Following a brief review of the historical development of oil and gas in the United States until the drilling of the Drake well, the more important opinions that have been expressed concerning the geology of oil and natural gas are presented. These include origin, migration, accumulation, reservoir traps, and their means of recognition. The progress of geologic thinking is traced through the main stages of its development, from the first theories pertaining to oil and gas occurrence, the technical advancements that have been made, and the effect of economic demand upon the employment of geologists by industry, to the present trends in thought and techniques.

Recent geologic thinking has been dominated by the recognition that reservoir traps could exist without dip reversals due to deformation, and could result from a lithologic change, giving a stratigraphic-type reservoir. Specializations in research were required which would give new and sufficient data concerning all phases of sedimentation to permit a reconstruction of depositional environment and deformational history--historical geology under the microscope. Discovery of new areas of production will require the application of existing geologic knowledge and techniques plus new geologic knowledge and techniques, and especially their increased coordination.