MODE OF OCCURRENCE. Natural gas is found most often in sandstone and shale, but may also occur in limestone. Two types of accumulation may be recognized, viz.: in impervious rocks, such as shales and limestones, and in porous rocks, such as sandstone. The former type or shale gas usually forms small wells of varying pressure, and is not necessarily associated with petroleum; the deposits have staying qualities, and do not depend on the structural arrangement of the containing strata. Sandstone or reservoir gas, on the other hand, is found in great wells, often accompanied by oil, and while the rock pressure in any one area is fairly constant, at the same time. these wells usually give out sud denly. The structure of limestone and sandstone formations containing natural gas, and, indeed, also petroleum, is often that of a low anticlinal, the gas and oil being found at and near the crest, respectively, while on either flank there is often an abundance of water. This theory of gas accumulation is known as the 'a-nticlinal theory,' and was developed by Profs. E. Orton and I. C. White. It has been noticed in all gas fields that when the reservoir is tapped the gas usually rushes out as though under great pres sure, this being spoken of as rock pressure. Prof. E. Orton believed that this pressure was hydro static, and due to the head of water in the rocks overlying the gas, the amount of pressure in the Ohio field being equal to a column of water whose height was equal to the elevation of Lake Erie above the gas-bearing stratum. While this
theory may hold in some cases, still I. C. White has pointed out that in others the rock pressure is much greater than the artesian pressure in the same region, and furthermore that the ex haustion of the gas is not always followed by a flow of water. In such cases the rock pressure must be due to the expansive force of the gas. The original rock pressure varies in different fields, and is not infrequently as high as 300 or 400 pounds per square inch at the mouth of the well, and in some wells may exceed 1000 pounds per square inch. Several of the newer wells in West Virginia having a depth of from 2700 to 3200 feet showed a rock pressure ranging from 1000 pounds to 1300 pounds per square inch. A decrease in pressure is always likely to follow with time, as in the case of the first well opened at Findlay, Ohio, where the pressure fell from 450 pounds in 1886 to 170 pounds in 1890. In the early days of gas-well drilling the supply ap peared so inexhaustible that the newly drilled ells were often allowed to blow off gas for several days or weeks before attempts were made to cap them.