Even a casual study of the stratigraphic and structural conditions adjacent to producing oil and gas horizons quickly brings out the fact that many if not most producing zones are intimately associated with unconformities. As one of a group of papers inquiring into this phenomenon, this paper will deal with the oil and gas production of the Mid-Continent region which is associated with unconformities. The purpose is to present as many facts as possible in order that the extent of this relationship may be established and that the varying conditions under which it occurs may be determined.
The term “unconformity” as here used follows the definition recently used by Twenhofel. He states2
UNCONFORMITIES. An unconformity represents a break in the geologic sequence and is a surface of erosion or non-deposition separating two groups of strata. If the strata below an unconformity are not parallel to those above, it is a non-conformity. … An unconformity separating strata which are nearly parallel is a disconformity, and represents a break in the geologic sequence of formation value. A diastem indicates a break of less magnitude than a disconformity and is represented elsewhere by a part of a formation.
The time value of unconformity may range from that necessary to deposit a single formation, to such an enormous period of time as that represented by the unconformity between the pre-Cambrian and Pleistocene.
The reader is assumed to be familiar with the general stratigraphy of the Mid-Continent region and only such description of
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.