The Applications of High-Resolution Sequence Stratigraphy to Paralic and Terrestrial Coal-Bearing Strata: Two Case Studies from the Western North China Paleozoic Basin and the Tulufan-Hami Jurassic Basin

Through the studies of high-resolution sequence stratigraphy on paralic Paleozoic coal measures of the western North China cratonic basin and terrestrial Jurassic coal measures of the Tulufan-Hami foreland basin, it is recognized that the depositional environment, coalbed thickness and maceral composition variations are mainly controlled by the relative sea/lake level changes. The main coal seam of Early Permian occurs on the bottom of the retrogradational sequence set of P11, which onlaps from the south(basinward) to the north(landward), and overlays the erosional surface of the Upper Carboniferous (2nd order sequence boundary). Along with the marine limestone beds thinning and pinching out to the north, coalbeds of four sequences combine northward into a very thick coal seam. There are no relationships between coal thickness variations and their underlying sandbody distribution because the depositional process is not continuous. Vitrinite, especially desmocollinite, of this coal seam, decreases landward to the north, and the inertinite and kaolinite interbeds increase. In the Tulufan-Hami basin vitrinite-rich and thicker coalbeds occur on the top of the retrogradational sequence, which develops during the lake level of the basin rising. Coalbeds developed in TST or retrogradational sequence sets are distributed more extensively and rich in vitrinite (especially desmocollinite), with better hydrocarbon potential than those of the coalbeds developed in HST or progradational sequence sets. Coalbeds of TST maybe is a new coal depositional model, and the previous coal depositional models [5, 6] are more suitable to the coalbeds developing in HST.