Sedimentary characteristics and coal accumulation patterns of the Permian shallow-water deltas in Shandong and its adjacent regions

Objective The Shanxi Formation, deposited during the critical transitional period from the sedimentary filling of epicontinental sea basins to the sedimentation of continental basins, exhibits well-developed shallow-water delta systems and significant coal-forming processes. This study aims to reveal the characteristics of sedimentary environment transformation during the late stage of sedimentary filling of epicontinental sea basins in North China. To this end, the Early Permian shallow-water delta systems in Shandong and its adjacent regions during that period was investigated, including their primary sedimentary characteristics, the predominant bases of sedimentary transformation, and the lithofacies paleogeographic characteristics of coal-forming processes.MethodsBased on data from outcrops, cores, wells, and logs in Shandong and its adjacent regions, this study determined the sedimentary microfacies in shallow-water deltas in detail and identified the assemblage characteristics of sedimentary systems. Using the single-factor analysis and multi-factor comprehensive mapping method, the evolutionary process of lithofacies paleogeographic features was examined, followed by the analysis of coal accumulation patterns and the establishment of coal formation modes.Results and ConclusionsThe shallow-water deltas in Shandong and its adjacent regions can be divided into an upper plain, a transition zone, and a lower plain with the high storm-tide line and mean low tide line as boundaries. Since channel sand bodies act as the fundamental framework of shallow-water deltas, the three parts can be identified based on the paragenetic assemblage relationships of channel sand bodies. Under the influence of tidal action, tidal-flat deposits are commonly observed in the sequences of shallow-water deltas, emerging as a key marker used to distinguish shallow-water deltas from general deltas. In this study, five sequences of sedimentary cycles were determined, revealing the evolutionary process where seawater retreated from the southeastern part and the northern portion transitioned into a continental environment first in the study area. During this process, the widely developed interdistributary bays provided favorable sites for the accumulation of peat bog. The coal seams in the upper plain tend to show a discontinuous distribution due to the frequent migration of distributary channels, while those in the lower plain prove thin under the tidal influence. The stable, primary mineable coal seam No.3 in the study area occurs in the transition zone between the upper and lower plains, demonstrating great thicknesses and a wide distribution. This coal seam serves as an ideal indicator for coal seam correlation in the study area, especially being well-developed in southwestern Shandong. The findings of this study further clarify the coal-controlling patterns of the transition zone in shallow-water delta systems, providing a critical geological basis for the precise exploration and regional correlation of coal resources in Shandong and its adjacent regions.