The continually rising standards of petroleum geoscience and oil search require an ever greater knowledge of the fundamental nature of petroleum, its origin, and its behavior in the rocks of the earth’s crust. In the early stages of the petroleum era, most oil men were not much concerned about the how, why, and when of oil and gas; all they cared to know was where. Those years are gone, and it is now more and more recognized that the how, why, and when of petroleum are critical keys to where it should be sought.