Sedimentary facies development and co-evolution of Eastern and Western marine systems in the late carboniferous Ordos Basin

Abstract

The sedimentary characteristics and evolution of Late Carboniferous marine systems in the eastern and western Ordos Basin are crucial for predicting source-reservoir assemblages and hydrocarbon exploration. This study systematically investigates the sedimentary facies develop-ment and evolutionary patterns through integrated field section observations, core analysis, seismic and well-log interpretations, lithofacies examination, and paleocurrent measurements, ultimately establishing a comprehensive depositional model. Results demonstrate that the Late Carboniferous succession contains 16 lithofacies types grouped into 7 distinct associations, representing tide-dominated delta plain distributary channels, delta front subaqueous distributary channels, estuarine-tidal flat complexes (including tidal channels, sand bars, and mudflat-mixed flats), barrier island-lagoon systems, and shelf deposits. The depositional environment evolved from a transitional deltaic-clastic coastal system through three phases: (1) During Yang-3rd Member deposition, the central-eastern basin remained an erosional paleo-uplift with localized sedimentation along the western margin that was dominated by tide-dominated delta-tidal flat-barrier island systems; (2)Yang-2nd Member period witnessed intensified rifting that expanded the Qilian marine domain, establishing estuarine-tidal flat deposition throughout the study area while the eastern North China Sea began receiving sediments, developing tide-dominated deltas in the Fugu-Shenmu area and tidal flat-lagoons in the central-eastern basin, all separated by the central paleo-uplift; (3) During the deposition of the Yang-1st Member, weakened rifting in the west and enhanced uplift in the north caused the central paleo-uplift to shrink southward. This process eventually connected the eastern and western marine realms, leading to the development of distinct depositional systems: tide-dominated deltas in the north, tidal flat-barrier island-shelf systems in the west, and tidal flat-barrier island-shallow shelf environments in the east, under conditions of sufficient sediment supply.