Abstract: The Nuobei area is still in the initial stage of petroleum exploration in the east part of the Sanhu Depression, Qaidam Basin.
In this paper, the characteristics of the gas source rocks and the living environments of the methanogens in the Quaternary Qigequan
Formation from the Nuobei area have been studied, based on a vast amount of data for lithology paleontology, elemental geochemistry,
organic geochemistry etc. We discussed the sedimentary environments and geochemical conditions that influenced the formation of
biogenic gas, and then to predict the favorable exploration blocks. The results show that the gas source rocks in the Nuobei area contain
low organic matter content which are dominated by type Ⅲ kerogen and still at immature stage. The Qigequan Formation was deposited
under the low temperature and alkalinity, high salinity, and strong anoxic conditions, which were favorable for the formation of the gas
source rocks. The geochemical environment affected the flourish of the Methanogens, for instance, the low temperature and high salinity
inhibited the activities of the Methanogens, and the neutral-alkaline and strong reducibility environment were suitable for Methanogens
to flourish. The inhibition effect of salinity for the Methanogens is gradually strengthened from south to north as affected by surface
runoff. The biogenic gas accumulations in the Nuobei area were dominated by vertical gas migration. According to the above analyses,
we suggested that the Central Depression is the most favorable exploration area in the Nuobei area, followed by the North Slope. In
these areas, the widely developed lacustrine mudstones supplied nutrient for Methanogens, the weak compaction during the early
diagenetic stage provided large amounts of pores in reservoirs, and the thick sedimentary strata and gypsum deposition increased the
sealing capacity of the cap rock.

Key words: biogenic gas, influence factor, Qigequan Formation, Nuobei area, Sanhu Depression, Qaidam Basin