湘鄂西地区奥陶系—志留系滑脱层古流体对页岩气保存的意义

Significance of paleo-fluid in the Ordovician–Silurian detachment zone to the preservation of shale gas in western Hunan–Hubei area

目前,对于中—上扬子地区奥陶系—志留系滑脱层古流体的研究热点集中在高密度甲烷包裹体的成因、地层压力演化过程等,而有关古流体对页岩气保存的意义则较少涉及。为此,通过观测湘鄂西地区奥陶系—志留系露头和钻探岩心的裂缝脉体地质特征,分析裂隙形成与滑脱层的关系,应用裂缝脉体包裹体组分、均一温度测试资料研究古流体的形成环境和滑脱层对页岩气保存的意义,并建立了滑脱层页岩气聚散模式。研究结果表明:(1)滑脱层岩性主要为奥陶系—志留系之交的硅质岩夹页岩,地层岩石破碎,发育小型揉皱,滑脱层上下硅质岩层共轭垂直节理较发育;(2)滑脱层脉体发育多期次、多类型包裹体,纯水溶液包裹体尤为发育;(3)研究区古大气水沿着滑脱层最大下渗深度超过4 000 m,滑脱带早期捕获了超压条件下的高密度甲烷包裹体,后期捕获常压环境的甲烷和氮气包裹体;(4)记录晚期流体活动的包裹体离子组分具有钠氯系数高、脱硫系数大、变质系数低的特点,表明滑脱层页岩封闭性变差;(5)研究区滑脱层页岩气聚散可分为向斜、宽缓背斜、紧闭背斜等3种模式。结论认为,研究区滑脱层为渗透层,滑脱构造发育的向斜、紧闭背斜不利于页岩气的富集,而宽缓背斜则相对有利于页岩气的聚集。

The studies on the paleo-fluid in the Ordovician–Silurian detachment zone in the Middle–Upper Yangtze area focus on the origin of high-density methane inclusions and the evolution process of formation pressure, but rarely deal with the significance of paleo-fluid to shale gas preservation. In this paper, the relationship between fracture formation and detachment zone was analyzed by observing the Ordovician–Silurian outcrops in western Hunan–Hubei area and by investigating the geological characteristics of fracture veins in the drilling cores. Then, the significance of paleo-fluid forming environment and detachment zone to shale gas preservation was studied by using inclusion compositions of fracture veins and homogenization temperature test data. Finally, accumulation-dispersion modes of shale gas in the detachment zone were established. The following results were obtained. First, the detachment zone is lithologically composed of silicate with intercalated shale at the Ordovician–Silurian interface. In the detachment zone, rocks are broken and small crumples are developed. The conjugate-vertical joints are relatively developed in the silicate above and below the detachment zone. Second, multistage and multi-type inclusions, especially the aqueous inclusions, are developed in the veins of the detachment zone. Third, the infiltration depth of ancient meteoric water along the detachment zone in the study area is over 4 000 m. High-density overpressure methane inclusions were captured in the detachment zone in the early stage, while normal-pressure methane and nitrogen inclusions were captured in the late stage. Fourth, the ionic constituents of inclusions recording the fluid activity in the late stage was characterized by high sodium chloride coefficient, high desulfurization coefficient and low metamorphic coefficient, and it is indicated that the sealing capacity of the shale in the detachment zone gets worse. And fifth, the accumulation and dispersion of shale gas in the detachment zone within the study area is divided into three modes, i.e., syncline, broad anticline and closed anticline. In conclusion, the detachment zone in the study area is permeable. Synclines and closed anticlines developed in detachment structures are unfavorable for the accumulation of shale gas, while broad anticlines are favorable.