Oil and natural gas have been discovered in many different formations, at depths down to 2 miles below the earth’s surface, and in various types of geologic structure, but we do not yet know when or how these fluids came to be stored in the underground reservoirs where they are now found. Although we are practically certain that the oil and gas originated from organic material, as discussed in Part II of this volume, we do not know whether the process occurred within a comparatively limited geologic time, or over a much longer period, perhaps intermittently; nor do we know how the source material was originally distributed in the sediments. The many factors and questions involved in the migration and accumulation of these hydrocarbons constitute one of the major problems in petroleum geology. Several phases of this problem are discussed in the following seven papers.
Alex. W. McCoy and W. Ross Keyte present a comprehensive summary of the problem as a whole. They discuss the function of microorganisms; the factors of heat, pressure, and chemical action; source beds; circulation of subsurface waters; compaction of sediments; the oil-water contact; the time of accumulation, and other topics. Their treatment is largely impartial, although they are clearly in favor of the theory which proposes a local origin of oil closely associated with existing reservoirs. This paper forms an excellent introduction to the whole subject of migration and accumulation.
Frank R. Clark strongly advocates the theory that petroleum has originated essentially in place, and
Figures & Tables
The AAPG volumes of Structure of Typical American Oil Fields preceed this book, which was written as a sequel to those, and at first conceived as a third volume of the earlier work. This book is designed to review, modify and, if possible, clarify ideas with regard to the fundamental concepts of oil geology, utilizing, for this purpose, the material presented in the two earlier data-based volumes. To conform to the original standard set for it, this book has been kept relatively free from factual data and has been compiled rather as a summation, based upon the best available evidence, of present knowledge of the science. This volume does not include a discussion of the technique of field or laboratory geology, but does include papers divided into 7 parts: History; Origin and evolution of petroleum; Migration and accumulation of petroleum; Relations of petroleum accumulation to structure; Porosity, permeability, compaction; Oil-field waters; and Subsurface temperature gradients.