Abstract: The kinetics and geochemistry characteristics of formation water in Sanhu area are investigated in order to reveal the effect of Quaternary formation water in Sanhu depression on biogas migration and accumulation, and the effect of formation water on biogas migration and accumulation is determined clearly by the features of biogas migration and accumulation. The study shows that the potential energy of the formation water in Sanhu depression in south is greater than that in north. Laterally, the salinity of formation water is lower in depression margin than in center, and 3 high-salinity districts appear. These high-salinity districts should be the favorable places for biogas precipitation and accumulation, for instance, the formation of Tainan and Sebei gas fields. Vertically, the salinity of formation water is increasing from the top down, with up and down multi-cycle variation. Such a "south-high and north-low" potential energy of formation water allows the biogas dissolved in water to migrate from the south to the north and accumulate toward slope zone in the north. And the multi-cycle variation results in the biogas precipitation at high-salinity section and dissolution at low-salinity segment, shaping multi-group superposed gas reservoirs. Also, the amplitude of multi-cycle variation of the formation water salinity is proportional to the biogas enriched extent.

Key words: Qaidam basin, Sanhu depression, Quaternary, formation water, biogas immigration and accumulation