preceded by careful geologic studies as it is that railroad construction should be based on surveys. These studies should include such subjects as topography, stratigraphy, structure, and surface evidence of petroleum in the regions to be tested. The work divides itself into two stages—preliminary reconnaissances and detailed surveys.
The preliminary reconnaissance should consist in procuring all the available published and hearsay evidence regarding the occurrence of oil or gas seepages or hydrocarbon deposits in the region; in making preliminary geologic surveys to determine from which formations the oil is to come and the areal distribution of these formations; in determining those general regions in which the surface evidence is supposed to be most favorable for the accumulation of hydrocarbons; and in determining the best routes and methods of transportation.
The second stage includes detailed geologic surveys of those regions where the surface evidence indicates that petroleum is most likely to be found and the location of test holes at favorable points. By working out the surface distribution and structure of the formations it is usually possible to select the areas offering the best chances of success. Geology should always be the dominant factor in determining the location of test holes, although modifications to meet natural conditions must sometimes be made.