Paleoenvironmental and vegetational controls on tar yield in the Jurassic Xishanyao Formation coals of southern Junggar Basin, China

Tar-rich coal is an important resource for coal-measure oil and gas exploration and clean energy conversion, yet seam-scale variations in tar yield and their controlling mechanisms remain poorly understood. This study investigates the Middle Jurassic Xishanyao Formation coals in the southern Junggar Basin by integrating organic petrography, palynology, geochemistry, and molecular-structure analyses. Results show an upward increase in tar yield, volatile matter yield, and hydrogen index, indicating relatively enhanced liquid-product potential during pyrolysis in the upper coal interval. Palynological analysis identified two assemblages, characterized by a shift from gymnosperm-pollen dominance in the lower seams (55.00–58.33%) to higher fern-spore proportions in the upper seams (50.00–58.82%). The lower coal seams were mainly formed in delta-plain forest swamps during the lowstand systems tract (LST), accompanied by stronger clastic input and water-level fluctuations. Their coal-forming materials were dominated by woody gymnosperms, while fluctuating water tables and greater oxygen exposure were associated with relatively abundant inertinite and more aromatic molecular structures. With progressive lake transgression, peat-forming mires evolved into lakeshore swamps characterized by more stable water coverage and more reducing conditions, which favored the preservation of labile organic matter. FTIR results show that high-tar-yield coals have higher CH 2 /CH 3 ratios and A-factors, but lower aromaticity and aromatic condensation, indicating molecular structures enriched in aliphatic chains and weakly condensed aromatic units. These features suggest that long-chain aliphatic side chains and bridge bonds provide a favorable molecular basis for liquid-product release during low-temperature pyrolysis, whereas highly condensed aromatic structures and certain oxygen-containing functional groups tend to inhibit tar generation. Overall, the higher tar yields of the upper coals were associated with changes in vegetation input and improved preservation of aliphatic-rich organic matter under more persistently water-covered, oxygen-restricted mire conditions during lake transgression. These findings provide a geological basis for interpreting tar-rich coal and identifying high-tar-yield intervals in comparable low- to medium-rank humic coal successions formed in terrestrial lacustrine settings.