Reconstruction of Carbon Isotope of Kerogen in Shunbei Area,Tarim Basin and Discussions on Hydrocarbon Generation Model of Lower Cambrian Yurtus Formation Source Rock

Cao Zicheng; Yun Lu; Ping Hongwei; Chen Honghan; Geng Feng; Han Jun; Huang Cheng; Lyu Haitao; Jiang Huashan; Liu Yongli

doi:10.3799/dqkx.2025.182

Abstract：The source rocks of the Lower Cambrian Yurtus Formation have been considered as the main source rocks of marine oil and gas in the Tarim Basin.Currently,the understanding of source rocks of Yurtus Formation is mainly based on the sample analysis in outcrops in the basin margin and drilling core samples in uplift areas of the basin,while the characteristics of source rocks in slope and depression areas are less understood.Understanding the carbon isotope characteristics of kerogen in source rocks of Yurtus formation is of great reference significance for determining the hydrocarbon generation mechanism of source rocks and establishing the correlation between oil and gas sources.In this paper,the organic geochemistry and carbon isotope geochemistry of Ordovician crude oil and natural gas samples from different fault zones in Shunbei area are studied in detail.Based on the quantitative evaluation of the influence of thermal maturity on carbon isotopes of crude oil and natural gas,the initial carbon isotope composition of crude oil and natural gas is reconstructed.The carbon isotopic composition of kerogen from crude oil and natural gas in Shunbei area was recovered by the fractionation between kerogen and oil and gas during hydrocarbon formation.The results show that the carbon isotopes of oil-derived kerogen are mainly in the range of-32.3‰ to-28.8‰,and that of gas-derived kerogen is mainly in the range of-33.1‰ to-29.8‰.The crude oil and natural gas are mainly derived from the mixed source of benthic algae and planktonic algae,and part of crude oils in the Shunbei area are also from the main source of planktonic algae.The light carbon isotope characteristics of kerogen indicate that the oil and gas in Shunbei area mainly come from the source rocks of Yurtus Formation.Based on changes in the assemblage of hydrocarbon forming organisms,the source rocks of Yurtus Formation are divided into oil-generating source rocks with planktonic algae as the main source(δ13C>-30‰),oil-gas generating source rocks with benthic and planktonic algae as the main source(-33.5‰<δ13C<-30‰)and gas-generating source rocks with benthic algae as the main source(δ13 C<-33.5‰).With the increase of thermal maturity of source rocks,the early oil is generated by planktic algae,while the late oil(mainly volatile oil-condensate oil)contributes more,resulting in the early oil isotope is heavier,while the late oil has relatively light carbon isotope characteristics.At the same time,the content of oil source kerogen gradually decreases,while the relative content of gas kerogen(benthic algae)gradually increases,and the total carbon isotope of kerogen in source rocks gradually becomes lighter.Therefore,the different source rock types and hydrocarbon generation processes of kerogens from different sources leads to the complex carbon isotope characteristics of the oil and gas generated from the source rocks of Yurtus Formation(such as the reversal of carbon isotope of the components of the reservoir crude oil group,the reversal of carbon isotope between the chloroform extract of the source rock and kerogen,etc.).The research results can provide a new constraint for the prediction of ultra-deep oil and gas phase state in Tarim Basin.