Abstract
Accurate identification of the lithofacies and sedimentary facies of coal-bearing series is significant in the study of peat accumulation, coal thickness variation and coal-measured unconventional gas. This research integrated core, logging and 3D seismic data to conduct a comprehensive seismic–geological study on the sedimentary evolution characteristics and peat accumulation regularity of the Shanxi Formation in the Xinjing mining area of the Qinshui Basin. Firstly, the high-resolution sequence interface was identified, and the isochronous stratigraphic framework of the coal-bearing series was constructed. Then, the temporal and spatial evolution of sedimentary filling and sedimentary facies was dynamically analyzed using waveform clustering, phase rotation, stratal slice and frequency–division amplitude fusion methods. The results show that the Shanxi Formation in the study area can be divided into one third-order sequence and two fourth-order sequences. It developed a river-dominated deltaic system, mainly with delta plain deposits, and underwent a constructive–abandoned–constructive development stage. The locally distributed No. 6 coal seam was formed in a backswamp environment with distribution constrained by the distributary channels. The delta was abandoned at the later stage of the SS1 sequence, and the peat accumulation rate was balanced with the growth rate of the accommodation, forming a large-area distributed No. 3 thick coal seam. During the formation of the SS2 sequence, the No. 3 coal seam was locally thinned by epigenetic erosion of the river, and the thin coal belt caused by erosion is controlled by the location of the distributary channels and their extension direction. This study can provide a reference for the research on the distribution of thin sand bodies, sedimentary evolution and peat accumulation regularity in the coal-bearing series under the marine–continental transitional environment.
Highlights
The study of sedimentary facies is important for the in-depth understanding of the peat-forming environment, peat accumulation regularity and unconventional gas in coal measures during the formation and evolution of a coal-bearing basin [1–6]
Incised valley filling deposits in the low-stand stage and their bottoms are a kind of erosional unconformity, which can be taken as the third-order sequence boundaries
The Shanxi Formation in the study area mainly developed delta plain sub-facies, including sedimentary microfacies, such as distributary channels, natural levees, interdistributary bays and crevasse splays, and it can be divided into a third-order sequence, SQ1, and two fourth-order sequences, SS1 and SS2
Summary
The study of sedimentary facies is important for the in-depth understanding of the peat-forming environment, peat accumulation regularity and unconventional gas in coal measures during the formation and evolution of a coal-bearing basin [1–6]. The distribution characteristics of sand bodies and sedimentary facies near coal seams usually have significant effects on peat accumulation, coal thickness variation, coalbed methane enrichment and resource recoverability [8,10–12]. It is significant to carry out fine characterization of the sedimentary facies of coal-bearing series under small-scale conditions and their controlling effect on coal seams. The sequence classification, lithology and sedimentary facies distribution characteristics of coal-bearing strata can be studied based on field outcrops, well logging curves and core data [2,13]. Seismic stratigraphy is more suitable for basin analysis, thick layer sequence classification and early- to middle-term resource exploration and evaluation, but the control accuracy of small-scale sedimentary units and thin sand bodies is still insufficient because of the limitation of seismic resolution
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