Abstract: In order to clarify the accumulation mechanism of organic matter in shale under the gradually arid palaeoclimate in the Late of Middle Jurassic for Northern Qaidam Basin, this paper systematically analyzed the palaeoclimate characteristics and its relation with the supply and preservation of organic matter by means of organic geochemistry, inorganic geochemistry and mineral composition analysis of shale samples for Seven Member of Dameigou Formation from some wells in Yuqia area. Compared with the lower gray-black shale, the upper gray-brown shale is characterized by higher organic matter content and sapropelic organic matter type. The higher contents of carbonate minerals and Sr/Cu, and the lower palaeoclimate index C value, with wide range, indicate gradually arid palaeoclimate with high frequency fluctuation. Apparent difference of C value for gray-brown shale in lateral direction reflects that the palaeoclimate at the same period is also stable. Lower Al and Ti contents indicate weak input of terrigenous detrital matter. Higher P/Ti and Ba/Al indicate higher palaeoproductivity. Lower Pr/Ph and higher pyrite content indicate more reducing water conditions. Higher Sr/Ba and mostly high gammacerane index reflect water salinization of the lake and water stratification in most areas. Based on the above proxies, it can be concluded that a certain degree of arid palaeoclimate is favorable to the decrease of terrigenous matter input, the increase of palaeoproductivity, the enhancement of water reducibility and salinity, and the stratification of most areas, which can facilitate sapropelic organic matter accumulation.

Key words: gradually arid palaeoclimate, high frequency fluctuation, vertical and lateral attributes, Seven Member of Dameigou Formation, geochemistry characteristics, mineral composition, organic matter accumulation