The carbon isotope fluctuation and its origin interpretion during the Early to Middle Jurassic transition period in the Shuanghu area of Qiangtang Basin

Deep-water black shale strata cropped out in Balingxiang area, Shuanghu County of Northern Tibet, including Lower Jurassic Quse Formation and the Middle Jurassic Sewa Formation. The occurrence of ammonite fossil indicates a Pliensbachian-Toarcian stage of Quse Formation and a Aalenian-Bajocian stage of its overlying strata Sewa Formation, which were comfortable with the highest degree of fossil control in the Tibetan Tethys region. The Soubucha J₂/J₁ boundary section is measured in the field, 148 samples are collected at 2-meter intervals, and the analysis of inorganic carbon (δ¹³C_carb) and organic carbon (δ¹³C_kero) is carried out in the laboratory, to reveal the paleo-oceanic evolution from the late Early Jurassic to the Early Middle Jurassic. It shows a positive shift of δ¹³C_carb in the Quse Formation while a negative shift of δ¹³C_carb in the Sewa Formation and a step negative excursion across of J₂/J₁ boundary. According to fractionation equation calculation, marine phytoplankton was abundant in the Toarcian stage with a positive shift of δ¹³C_DIC and the concentration of nutrient NO₃ in seawater was low, while the opposite was the case in the Aalenian stage. The sedimentary organic matter (kerogen) carbon isotope δ¹³C_kero is consistent with the variation trend of δ¹³C_carb above and below the J₂/J₁ boundary, characterized by the gradual decrease from the high value while the crest and trough of the change curves of δ¹³C_carb and δ¹³C_kero are not synchronized, owing to the relative content of organic matter variation in the marine and the terrestrial provenance. Based on the carbon isotopic mass balance equation, this study describes quantitatively the variation process of the organic matter ratio between marine and terrestrial source in the Sobucha boundary section, and discusses the significance of the source rocks of the Quse Formation and the Sewa Formation in oil and gas exploration.