羌塘盆地双湖地区早中侏罗世过渡期碳同位素波动及成因研究

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

关键词:  下侏罗统  中侏罗统  地层界线剖面  无机碳同位素  有机碳同位素  古海洋与烃源岩  藏北羌塘地区

Key words:  Lower Jurassic  Middle Jurassic  J₂/J₁ boundary section  Inorganic carbon isotope  Organic carbon isotope  Paleo-ocean and hydrocarbon source rocks  Qiangtang Basin in northern Tibet

摘要:

藏北双湖县巴岭乡地区出露一套深水相黑色页岩地层，包括下侏罗统曲色组和中侏罗统色哇组二个组地层单元。根据菊石化石控制的生物地层时代，下伏曲色组划归Pliensbachian-Toarcian 阶，上覆色哇组级代表Aalenian-Bajocian 期沉积，二者之间为连续沉积，是目前西藏特提斯域菊石化石控制程度最高的中下侏罗统地层。野外实测了索布查J₂/J₁界线剖面，按2m间距采集了148件样品，室内开展了无机碳（δ¹³C_carb）和有机碳（δ¹³C_kero）分析，目的是揭示早侏罗世末期到中侏罗世初期这一时段的古海洋演化过程。研究结果表明，曲色组沉积期古海水δ¹³C_DIC偏正，而色哇组δ¹³C_DIC偏负，J₂/J₁界线上下δ¹³C_carb值显示阶步式负向偏移的特点。根据相关分馏方程计算，Toarcian期海洋浮游植物繁盛，δ¹³C_DIC偏正_，海水营养盐NO₃浓度偏低，而Aalenian期海洋浮游植物衰减，δ¹³C_DIC偏低，NO₃浓度升高。沉积有机质或干酪根碳同位素δ¹³C_kero在J₂/J₁界线上下与δ¹³C_carb变化趋势一致，也具有由高值逐渐偏低的特点，但δ¹³C_carb和δ¹³C_kero变化曲线的波峰和波谷并不同步，这是因为海源和陆源有机质相对含量高低变化所致。文中根据碳同位素质量平衡方程，定量的描述了索布查界线剖面陆源和海源有机质比例的变化过程，讨论了曲色组和色哇组烃源岩在油气勘探中的意义。

Abstract:

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.