Geochemistry of natural gas accumulation in Sulige large gas field in Ordos Basin.
鄂尔多斯盆地苏里格大气田天然气成藏地球化学研究

The Sulige large gas field is located in the northwestern Yishan slop in Ordos Basin and belongs to the typical low permeability sandstone gas reservoir.Through the analyses of components and carbon isotopes of natural gases,it is demonstrated that the Upper Paleozoic natural gases in the Sulige large gas field are characterized by major dry gases and minor wet gases.The methane contents are in the range of 82.729%～98.407% and the dryness coefficients are 84.7%～98.8%.The carbon isotope of methane and ethane are ranged from-36‰ to-30‰,from-26‰ to-21‰,respectively.This indicates these gases are mainly belonged to the high maturity coal-formed gases.The components and carbon isotopes of natural gases from different wells are varied little in the Sulige gas field,reflecting the similarity of the genesis and reservoir-forming process.Based on the analyses of microscopic observation of fluid inclusion,homogenization temperature of inclusion,hydrocarbon-bearing inclusion abundance,quantitative grain fluorescence(QGF),and laser Raman of fluid inclusion,there are developed different types fluid inclusions,including aqueous inclusion,gas inclusion,oil inclusion,and CO2 inclusion in Upper Paleozoic reservoirs in the Sulige large gas field.These inclusions are mainly distributed in quartz overgrows,fissures and cementations.The measured homogenization temperature of fluid inclusions shows the continuous single peak,which their homogenization temperatures are ranged from 80～180℃ and the main peak temperature is 100～145℃.The abundances of hydrocarbon-bearing inclusions are not high(mainly 1%～5%) and the QGF intensity is relatively lower(1～10pc) in the Upper Paleozoic sandstone reservoirs.It is suggested that natural gas in the Sulige large gas field may be charged by a continuous process,and experiences one main gas accumulation period,mainly occurring at the Late Jurassic and Early Cretaceous.The kinetic results of gas generation and carbon isotope show that natural gas in the Sulige large gas field are mainly derived from Carboniferous-Permian coal measure source rocks in the Sulige region,belonging to the near-source gas charging and cumulative gas accumulation.