Sequence paleogeography and coal accumulation in epicontinental basin

Based on an investigation of the Benxi-Shanxi formations in the southwestern Shandong province,we studied the forming mechanisms of key stratigraphic boundaries in an epicontinental basin and the coal-accumulating characteristics in the paleogeographic background of isochronous stratigraphic frameworks.In total,we sequenced three types of stratigraphic boundaries(i.e.,regional unconformities,the shifting surface of transgressions caused by basin basement subsidence,and fluvial erosional surfaces including exposed surfaces as well as correlative conformities,which are caused by forced regression)and proposed a model for coal-accumulation.The results show that in the high accommodation space of the barrier coast system in the lower Benxi-Taiyuan formations,the base level was higher than sedimentary boundaries,parasequences were bounded by a marine flooding surface and formed during the semi-cycle of base level falling that was influenced by high-frequency fluctuations of base level,and coal seams were thin with multistoried patterns.In this region,the coal accumulating center was located in the transitional zone of the barrier island and tidal-flat.In the low accommodation space of the fluvial delta in the upper Shanxi formation,the base level coincided with sedimentary boundaries,parasequences formed during the semi-cycle of base level rising were bounded by erosional surfaces and a marine flooding surface,and the coal accumulating center was located in the lower deltaic plain and interdistributary lacustrine environment.In this region,the surplus accommodation space was relatively high.

A Comparative Study of the Coal-forming Characteristics of Marginal Sea Basins and Epicontinental Sea Basins

The coal-forming characteristics,as well as the similarities and differences between epicontinental sea basins and continental marginal sea basins developed during different time periods,were analyzed in this study by adopting comparative analysis thoughts and methods.The results obtained in this study revealed that epicontinental basins and marginal sea basins are both characterized by the main development of thin coal seams or extremely thin coal seams.In addition,changes in sea levels were determined to be the main controlling factors for coal formation,and there were similarities in the continent-sea interactions and coal-forming sedimentary systems of the different basins.However,there were also significant differences observed in the sea level change events,basin basement structural characteristics,coal seam stability levels,accumulation and aggregation characteristics,and the migration patterns of coal-forming materials.For example,the marginal sea basins in the South China Sea were found to be characterized by strong tectonic activities,diversity and complexity.The basin structures showed complex patterns of depressions,uplifts and concave or sag uplifts,which tended to lead to greater complexity in the paleogeographic patterns of the coal formations.This had subsequently resulted in complex coal-forming processes and paleogeographic characteristics,in which the coal-forming zones displayed bead-like distributions,and the enrichment areas and centers were scattered.The practical significance of studying the similarities and differences of the coal-forming characteristics between epicontinental basins and marginal sea basins is that the results can potentially be used to guide the predictions of coal-measure coal seam distributions in South China Sea,as well as provide valuable guidance for future explorations of natural gas reservoirs related to coal measures in the South China Sea area.