Abstract: The Lower Cambrian Niutitang Formation shale in the Upper Yangtze block is characterized with high total organic carbon (TOC) content, large thickness, and wide distribution, thus being identified as the potential favorable stratum for shale gas resources. However, the high nitrogen(N₂)content in this shale sequence increases the geological exploration risks. To reveal the genesis mechanism of N₂ in the Cambrian Niutitang Formation shale, analyses were conducted on the marine nitrogen cycle, inorganic nitrogen distribution and its controlling factors, as well as the pyrolysis patterns of nitrogen-containing compounds during the depositional periods of Niutitang Formation, and a comparison was made with the N₂-poor shales deposited in Wufeng Formation and Longmaxi Formation. Reconstruction of the marine nitrogen cycle demonstrates that during the deposition of the Niutitang Formation shale, ammonium assimilation in both shelf and slope facies resulted in the accumulation of ammonium (NH₄⁺) in sedimentary waters or pore waters, with the highest NH₄⁺ concentration observed in the slope facies. By contrast, during the deposition of the Wufeng Formation and Longmaxi Formation shales, the nitrogen cycle in the Yangtze Sea was dominated by nitrogen fixation, thus limiting the concentration of NH₄⁺ in sedimentary waters or pore waters. The content of inorganic nitrogen in the Niutitang Formation shale gradually increased from platform facies through shelf facies to slope facies, followed by a precipitous decline in basin facies. This spatial variation is controlled by the NH₄⁺ concentration and redox conditions in the marine environment. Pyrolysis analysis of nitrogen-containing compounds indicates that N₂ release in shale is dominated by inorganic nitrogen decomposition at a low thermal maturity stage, and by organic nitrogen cracking at a higher thermal maturity stage, between which the transition is marked by the equivalent vitrinite reflectance of 3.4 %. As indicated by a strong positive correlation between the inorganic nitrogen content and N₂ concentration, the inorganic nitrogen decomposition during thermal evolution may have played a key role in increasing the N₂ concentration in the Niutitang Formation shale. These findings are of reference significance for the exploration and research of shale gas in the Lower Cambrian reservoirs and other N₂-rich intervals in South China, and also provide valuable insights for the exploration and development of shale gas in other similar areas.

Key words: Upper Yangtze block, Cambrian, Niutitang Formation, Wufeng Formation, Longmaxi Formation, organic nitrogen, inorganic nitrogen, nitrogen genesis

摘要： 上扬子地块下寒武统牛蹄塘组页岩有机碳含量高、发育厚度大、分布面积广,是页岩气资源潜在的有利层系,但该套页岩中氮气(N₂)含量普遍较高,增加了地质勘探的风险。为揭示寒武系牛蹄塘组页岩的N₂成因,对牛蹄塘组沉积期的海洋氮循环、无机氮分布及其控制因素和含氮化合物的热解模式开展了分析,并与贫N₂的奥陶系五峰组—志留系龙马溪组沉积期页岩进行了对比。海洋氮循环重建表明,牛蹄塘组页岩沉积期,陆架相和斜坡相由于铵同化作用导致沉积水体/孔隙水中铵根(NH₄⁺)累积,尤其在斜坡相,NH₄⁺浓度最高,而在五峰组—龙马溪组页岩沉积期,扬子海的氮循环以固氮作用为主,沉积水体/孔隙水中的NH₄⁺浓度受限。牛蹄塘组页岩中的无机氮元素含量从台地相、陆架相到斜坡相逐渐增高,至盆地相呈断崖式下降,这种空间变化受控于海洋环境中的NH₄⁺浓度和氧化-还原环境。含氮化合物的热解实验分析表明,页岩中的无机氮热解并释放N₂主要发生在热演化程度相对较低的阶段,而有机氮热解并释放N₂主要发生在热演化程度更高的阶段,两者之间的界限大约在等效镜质体反射率为3.4 % 处。牛蹄塘组页岩中的无机氮元素含量与N₂含量的变化在空间上高度吻合,指示页岩中的无机氮热解可能对N₂含量的增加发挥了关键作用。研究对开展中国南方地区下寒武统及其他高N₂含量目的层系的页岩气勘查研究具有参考作用,对其他相似地区的页岩气勘探开发也具有借鉴意义。

关键词: 上扬子地块, 寒武系, 牛蹄塘组, 五峰组, 龙马溪组, 有机氮, 无机氮, 氮气成因