Sedimentary environment and organic matter enrichment mechanisms of Lower Cretaceous source rocks in the Hari Sag, Yingen-Ejinaqi Basin

In this study, we examined the Hari Sag in the Yingen-Ejinaqi Basin as a case study to investigate the factors controlling organic matter (OM) enrichment and the related mechanisms in small- to medium-sized faulted lake basins in northern China. Based on TOC and elemental geochemistry data from mudstone samples, we reconstructed the sedimentary environment and its evolution of the study area during the Early Cretaceous. We further investigated the mechanisms of OM enrichment in the source rocks and developed a corresponding enrichment model. The results reveal the sedimentary environment and evolutionary characteristics of the Early Cretaceous in the study area. Paleoclimate of the study area shifted from semi-humid to arid, and finally to semi-humid. Meanwhile, the redox conditions of the water column evolved from oxidizing to weakly oxidizing (or weakly reducing), then returned to oxidizing, and finally shifted to weakly oxidizing (or weakly reducing). Paleosalinity increased overall from fresh water to saline water conditions. Specifically the paleowater depth was relatively deep during the deposition of the first and second members of the Bayingobi Formation (K 1 b 1 and K 1 b 2 ), relatively shallow during the deposition of the third member of the Bayingobi Formation (K 1 b 3 ) and the Suhongtu Formation (K 1 s), and reached its maximum during the deposition of the Yingen Formation (K 1 y). Synsedimentary hydrothermal activity was common during the Early Cretaceous in the study area, with varying intensities among different stratigraphic intervals. Changes in regional tectonic and climatic environments jointly influenced the paleosalinity and the paleowater depth, and other factors, determining the lake paleoproductivity of each stratigraphic interval in the study area. Redox conditions and paleosalinity influenced OM preservation, thereby determining the degree of OM enrichment in each interval. Synsedimentary hydrothermal activity also enhanced OM enrichment to a certain extent, and variations in hydrothermal intensity across different sedimentary periods led to differences in OM enrichment. Based on this, we further established an OM enrichment model for small- to medium-sized faulted lake basins dominated by tectonic movement, paleoclimate, and hydrothermal sedimentation. Our findings provide a reference for similar studies and offer a basis for source rock prediction in analogous small- to medium-sized faulted lake basins.