流体包裹体记录的原油裂解成气温度:以湖北宜昌地区水井沱组为例

Fluid Inclusion Record Oil Cracking Tempera⁃ture:Case Study of the Shuijingtuo Formation in the Yichang Area,Hubei

原油裂解气是中国南方海相页岩气的主要来源之一,确定原油向天然气转化的温度对页岩气成藏研究具有重要意义。前人主要基于生烃模拟实验和动力学方程来模拟原油裂解的温度条件,但是依然缺乏直接的地质记录。本次研究通过对湖北宜昌地区水井沱组页岩中白云石脉和方解石脉流体包裹体岩相学观察、显微测温和激光拉曼光谱等手段来识别代表早期原油裂解的沥青+气两相包裹体,通过其同期盐水包裹体的均一温度来约束原油向湿气裂解的温度。结果表明早期油包裹体形成在白云石脉充填之后、方解石脉充填之前,与其伴生的盐水包裹体平均均一温度指示原油裂解的最低温度为142.1℃。分析认为,研究区白云石脉形成于加里东晚期至海西晚期,原油快速裂解成气在海西运动晚期(300 Ma)开始,方解石脉则形成于燕山早期(140~120 Ma);鄂西地区水井沱组台内冩湖区由于缺乏岩家河组沉积,构造作用造成水井沱组早期形成的页岩油气沿白云石脉向下进入灯影组顶部白云岩储层,对寒武系页岩气成藏以及天然气成藏具有重要的制约作用。

Gas from oil cracking is one of the main sources of marine shale gas in South China.Determina-tion of the temperature of oil cracking into gas is of great significance to the study of the process of shale gas accumulation.Previously,the temperature conditions of oil cracking were mainly based on hydrocarbon simulation experiments and dynamic equations,but the direct geological records are still not sufficient.Through petrographic observation,microthermometry and laser Raman spectroscopy,this study has identified the existence of the bitumen-bearing gas-liquid inclusions,indicative of early oil cracking and restricts the cracking temperature with the homogenization temperatures of the coevally trapped aqueous inclusions.The results show that the early-stage oil inclusions have been trapped later than the precipitation of dolomites and earlier than that of the calcites filling the veins,with the coeval aqueous inclusions homogenizing at 142.1℃,which indicates the lowest cracking temperature of the crude oil.Combined with other information,it is deduced that the dolomite has been precipitating from the late Caledonian to the late Hercynian orogeny,during which point the oil started cracking rapidly(300 Ma).Calcite veins have been formed at the early stage of the Yanshan orogeny(140~120 Ma).The Shuijingtuo formation in the lagoon facies in south Hubei lacks the deposition of Yanjiahe formation,therefore caused by tectonic movements,the migration of early shale oil generated from the Shuijingtuo formation into the dolomite reservoir of the uppermost part of the Dengying formation through dolomite veins,which provides essential constraints to the shale gas accumulation and other natural gas accumulation in the Cambrian strata.