Morphologies, patterns, and filling architectures at the confluence zones of the cretaceous shallow-water delta in the Songliao Basin, NE China

Abstract

Confluences are particular locations within distributary channel networks that undergo notable changes in hydraulic and morphological features, leading to distinct erosional and sedimentary characteristics. However, our understanding of the filling architecture of river confluences remains incomplete. To address this gap, it is necessary to explore typical deposits at ancient channel confluences. This study conducts a detailed analysis of parasequence set 4 (PSS4) from 36,886 wells that penetrated the Cretaceous shallow-water Daqing Changyuan delta, Songliao Basin, NE China. Based on the composition, fabric, texture, particle size, and sedimentary structures of cores, seven lithofacies, ten lithofacies associations, and two facies associations were identifed. The interpretation of the sedimentary environment indicated that Daqing Changyuan delta mainly comprises delta plain and delta front in the study area, and we provided a precise description of the distributary channel system covering 690 km2. The distributary channels in the delta show three confluence patterns: asymmetric confluence, symmetric confluence with a low angle, and asymmetric confluence with a high angle. The patterns give rise to varied three-dimensional reservoir architecture units, including a tributary mouth-bar, an upstream junction bar, a separation zone bar, a confluence scour bar, a fine-grained deposition wing, and a post-confluence mid-channel bar. Each confluence pattern has its own unit compositions and distributions, resulting in varying reservoir potentials.