Paleo-depositional environment, origin and characteristics of organic matter of the Triassic Chang 7 Member of the Yanchang Formation throughout the mid-western part of the Ordos Basin, China

• Published in: International journal of coal geology 2021-03, Vol.237, p.103636, Article 103636
• Main Authors: Qiao, Jinqi, Baniasad, Alireza, Zieger, Laura, Zhang, Chen, Luo, Qun, Littke, Ralf
• Format: Article
• Language: English
• Subjects: Chang 7 Member; Molecular geochemistry; Ordos Basin; Organic geochemistry; Organic petrology; Triassic
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2020.103636

A comprehensive geochemical and petrographic study was performed on 23 clay-rich shale samples collected from 18 wells drilled in the mid-western part of the Ordos Basin to investigate the characteristics and thermal maturity of organic matter as well as the paleo-depositional environment of the Chang 7 Member within the Triassic Yanchang Formation. The samples are characterized by moderate to very high organic carbon (TOC) contents ranging from 0.24% to more than 24.83% (2.72% on average) and total sulfur (TS) contents of 0.06% to 6.51%. Rock-Eval S1 and S2 values range from 0.13 to 3.70 mg HC/g rock and 0.62 to 92.50 mg HC/g rock, respectively, implying good to very good hydrocarbon generation potential for most of the samples. Rock-Eval data and petrographical observations reveal that the kerogen is mainly composed of type II to type III, consisting of a mixture of aquatic and terrigenous organic matter. The aliphatic fractions show variable monomodal to bimodal distributions of normal alkanes. Different parameters such as TOC/TS and molecular biomarker data imply that the Chang 7 member is a typical clay-rich source rock deposited in a sulphate-poor lacustrine to fluvio-deltaic environment under oxic to sub-oxic conditions with substantial input of organic matter derived from higher land plants, mainly conifers. Low values of gammacerane further indicate a low salinity of the paleo-lake. Average Tmax values of 445 °C, vitrinite reflectance values between 0.68% and 0.88%, biomarker parameters, such as sterane ratios of C29 ααα 20S/(20S + 20R) and C29 ββ/(ββ + αα), and maturity indices based on aromatic compounds, indicate a thermal maturity within the oil window for the studied samples, showing some regional variability within the mid-western Ordos Basin. •Detailed elemental and molecular composition, organic petrology of the Chang 7 Member•Thermal maturity within oil window uniform over a large area within the Ordos Basin•Lacustrine deposition at high sedimentation rates under oxic-suboxic bottom water•Kerogen originated from a mixture of higher land plants, bacteria and algae