Analysing of palaeoenvironment and organic matter enrichment: A case study from the Triassic Yanchang Formation in the southern Ordos Basin, China

Abstract

The organic-rich shale of the seventh Oil Member (Chang 7 Member for short) of the Middle-Late Triassic Yanchang Formation is extensively distributed in the southern margin of the Ordos Basin. Understanding the palaeoenvironmental conditions during the sedimentary period of the Chang 7 Member is crucial to exploring the main factors that control the enrichment of organic matter in high-quality source rocks. In this study, lithofacies analysis was conducted, and samples were collected from the N36 well located in the southern part of the Ordos Basin. Subsequently, element geochemical analysis was performed to reconstruct the palaeoenvironment, and total organic carbon (TOC) tests were conducted to analyse the characteristics of organic matter enrichment. The findings indicate that the sedimentary period of the Chang 7 Member was characterized by a relatively warm and humid palaeo-climate and a brackish water environment with oxygen-poor or sub-oxygen-rich conditions, and a gradual decrease in palaeo-water depth. The parent rock types were mainly felsic igneous rock and intermediate igneous rock, which provide limited nutritional elements for primary producers. The enrichment of organic matter in the Chang 7 Member is influenced by several factors, mainly including palaeo-salinity, palaeo-water depth, palaeo-redox conditions, terrestrial inputs, and palaeo-climate. The warm and humid palaeo-climate, higher palaeo-productivity, relatively higher palaeo-salinity, lower terrigenous input, deeper palaeo-water depth, and better preservation conditions are conducive to the development of high-quality source rock in the Chang 7 Member.