Longshore changes in the microfacies and distribution of clastic barrier coastal sandbodies: a case from the Benxi formation in the Ordos Basin, China

The Benxi Formation in the Ordos Basin is an important gas-producing layer in the Yanchang gas field. There are critical changes in the microfacies type and extension trend of sandbodies, which affects the understanding of reservoir formation and the deployment of development wells. Based on many core and thin section observations, as well as statistical analyzes of sandstone thickness and percentage of sandstone content from 1446 wells, we analyzed the coastal changes and control factors of the microfacies type, planar morphology and distribution of the Benxi Fm sandbodies in the field. The results indicate that the Benxi Fm of the Yanchang gas field is generally composed of terrigenous coastal clastic barrier deposits in a medium to small tidal range environment, but the wave action in the southern region is weak, forming typical barrier sandbars and backbarrier deposits. The wave actions in the north change more strongly, forming three major microfacies sandbodies: reworked barriers, shoals and incised valley filled tidal channels. The barrier sandbodies are arranged in a belt or bead-like pattern parallel to the shoreline and are distributed longshore near the provenance. The oval-shaped shoal sandbodies are dispersed further from the shoreline, and the filling of tidal channel sandbodies is controlled by ancient geomorphic grooves. This research has important guiding significance for the identification and evaluation of reservoirs in the Yanchang gas field and similar geological settings.