Coal Seam Correlation in Terrestrial Basins by Sequence Stratigraphy and Its Implications for Paleoclimate and Paleoenvironment Evolution

Coal correlation cannot only guide coal and coalbed methane exploitation, but may also provide valuable information for the evolution of the paleoclimate and paleoenvironment during coal-accumulation periods. It is difficult to undertake coal correlation in a terrestrial basin due to multiple superimposed coal seams and a lack of effective marker beds. In this study, based on the analyses of coal-bearing sedimentary environments and the observations of field outcrops and drilling cores, three key sequence surfaces of the Middle Jurassic Xishanyao Formation were identified in the middle part of the southern Junggar Basin. Three third-order sequences (S.III1, S.III2 and S.III3) and ten fourth-order sequences were divided within the Xishanyao Formation. Regional coal correlation was finally identified, combined with the layered vertical pattern coal and its interval thickness, coal maceral, and coal quality data. In terms of the coal correlation within a specific sequence set, the interval of the B2–B3 coals was approximately three times thicker than that of the B3–B5 coals, which was the main marker for regional coal correlation in the Lower Xishanyao Formation. Furthermore, the phenomena of steadily developed B4 coal seam (2–3 m in thickness) across the Liuhuanggou coal mining area (eastern study region) and the large-small-large variation for intervals between the B8 and B9, B9 and B10, and B10 and B11 coal seams in the Manasi coal mining (western study region) were also an effective means for local coal correlation. Increasing vitrinite and sulphur content and decreasing inertinite content in a third-order sequence were found, which were likely to be closely related to lake level fluctuations. Based on the variation of coal macerals, the wildfire frequency gradually decreased from the bottom to the top of the Xishanyao Formation. The predicted O 2 level of the S.III1, S.III2, and S.III3 of the Xishanyao Formation was 26.36%, 24.22%, and 22.51%, respectively. Except for global paleoclimate effects, the provenance direction changes caused by the Bogda Mountains uplifts would result in a regular variation of coal maceral and coal quality across the study region.