Grain-size characteristics and sedimentary environmental significance of terrestrial red sandstone in the Dongying Depression with a gentle slope zone

Objective In the gentle slope zone of the Dongying Depression, the first member of the Kongdian Formation and the lower part of the fourth member of the Shahejie Formation can be split by a distinctive assemblage of reddish sediments, which originated from both fluvial and lacustrine processes. Extensive research has substantiated the significant exploration potential of the Palaeogene red bed in the Dongying Depression. Nevertheless, the sedimentary facies type of the red bed remains a subject of debate among researchers. Methods To address this issue, we present a comprehensive analysis of wells W1, W2, and W3, which are located in the Wangjiagang area within the southern gentle slope zone of the Dongying Depression. This analysis mainly focused on evaluating the grain size distribution, grain size parameters, and grain size probability accumulation curves of the red bed. Results This study revealed the distinctive characteristics of the red sandstone reservoirs in the study area. These include thin single sand body thicknesses ranging from 1 to 7.7 m and fine particle sizes, changing from 2.24ϕ to 4.73ϕ. The grain size distribution is mainly characterized by unimodal positive skewness curves, suggesting the dominance of the relatively coarse fraction. The grain-size parameter discriminant function and bivariate grain-size parameter plots suggest a fluvial environment. The grain size accumulation probability curves of most samples exhibit a significant transition component (over 50%), and the graphs in the C-M bivariate plot are parallel to the C=M baseline, reflecting the characteristics of density flow. Moreover, the logging curves mainly exhibit box and bell shapes with medium to high amplitudes, indicating that the sedimentary processes primarily involved vertical accretion and lateral accretion. The core analysis revealed the presence of blocky bedding indicative of rapid sediment accumulation, parallel bedding characteristic of unidirectional flow, and small cross-bedding suggestive of weak flow. Conclusion In conclusion, it is believed that the red sandstone resulted from distributive fluvial systems deposited with both palaeo-geomorphic features and hot-arid palaeo-climate factors in the study area. From upstream to downstream, the hydrodynamic conditions gradually weakened, the bifurcation of river channels strengthened, the flood erosion ability decreased, the single sand body thickness and sediment grain size decreased progressively, and the fluid properties transitioned from density flow to traction flow. The novel insights of this study offer a fresh perspective for the exploration of petroleum in the red sandstone of the study area. In forecasting such reservoirs, particular emphasis should be placed on analyzing palaeo-provenance and paleo-current while enhancing the identification and characterization of individual-stage flood channels.