West of Mississippi River natural gas, in commercial quantities, was first discovered in Miami County, Kansas, in 1877. This and subsequent supply discoveries were from rocks, mostly sandstones, of Pennsylvanian age. As most of these sandstones are lenticular, few gas occurrences are controlled by folding. It is estimated that about 350 square miles of gas-bearing rocks have been discovered since 1900.

Occurrences of natural gas in rocks of Pennsylvanian age in eastern Kansas are here, for analytical purposes, divided into post- and pre-Fort Scott. The supply contributed by post-Fort Scott rocks has been small, but pre-Fort Scott rocks have been the major source of natural gas supply. The pre-Fort Scott section of Pennsylvanian rocks is divided into an upper and a lower division. The upper division, consisting of the upper 350 feet of Cherokee formation, covers all of eastern Kansas. In its upper 50 feet are found most of the gas- and/or oil-bearing shoestring sandstone bodies; in the lower part the oil-bearing sandstones of Greenwood and Butler counties. The supply of gas from these formations has not been of major importance. The lower division, with a maximum thickness of about 3 0 feet, contains many large sandstone bodies (also lenticular) from which natural gas was the dominant product. The important fields, producing gas from the lower division, were located in Allen, Neosho, Wilson, Montgomery, and Labette counties. They are now either abandoned or nearly exhausted.

At the contact between the Pennsylvanian and Mississippian systems occurs the Burgess (Hogshooter) sandstone. This sandstone, also lenticular, is extremely erratic in distribution. Where productive, notably at Elk City in Montgomery County, it is very prolific but short-lived.

Limestones in the upper 50 feet of the Mississippian system have produced gas in eastern Kansas, notably in Elk and Chautauqua counties on the Longton anticline.

Rocks of Ordovician age have yielded very little gas in eastern Kansas.

Occurrences of gas in post-Fort Scott rocks are up-dip accumulations, commonly found on plunging anticlines or domes. In pre-Fort Scott sandstones, the entire body may be gas-filled with bottom water, if it parallels isopach lines of the Cherokee formation, but it may be distinctly up-dip with edge and bottom waters where the body pitches with the regional dip. The upper surfaces of pre-Fort Scott sandstone bodies show structural closure (lenticularity); the overlying Fort Scott limestones usually show a similar structure, but the underlying Mississippian rocks are usually undisturbed excepting for regional tilting and warping. Gas-bearing areas in rocks of Mississippian age are restricted to anticlines and domes, most of which were folded and partly eroded before deposition of Cherok e sediments. The thickness of Mississippian rocks is everywhere reduced over these folds.

Most of the important gas fields in Kansas were discovered and developed prior to the use of surface or subsurface geology. A few important gas fields have been

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found on surface structure forms, notably at Elk City in Montgomery County; a few were recognized as major subsurface folds before their importance as gas fields was known, notably the Longton anticline.