Differences in depositional environments and formation conditions of the ultra-deep Middle and Upper Permian saline lacustrine shales/mudstones of the Dongdaohaizi depression in the Junggar Basin

<p indent="0mm">There is abundant hydrocarbon resources in the Permian strata in the Junggar Basin. However, relevant researches are mostly focused on the shallow layer of the strata in the margin of the Junggar Basin. In order to explore formation environments of the ultra-deep Shangwuerhe and Xiawuerhe formations in the central area of the basin, in this paper, we have undertaken the elemental geochemical analysis of 15 rock samples from drilling cores in the Dongdaohaizi sag and have discussed their provenances, paleotectonic environments, paleoclimatic conditions, and paleowater environments of sedimentation. The results indicate that the provenance of the Xiawuerhe formation is similar to granitoids (acidic rocks), while the provenance of the Shangwuerhe formation is similar to andesitic rocks (intermediate rocks). These sedimantary rocks were formed under the paleotectonic setting of the continental island arc. Moreover, the paleoclimate environment during the sedimentation of the Shangwuerhe and Xiawuerhe formations was cold and dry. There was relatively weak chemical weathering. There was a warming trend for temperatures of the water systems of sedimentation from the Xiawuerhe formation to the Shangwuerhe formation. At the same time, there was generally a saline environment during the sedimentation of the and the paleosalinity was decreased from the water system of the Xiawuerhe formation to that of the Shangwuerhe formation. The paleoredox conditions of sedimentary water systems were also transitioned from a suboxic environment for the water system of the Xiawuerhe formation to an oxic environment for that of the Shangwuerhe formation.