Composition characteristics and genetic mechanism of ultra thick coal seams: A case study of Middle Jurassic in Yuqia Area, Northern Qaidam Basin

Thick coal seams are widely distributed in the world, which contain rich geological information and play an important role in the global carbon cycle. The genetic mechanism of thick coal seams has recently gained attention in coal geology research. In order to reveal the composition characteristics and genetic mechanism of ultra thick coal seams in the isochronous sequence stratigraphic framework, the ultra thick coal seam M7, developed in lacustrine transgressive system tract of Middle Jurassic sequence S1 in the northern Qaidam Basin, was continuously sampled at equal intervals, and then the macerals and minerals were identified. A comprehensive analysis was conducted based on the recognition principle of key surface in coal seam, and the paleo-environmental indication of maceral and mineral. The results showed that the content of vitrinite is the highest (48.5%), followed by inertinite (37.22%), and liptinite is fairly low (3.32%). The content of minerals is relatively stable, with an average value of 10.96%. Based on the alternated development of coal seams-detrital deposits, and the content variation of macerals and minerals in vertical direction, five key surfaces were identified, including terrestrialization surface, erosional subaerial exposure surface, paludification surface, give-up transgressive surface, and accommodation reversal surface. The vertical evolution of swamp was analyzed by selecting boreholes at the landward and basinward locality along the sedimentary trend compared to the Yuqia coal mine. Four sedimentary cycles were identified in the ultra thick coal seam M7 of Yuqia coal mine. In landward YQ6–22 and basinward YQ6–12, three sedimentary cycles were identified due to the separation of M7–1 to M7–3 by terrigenous detrital and lacustrine deposits. High resolution sequences were found in ultra thick coal seams, which were divided into four fourth-order sequences. Combined with the sequence boundary corresponding to the key surface in coal seam, the genetic mechanism of ultra thick coal seams was revealed. A multi-peat superposition genetic model in the sequence stratigraphic framework was established, which can be further applied to the research of thick coal seams in more geological times around the world.