Petroleum and natural gas. These valuable illuminants and fuels are considered here because, although they are found in traces in older strata, it is in the Ordovician that they occur for the first time in large quantities. They range throughout later formations down to the most recent.
The oil horizons of California and Texas are Tertiary; those of Colorado, Cretaceous; those of West Virginia, Carboniferous; those of Pennsylvania, Kentucky, and Canada, Devonian; and the large field of Ohio and Indiana belongs to the Ordovician and higher systems.
Petroleum and natural gas, wherever found, have probably originated from the decay of organic matter when buried in sedimentary deposits, just as at present in swampy places the hydrogen and carbon of decaying vegetation combine to form marsh gas. The light and heat of these hydrocarbons we may think of, therefore, as a gift to the civilized life of our race from the humble organisms, both animal and vegetable, of the remote past, whose remains were entombed in the sediments of the Ordovician and later geological ages.
Fig. 277. Diagram Illustrating the Conditions of Accumulation of Oil and Gas
a, source; b, reservoir; c, cover. What would be the result of boring to the reservoir rock at d? at d´? at d´´?
Petroleum is very widely disseminated throughout the stratified rocks. Certain limestones are visibly greasy with it, and others give off its characteristic fetid odor when struck with a hammer. Many shales are bituminous, and some are so highly charged that small flakes may be lighted like tapers, and several gallons of oil to the ton may be obtained by distillation.
But oil and gas are found in paying quantities only when certain conditions meet:
1. A source below, usually a bituminous shale, from whose organic matter they have been derived by slow change.