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