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 δ 13 C carb values for the Chang 7 1 to Chang 7 3 sub-members are 0.17‰, 0.33‰, and −2.8‰, respectively, and the average δ 18 O carb values are −14.1‰, −14.3‰, and −16.0‰, respectively. The Chang 7 3 sub-member exhibits anomalously low δ 13 C carb and δ 18 O carb values due to the influence of volcanic and hydrothermal activity. The carbon-oxygen isotopes, together with other element proxies, indicate that the Ordos Basin had semi-closed to semi-open hydrological conditions during the depositional periods of Chang 7 1 and Chang 7 2 , with an overall warm and humid climate. The climate during the Chang 7 2 depositional period was relatively stable, while the Chang 7 1 period showed a trend towards increasing aridity. The lake during the Chang 7 depositional period had high paleo-productivity, with sedimentary organic matter being of mixed origin, primarily from aquatic organisms with some contribution from C3-type terrestrial higher plants. The findings of this study provide support and reference for research on paleoclimate, paleoenvironment, and organic matter enrichment mechanisms in the region.