Differential enrichment mechanisms of organic matter in the Chang 7 Member mudstone and shale in Ordos Basin, China: Constraints from organic geochemistry and element geochemistry

Abstract

Source rocks in the seventh member of the Upper Triassic Yanchang Formation (Chang 7 Member) are characterised by lacustrine organic-rich mudstone and organic-rich shale in the Ordos Basin. There are obvious differences in organic and inorganic geochemical characteristics between mudstone and shale in the Chang 7 Member, indicating that they are significantly different in genetic mechanism. Presently, the difference of organic-matter enrichment mechanism between shale and mudstone in the Chang 7 Member is still obscure. In this study, we explored the organic-matter accumulation mechanisms in the Chang 7 Member mudstone and shale by investigating their parent-rock properties, palaeoproductivity, palaeoenvironment and dilution conditions using biomarkers and elemental geochemical data. According to our analysis results, the mudstone and shale parent rocks are composed of sedimentary rock and granite. The material source of the mudstone was mainly from the south of the Ordos Basin, whilst the source of the shale was mainly from the south and northeast of the basin. The organic-matter enrichment of the shale was affected by high palaeoproductivity and anoxic environment conducive to the preservation of organic matter. The palaeoproductivity during the deposition of the mudstone was moderate, and the dilution caused by the high sedimentation rate was the main reason for its low abundance of organic matter. Affected by frequent volcanic activities, a grater algae bloom occurred in the process of the shale deposition, which made the palaeoproductivity during shale deposition higher than that of the mudstone deposition. In addition, the degradation of a large quantity of algae resulted in a reducing environment during shale deposition. Therefore, the high palaeoproductivity and anoxic preservation conditions resulted in large amounts of aquatic organisms in the shale, resulting in high organic-matter abundance in the shale. The palaeoclimate during mudstone deposition was warmer and wetter than during shale deposition, which was more conducive to parent-rock weathering and the transportation and deposition of terrigenous detritus. Therefore, the organic matter of mudstone is a mixture of terrestrial higher plants and aquatic organisms. However, the high input rate of terrigenous detritus caused the dilution of organic matter in the mudstone. The mudstone was mainly deposited under weakly oxidising to weakly reducing conditions. The moderate palaeoproductivity, oxygen-containing preservation conditions and dilution caused by the high sedimentation rate made the aquatic and terrestrial organic matter in the mudstone easily oxidised, degraded and diluted. Therefore, the differences of primary productivity and preservation conditions make the organic-matter abundance of the mudstone lower than that of the shale.