THE SEARCH FOR OIL — LEES 239 



EXPLORATION METHODS 



I have already explained in general terms the geologic character- 

 istics of oil fields and I shall now describe briefly the principles and 

 working methods which lead up to the discovery of new oil fields. 



In the first instance a country to be investigated must be examined 

 in respect to its general geologic nature. Countries that are com- 

 posed exclusively of igneous or metamorphic rocks, that is to say, 

 of granite or schists or gneisses, have no possible hope of contain- 

 ing any oil, and attention is directed solely to areas having a cover 

 of sedimentary rocks (fig. 2). Tlie strata must have had a favorable 

 development for the original formation of oil, must contain suitable 

 porous rocks to act as oil reservoirs, and must be folded into a suit- 

 able pattern for the accumulation of important amounts of oil. Un- 

 fortunately, science has not yet progressed so far that we can with 

 certainty recognize primary oil-source rocks, but we can say that 

 certain groups of rocks may come into this category and that others 

 certainly do not; for this reason the oil geologist is greatly en- 

 couraged by outward indications of the actual, as distinct fi'om the 

 theoretical, existence of petroleum in the form of oil seepages, oil- 

 impregnated rocks, or of oil residues. 



A preliminary geologic survey investigates these various points 

 and, in the case of relatively unknown countries, it is usually neces- 

 sary to make a comprehensive geologic map at the outset and an 

 intensive study of structural and stratigraphic conditions must be 

 made. During the early stages of this exploration, aerial photog- 

 raphy has been used to great advantage during recent years. Not 

 only can topographic maps of suflBcient accuracy for the purposes 

 required be drawn from these photographs but, in addition, much 

 geologic information can be deduced from expert stereoscopic ex- 

 amination of pairs of photographs. Kock outcrops can be marked, 

 dip and strike determined, and faults detected; and the outline 

 geologic map thus produced is of great assistance to the subsequent 

 ground survey. In many instances, particularly where outcrops 

 are poor, structural deductions, which would be difficult or even 

 impossible to make on the ground, are possible from a study of 

 drainage pattern or of hill forms from the photographs. 



A geologic survey is dependent on sufficient rock outcrops, and 

 in many foothill zones in front of main mountain systems, par- 

 ticularly in arid countries, the solid geology is well exposed. But 

 sedimentary basins frequently fall in low-lying areas or alluvial 

 plains, and in this case the solid rock structure is completely obscured 

 from view by a mantle of alluvium, marsh, or tropical forest. When 

 this is the case the capacity of the geologist is limited and he has 

 to call on the assistance of the geophysicist for further help. By 



