High-resolution carbon and oxygen isotope records in the Chang 7 organic-rich shale of the Yanchang formation, Ordos Basin, China and their geological significance

The Late Triassic Chang 7 Member lacustrine organic-rich shale in the Ordos Basin is the most significant source rock for the Mesozoic oil system in the basin. This study utilizes high-resolution source rock samples to systematically analyze the organic carbon isotopes and carbonate carbon-oxygen isotopes of the Chang 7 Member source rocks. The results indicate that the average δ13Ccarb values for the Chang 71 to Chang 73 sub-members are 0.17‰, 0.33‰, and −2.8‰, respectively, and the average δ18