ORIGIN AND GEOLOGICAL OCCCRRENCE. The geo logical history of petroleum and natural gas are closely connected, so that what is said of one practically holds true of the other. Petroleum is always found in sedimentary rocks. For many years it was known only in sandstones or shales, and the term oil-sand was applied to the contain ing strata. Subsequently oil was struck in lime stone in (Ain, thus ((wining a new type of occur rence, although one which has since proved to be rather unique.
Petroleum is considered by most geologists to have been derived by the destructive distillation of either animal or vegetable matter contained in the rocks. The products of this distillation have in some ease-, sec kited in the strata in which they were formed, while in others they have escaped upward into the overlying beds, in some instances even reaching the surface.
Petroleum occurs in all geological formations, from the Lower Silurian or Ordovieian up to the Tertiary; it is chiefly of importance in the Silu rian, Devonian. and Tertiary rocks. The relation of the distribution of oil to geological structure was not recognized until as late a date as about 1880. at which period geologists began to tind the cane of oil accumulation and pre.aire. Prof. Edward I Whin, of (MM. was the most prominent investigator in this field. In all regions where pe troleum occurs the strata are not only disturbed, but they are bent into anticlinal or arch-shaped folds. if the rocks are porous the gas tends to col lect at the summit of the Anticlinal fold or arch. while the oil collects in the flanks of the fold. Salt water is usually associated with the gas and oil, and, being heavier. aCellInulate: in the flanks of the anticlinal or in the neighboring synclinal folds. There is little use of searching for oil in regions where the strata are flat; and it is rarely found in highly folded regions. for where the
flanks of the fold have a dip of more than 10° the bending of the rocks is often sufficient to create eraeks through which the oil or associated gas will escape to the surface. The rock in which the oil is found is spoken of as the reservoir, or oil sand, and it is essential that this rock should be porous. The degree of porosity not only influ ences the quantity of oil which the rock can hold. but it may also influence the rate of llow of the well. Some wells may yield as little as 13 bar rels per day; others may reach a production of .10,000 or 01000 barrels per day. The porosity of the oil-bearing formation may also change from place to place and would account fur the location of a profitable well RI one point and a barren one a short distance from it. in order to prevent the escape of oil from the containing stratum it should also be overlain by a rock of more or less impervious nature. In many wells the petridenin flows to tlne surface under pressure. Professor Orton believed that the oil was under hydrostatic pressure: aceording to his theory, in different wells I if the same basin or pool ought to be nearly constant. In any re gion, however, the pressure usually diminishes with time. While Orton's themT may be true for ()hi°. it seems doubtful whether hydrostatic pressure will account for the great oil and gas pressure found in some regions. The quantity of oil whielt a given territory can yield is often very great, since -unic sand will hob) as much as one-eighth of its hulk in oil tinder pressure. This means that there is 1..1 inches of oil to every vertical foot of oil-sand, or nhola annl clIbie feet per acre.