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.
One of the sources of oil which is likely to become important in the future is oil shales,--that is, shales from which oil product can be extracted by distillation. These have already been referred to on previous pages. Such shales are now mined only in Scotland and in France to a relatively small extent, but there are immense reserves of these shales in various parts of the world which are likely to be drawn upon when commercial conditions require it. In the United States alone it is estimated that the oil shales are a potential source of oil in amounts far greater than all the natural petroleum of this hemisphere.[30] The solution of the problem of extraction of oil from shales is fairly well advanced technically, and the problem has now become principally one of cost. In order to recover any large amount of oil from this source, operations of stupendous magnitude, approximately on the scale of the coal industry, must be established. As long as there are sufficient supplies of oil concentrated by nature to be drawn upon, it is unlikely that oil shale will furnish any considerable percentage of the world's oil requirements. With the great increase in world demand for oil, however, which may very possibly outstrip the available annual supply in the future, and particularly with the increase in the United States demand relative to domestic supplies, exhaustive surveys of the situation are being made with a view to development of oil shales when warranted by market conditions.