Keywords: 
• deep natural gas reservoirs

Abstract: Through observation of petrological characteristics of fluid inclusions under microscope, the types, shapes, occurrence and colors of fluid inclusions in deep natural gas reservoirs in Minfeng subsag, Dongying sag were described in this paper. The characteristics of hydrocarbonbearing inclusions indicate that evolution of oil and gas in Minfeng area reached high maturity stage. The Raman spectra of hydrocarbonbearing inclusions imply that the accumulation of deep natural gas in Minfeng subsag may be relative to methanerich hydrocarbon fluids. Based on the microthermometry analysis of aqueous inclusions which coexist with hydrocarbonbearing inclusions and reconstruction of burial and thermal history, there were two oil/gas entrapment stages were divided in Minfeng subsag. The first stage mainly occurred during Oligocene Dongying Period, but having not too much contribution. The second accumulation mainly occurred in Pliocene Minghuazhen Period. The latter is the main accumulation period, and mostly occurred smaller than 10 Ma. The analysis of Raman spectroscopy show that hydrocarbonbearing fluids coexist with carbon dioxide in fluid inclusions in deep natural gas reservoirs, confirming that there were acid fluids existed during the accumulation of natural gas. Acid fluids can dissolve calcite cement formed in early diagenesis, consequently improving the growth of induced porosity in deep reservoirs and favor the accumulation of natural gas. 