Origin and Evolution of Petroleum
The organic theory of the origin of petroleum is so generally accepted by geologists that it has been deemed unnecessary to review the various theories of inorganic origin which have been proposed, or to discuss the difficulties of applying these theories to the formation of our commercial petroleum deposits. The following brief statement by David White, 1 presents the fundamental concepts of the origin of petroleum deposits, now almost universally held.
Opinions differ as to the origin of petroleum, and several theories have their following. The problem of the genesis of the oil itself is the most fundamental and important of the many problems of petroleum geology.
Most American geologists are of the belief that oil is derived from organic matter deposited in sediments as slimes (sapropels), oozes, or water plant débris, together with animal matter, in relatively tranquil or stagnant water and now lithified into shales or limestones. The vestiges of plant remains recognized under the microscope consist mainly of algal filaments, one-celled micro-algae, covering of pollen grains, spore envelopes, coloring matter from plant and animal cells, fragments of woody cell walls, chitinous débris, wax, resin fragments, et cetera. Of these, algae appear the most important. Animal products resulting from smothered decomposition of the organic débris most probably enter into the organic colloids in the sedimentary deposit. Diatom cases are very abundant in shales associated with certain oil deposits, especially in California. It is believed by most geologists that the deposition of source rocks of oil took place in