Sequence-paleogeography and coal accumulation of the Paleogene Hunchun Formation in the Hunchun Basin

The Hunchun Basin is an important coal-bearing region in Jilin Province, northeastern China. The analysis of the sedimentary paleogeography of the Paleogene Hunchun Formation and coal-accumulation law in the sequence framework provides a theoretical basis for future coal and coalbed methane exploration. The lithofacies, sedimentary systems, sequence paleogeography, and coal accumulation characteristics were analyzed based on regional geology, drilling cores and well logging data. The results show five rock types, including gravel, sandstone, siltstone, mudstone, and combustible organic rock, and twelve lithofacies. The Hunchun Formation was divided into three third-order sequences by three sequence interfaces, including the regional unconformity surface, the river incised valley washed surface and the sedimentary system abrupt surface, and the isochronal stratigraphic framework of the Hunchun Formation was built. Three maps of lithofacies paleogeography were reconstructed according to the strata thickness, sandstone thickness, sand-mud ratio, and the distribution of coal seam thickness in each sequence. In the Hunchun Formation, four sedimentary systems (alluvial fan, braided river, braided river delta, and lacustrine), twelve sedimentary subfacies, and thirteen sedimentary microfacies were identified with provenances in the north and south sides of the study area. Within the stratigraphic framework of each sequence, there were laws for the coal seam distribution in the Hunchun Basin. From sequence Ⅰ to sequence Ⅲ, the coal accumulation first increased and then decreased. The thick coal seams were mainly developed in the period in which the clastic sedimentary system was weakened significantly, such as lacustrine transgressive system tracts and highstand system tracts. There are three coal-accumulation centers in the Hunchun Formation, located in Baliancheng, Banshi, and nearby 54-9 drilling well in the western, southern and eastern areas, respectively. There was a good corresponding relationship between the depositional center and the coal-accumulation center, and both of them were changed from the west to the east and then to the west again from sequence Ⅰ to sequence Ⅲ. The favorable paleogeographic units of coal accumulation were the sheet flow in the fan end, flood plain in the braided river, interdistributary bay in braided delta plain, and shore lacustrine.