Characteristics and origin of tuff-type tight oil in Jimusaer sag, Junggar Basin, NW China

Based on core and thin section observation, and combined with X-ray diffraction, electron probe and geochemical analysis, the characteristics and origin of tuff tight oil of the Permian Lucaogou Formation in the Jimusaer depression, Junggar Basin were examined. There are tuff, tuffite and tuffaceous dolomite interlaid in the sedimentary rocks of the Lucaogou Formation. The alkali feldspar and quartz in the tuff are rich in strontium, which, according to mineral geochemical analysis, are originated from alkaline-peralkaline magmatic rocks. The Sr isotope ratio (Sr-87/Sr-86) of dolomite is similar to that of mantle, which shows minerals of tuff were formed in volcanism-related hydrothermal environments. High Sr/Ba and delta C-13 of dolomite, and occurrence of halite under microscope suggest that the Lucaogou Formation was deposited in high salinity environment. High content of magnesium and iron ions related to volcanism and hydrotherm, and high salinity environment contributed to the precipitation of dolomite. The distribution of tight oil, organic matter abundance of source rock and reservoir physical properties show that it is the rich nutrients provided by volcanic activity that controlled the formation of high abundance source rock; tuff, tuffaceous dolomite with plenty of residual inter-granular pores and dissolved pores are favorable reservoir sections; and tuff controls the formation and distribution of tight oil.