Research On The Sequence Stratigraphy Of Badaohao Coal Basin, Volcanics Horizon In West Liaoning Province And The Coal-searching Prospect

Ther are 18 Mesozoic basins distributed in Zhangwu-Heishan region, west Liaoning province. Most of which are severely covered with the Quaternary strata or volcanic rocks. This paper takes Badaohao Basin in this region for example and combines the seismic stratigraphy, sequence stratigraphy, well logging, correlation of coal seam and strata, and other techniques to research its sequence division, sedimentary facies correlation and the coal accumulation rule. Volcanic rock is widely distributed in this region. Over 20 volcanic rock samples are collected, 10 of which the SHRIMP U-Pb isotope dating analysis is conducted to. The research result shows:Badaohao Formation - coal-bearing strata - in Badaohao Coal Basin is of early Cretaceous Period and can be correlated to Shahai Formation in Fuxin Basin. The seismic profiles, well logging and core identification show that Badaohao Formation formed and is preserved in small-scaled tilting structure with synsedimentary faulting events in the west. This basin is deep in the west and shallow in the east, and shows obvious unsymmetry. Badaohao Formation can be divided into 3 third-order sequences. The first sequence in the lower part is one subaqueous-fan~fan-delta depositional system which was caused by alluvial fan intruding lake basin; the lake transgressive system tract and high-stand system tract each contain one coal-seam group; and thick coal seams develop at the edge of subaqueous fan retrograded by lake transgressive system tract and in the region appropriately covered with water in the fan delta of high-stand system tract. At the early stage, due to the uplift of Yiwulv Mountain to the west of the basin, the alluvial fan intruded the lake basin from the southwest of the Basin; and the deep part of the lake in the northwest of the basin is filled by coarse clastic sediments in turbidite fan to form the coal-forming environment, develops thick coal seams and overlaps eastwards from the place near the west basin margin. As the alluvial fan continued filling the deep part of the lake, the part which was covered with deep water in the lake basin migrated from the west to the east and the coal-forming swamp moved eastwards, so the coal seam is covered with a prograding fan-delta sedimentary system and the coal seam in Sq1 lake transgressive system tract is more widely distributed than that in high-stand system tract. As the lake basin shoals and silts up, the color of clastic sediment gradually changes from black, black gray, gray and grayish green to purplish grey and red purple, meaning that the subaerial alluvial fan deposit of late stage has replaced the fan-delta deposit and that the coal-forming process ends.The U-Pb isotope ages of the 11 volcanic rock samples are 122Ma~132Ma and mostly 125Ma~127Ma, which belongs to the middle period of early Cretaceous epoch. 3 volcanic eruptions happened: the 1st and 3rd are fissure eruptions of intermediate-basic basaltic andesite and are resulted from the structures of active period, while the 2nd belongs to central eruption of neutral trachyandesite and is derived from the structure of stabilized period. They are almost cotemporary with the volcanic eruptions of Yixian Formation near Yi County in the south of Fuxin Basin. Therefore, it is unpromising to find Mesozoic coal seams under volcanic rock unless thrust structure exists.The research shows that these basins alternate between uplift and concave and that their spreading directions follow certain rule. The 6 NEE-trending basins are of early Jurassic epoch, some of which may contain coal-bearing strata and minable coal seams of Beipiao Formation. The 12 NNE-trending basins are all of early Cretaceous epoch, among which Badaohao, Leijia and Xielintai Basins have relatively good coal-bearing property and can serve as coal-searching prospect area.