Geochemical characteristics of coal-measure source rocks and coal-derived gas in Junggar Basin, NW China

Abstract

Carboniferous and Jurassic coal-measure rocks with their relative coal-formed gas are developed in Junggar Basin. Carboniferous source rocks have high organic material abundance, Type III organic material, and high maturity. Jurassic source rocks are widely distributed with big thickness and great maturity variety. The Middle and Lower Jurassic are the predominant sources of natural gas, and the potential is large. Carboniferous coal-formed gas is dry gas, its carbon isotope is heavy, the methane carbon isotope composition is about −30‰, the ethane carbon isotope composition is above −28‰, the carbon isotope series is positive (δ13C1δ13C2δ13C3δ13C4). Gas reservoir is self-generating and self-preserving. Gas from the Jurassic source rocks has wide distribution and is wet. Methane carbon isotope composition varies greatly, and the carbon isotope reversal of butane and propane is common, reflecting that natural gas has different maturity and the mixing of different-maturity natural gas originated from the same source results in the light isotopic values of heavy hydrocarbons. The potential of the Carboniferous rocks is huge, and is the major exploration area. Gas from the Jurassic source rocks is distributed widely and the southern margin of the basin is a favorable area.