Paleoenvironment reconstruction and differential OM enrichment mechanism of the Upper Triassic Chang 7 member source rocks in the Ordos Basin

The organic-rich mudstones within the lacustrine deposits of the Chang 7 member in the Ordos Basin are recognized as the primary source rocks of the Mesozoic petroleum system. Previously, scholars have extensively investigated the formation processes of high-quality source rocks in the deep lake facies of the southern part of the basin. However, systematic studies on the differential enrichment of organic matter in the Chang 7 member across the entire basin are lacking. In this study, geochemical analyses, including rock pyrolysis, total organic carbon (TOC) content, saturate hydrocarbon gas chromatography-mass spectrometry, and major and trace element analyses, are performed to determine the hydrocarbon generation potential of the Chang 7 member source rock in shallow lake facies and illustrate the differences in the organic matter enrichment of this member between the deep and shallow lake facies. The shallow lake facies Chang 7 member source rocks in the northern part of the basin are generally of medium to high quality. These rocks are mature and feature II₁-II₂ organic matter types, which can be considered as effective source rocks for oil-gas accumulation in suit. In shallow lake facies, a warm and humid climate facilitated the input of nutrients from terrigenous clastics for primary producer, which resulted in higher organic matter abandunce in the shallow lake facies of the Chang 7₁₊₂ sub-member interval compared to coeval deep lake sediments. Conversely, the deep lake facies of the Chang 7₃ sub-member interval, influenced by volcanic and hydrothermal activity, exhibited significantly higher organic matter abundance. Frequent and intense volcanic activities introduced volcanic ash and hydrothermal inputs, which reduced gas exchanges between atmosphere and watermass, as well as increased water salinity, fostering a stratified and anoxic-euxinic watermass that promoted organic matter preservation. However, shallow lake regions remained less affected by volcanic influences due to long distance from volcanic and hydrothermal activity area, with organic matter accumulation primarily driven by terrigenous nutrient supply under warm-humid climatic conditions.