Sequence stratigraphy and coal accumulation of barrier coastal system under epicontinental-sea environment: A case study of the Late Paleozoic Taiyuan Formation in Bohai Bay Area

The Bohai Bay Area, located in the east of North China, is an important area for coal production in China. There are many coal seams available for industrial exploitation in the Late Paleozoic, among which the marine coal seams developed in the epicontinental-sea environment are the most widely distributed. In order to study the sedimentary sequence and coal accumulation model of the barrier coastal system in the epicontinental-sea environment, this paper studies the sedimentology and sequence stratigraphy of the Late Paleozoic Taiyuan Formation in the Bohai Bay Area, combining with drilling and field profile data. Based on the identification of lithology and sedimentary facies, the study of sequence stratigraphy is carried out to reveal the coal accumulation model of the barrier coastal system in the sequence stratigraphic framework. Taiyuan Formation is mainly composed of yellowish brown, gray black mudstone and yellow green and gray white sandstone, with several layers of marine limestone and coal seams. The sedimentary facies are mainly tidal flat facies, lagoon facies, barrier island facies and platform facies. Against the background of the barrier coastal system, the base level and the sedimentary interface are basically the same, and the sequence boundary is mostly sedimentary interface. According to the two types of subaerial unconformities(SU) including sedimentary facies transition surface and incised valley scour surface and transgressive direction transition surface(TDS), Taiyuan Formation can be divided into Sq1, Sq2 and Sq3. The third order sequence is divided into LST, TST and HST system tracts by maximum flooding surface(MFS) and maximum regression surface(MRS), representing eight fourth order sequences. During the period of Sq1, the internal area of the basin fails to provide a stable coal accumulation environment, and the thickness of the coal seams is relatively thin. During the Sq2 period, the influence of sea level on the Area increases, the coal accumulation reaches the peak, and the overall thickness of the coal seams increases and connects into slices. During the period of Sq3, the coal accumulation relatively weakens, and the distribution range of coal accumulation centers contracts, developing in isolation. The statistics of cumulative thickness of the coal seams in drilling data reveal that thick coal seams are mainly developed in the eastern part of Jizhong depression area, Huanghua depression area and Jiyang depression area. The coal accumulation model of Taiyuan Formation in the study area can be divided into five stages: LST, early TST, middle TST, late TST and HST. Under the control of the growth rate of accommodation space and peat accumulation rate, peat flat and lagoon are favorable coal accumulation centers.