PETROLEUM GEOLOGY — LEVORSEN 231 



surface information for the reason that many of the discoveries of 

 the past, which have been attributed altogether to geophysical meth- 

 ods, would or could have been made by ordinary geological tech- 

 niques at a much lesser cost. 



Obviously, there is much yet to be learned from surface mapping. 

 One important factor is the availability of aerial photographs in 

 nearly all regions, which force into the discard the methods and 

 standards of accuracy that prevailed even a decade ago. Another 

 factor, which has been the cause of many failures of interpretation, 

 is the almost universal presence of unconformities. Wlien a surface 

 fold is projected through two or three unconformities, one or more 

 of which is associated with diverging thickness of sediments, the 

 expression of the fold at the surface has but a faint resemblance to 

 what is found at 5 or 10 thousand feet below it. A third factor that 

 applies to surface geology is that most of the early work was solely 

 concerned with a search for anticlines and other types of favorable 

 deformation. Many regions are therefore being reworked and the 

 detailed stratigraphic information, which was neglected in the first 

 surveys, is now being added. In my own experience, I well know 

 that the year I spent in mapping " structures " in the San Juan Basin 

 of New Mexico could be repeated to better advantage by mapping the 

 stratigraphy and its relation to possible oil accumulation. 



This brings us to another trend which is apparent, and that is a 

 growing awareness of the importance of geologic history in the search 

 for new oil fields and new oil provinces. It is expressed in the deep 

 interest of geologists in unconformities, overlaps, wedge belts of 

 porosity, paleogeology, lateral and vertical gradation of porosity and 

 permeability, facies changes, and in all of the varied phenomena of 

 sedimentation. We may think of the rocks between each regional 

 unconformity as being in layers of geology — each layer having its 

 own peculiarities of sedimentation, structure, stratigraphy, and value 

 as a potential oil-producing unit. In some regions, notably west 

 Texas and northern Louisiana, as many as four such layers are being 

 explored and each found productive. Many other developed areas 

 have shown two or three such separate and distinct layers of geology, 

 and we may, therefore, conclude that much of the unexplored terri- 

 tory of the United States, undeveloped because it was thought to be 

 uninteresting and monotonously uniform from a shallow structural 

 viewpoint, now takes on value, because it likewise may be underlain 

 by two or more additional layers of geology. Thus, the first clues 

 to the possibilities of a new region may not appear until one starts 

 exploring 5, 6, or 8 thousand feet below the surface, and thinking 

 through one, two, or three regional unconformities. The almost 

 infinite number of combinations of structure and stratigraphy that 

 have in the past been found to produce oil, if projected into the 



