This Book is intended primarily for students who have had the basic courses in geology. Geology enters into many of the problems of the geophysicist, the reservoir and production engineer, and the wildcatter, and it is hoped there is also something of interest here for each of these. The order of presentation is what to me seems most logical: first, and of most importance, the reservoir, with particular emphasis on the trap; next the reservoir conditions of temperature and pressure and the different reservoir fluids; then the speculative ideas on origin, migration, and accumulation; and finally some of the ways of applying what has been considered in the search for new pools and provinces. In the search for new pools, we must remember, most of the ideas that concern the geology of petroleum are translated sooner or later into economic values, for they enter into one's judgment on many questions in exploration, such as whether or not to lease, to drill, to test, or to abandon. The book therefore includes some of the practical applications, along with the theoretical analyses, of the geological elements involved in petroleum exploration. We are learning that the geology of petroleum is essentially the geology of fluids. Geologic concepts enter into each phase of the operation more during the prospecting, it is true, than during the development; but even in the later period many operators continue to use geologic concepts until the pool is completely developed some, indeed, into the period of secondary recovery. The modern petroleum geologist must know something, therefore, about the reservoir fluids, the forces involved, and the manner in which petroleum is developed and produced.