Chemical and geologic evidence indicates a probable vegetable origin of much of the petroleum and natural gas produced in the fields of Pennsylvania, the oil coming from source beds which carry a rich micro-flora and the gas being in large part derived from coarse vegetable residua. Both oil and gas are believed to have been formed by a combination of bacterial decomposition and acid hydrolysis of the organic matter with subsequent chemical reactions in which the effect of catalytic elements was very important.
The fields are restricted to areas where marine reservoir rocks are present and where source rocks are in contact or closely associated with the productive sandstones. The writer believes that most of the oil and gas produced in the Pennsylvania fields originated in the source beds and moved into the reservoir rocks shortly after the close of the Mississippian period. Owing to the discontinuity or changing lithology of most of the productive sands and to the absence of faults a long range migration of the oil and gas through the reservoirs to the point of accumulation is rather improbable.
Four general types of accumulation have been recognized: (i) accumulations of intimate oil, gas, and water mixtures; (2) accumulations limited and controlled by the cementation of the reservoir rocks; (3) accumulations which are dependent on the permeability of the reservoir rocks; and (4) gravitational accumulations on anticlinal structures.
Although the occurrence of oil and gas in Pennsylvania conforms very closely to the carbon ratios of the coals, the writer can not subscribe to the theory that the hydrocarbons were generated by dynamic agencies.
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.