Unraveling gas charging and leakage for oil reservoirs in the Mahu sag of the Junggar Basin, NW China using concentrations and ratios of biomarkers, light hydrocarbons and diamondoids

•Maturity of Mahu oils was assessed by biomarkers, light hydrocarbons and diamondoids.•Biomarkers entered the reservoirs along with the initial oil charge.•Light hydrocarbons and diamondoids entered the reservoirs with the late gas charge.•Hydrocarbon compositions indicate large amounts of gas charging of the reservoirs.•Low gas to oil ratios for the reservoirs can be mainly ascribed to gas leakage.