Sedimentary characteristics of sandy braided river deposits and factors controlling their deposition: A case study of the lower Shihezi Formation in the northern ordos basin, China

Abstract

Fluvial sandstones have been identified as significant reservoirs due to their importance in global hydrocarbon exploration. The current study investigated the sedimentary characteristics and temporal evolution of sandy braided river systems as well as the factors controlling their deposition in the Lower Shihezi Formation (H1-1 to H1-4 sub-member) of the Xinzhao area in the northern Ordos Basin using well logs, core intervals, whole rock geochemical data, and other measured reservoir parameters. Within the braided river deposits, five architectural elements were identified: multistory braided channel, single-story braided channel, sandy braid bar, overbank, and floodplain. The evolution of architectural elements in the Lower Shihezi Formation is closely related to paleoclimatic conditions and tectonic activity. Climate influences sediment accumulation through its effect on terrestrial detrital input, and tectonism influences stacking patterns and architectural style by influencing subsidence rates and accommodation changes. High sediment deliveries and high-energy hydrodynamic conditions in warm and humid climates, combined with low subsidence rates and low accommodation/sediment supply (A/S) ratios, can result in the formation of highly amalgamated braided channels and braid bars as well as complex vertical stacking patterns and aggradational processes in the H1-2 and H1-3 sub-members. Conversely, the dry and hot climates, high subsidence rates and associated high A/S ratios are responsible for the low detrital influx input, stacking of isolated sand bodies, and high preservation of overbank and floodplain deposits in the H1-4 sub-member. Considering the accommodation-based systems tracts, we contend that climate and tectonism both have a significant impact on the evolution and architectural styles of sandy braided river deposits. It is suggested that climates control the stacking pattern of fluvial sand bodies during high accommodation systems tracts, whereas both climates and accommodation spaces influence architectural styles and the degree of sand body amalgamation during low accommodation systems tracts. Considering the thick, highly amalgamated sand bodies with better reservoir properties and high vertical and lateral connectivity in the H1-2 and H1-3 sub-members, we posit that these stacked multistory sand bodies should be the preferential targets for hydrocarbon exploration. The findings of this study can further elucidate the architectural styles and factors that influence the deposition of sandy braided river systems under complex tectonic and climatic conditions.