Geology of Natural Gas in the Joban Coal Field

Abstract

The natural gas of the Joban coal field is accumulated mainly in the coal-bearing Iwaki formation and the. Asagai formation of the Oligocene age. Minor accumulations are found in the Goyasu formation of Miocene age. The chief source of the gas is sub-bituminous to bituminous coal of the Iwaki formation. As the Iwaki formation consists mainly of sandstones, the greater portion of the gas has escaped from the coal seams without difficulty, and migrated upward.
Structurally the Joban coal field consists of faulted blocks, and the gas is trapped by the faults.Gas reservoirs are commonly found on the upthrown side of the faults. The reservoir rocks are thick sandstones which are low in permeability. The gas issues chiefly from fissures and fractures of the reservoir rocks. The Takai No. 5 well, which was drilled in the fault zone of the Takai fault, recorded a maximum production of 150, 000 cubic meters per day.