四川江油地区下三叠统飞仙关组白云化流体的锶同位素示踪研究

Sr isotopes are as one of effective tools to trace the sources of the dolomitizing fluids, and have increasingly become an indispensable method to the studies of the origin of dolomite. Based on the stratigraphy, sedimentology, mineralogy, petrology, elements, C, O and S isotope geochemistry, this study will systematically obtain the Sr isotope composition of the different types of dolomite in the Lower Triassic Feixianguan Formation, Jiangyou area of Sichuan Province and then establish and compare the Sr isotope curves of the Feixianguan Formation at each outcrop section that experienced different degrees of dolomitization. Moreover, combined with the studies of the regional tectonic, basin modeling, paleoclimate, sea-level change, sequence stratigraphic framework and fluid inclusions, this study will comprehensively identify the properties and sources of the dolomitizing fluids using the Sr isotope composition and their trends of the different types of dolomite, and the simulation calculation of the Sr isotope change in the mixing processes of the different fluids. Finally, this study will sum up the processes of the dolomitizing fluids, and conclude the dolomitizing systems and dolomitization mechanisms of the different types of dolomite. This study will provide a new approach to further understand the sources of the dolomitizing fluids and the origin of dolomite in the Lower Triassic Feixianguan Formation, Jiangyou area of Sichuan Province, and also provide new basic data to make progress in the development of the marine oil & gas exploration in the northwestern Sichuan Basin, even in the whole Sichuan Basin.

锶同位素组成作为目前白云化流体示踪研究的有效工具之一，日益成为各国地质学家研究白云岩（白云石）成因中不可或缺的方法之一。在地层学、沉积学、矿物学、岩石学、元素和碳氧硫同位素地球化学等研究的基础上，系统获取四川江油地区下三叠统飞仙关组不同类型白云岩（白云石）的锶同位素组成，建立和对比不同白云化程度、不同地区飞仙关组的锶同位素组成演化曲线，结合区域构造、盆地模拟、古气候、海平面变化、层序格架、流体包裹体等研究，利用不同类型白云岩（白云石）的锶同位素组成及其变化趋势、不同流体混合时锶同位素组成变化的模拟计算综合反演白云化流体的性质和来源，归纳总结白云化流体的作用过程，判断不同类型白云岩（白云石）对应的白云化系统与白云化机制，为进一步研究四川江油地区下三叠统飞仙关组的白云化流体来源、白云岩成因提供一个新的方法和途径，并为进一步促进川西北地区乃至整个四川盆地海相地层油气勘探的发展提供崭新的基础资料。

在地层学、沉积相以及岩石学研究的基础上，测试了江油地区下三叠统飞仙关组不同类型海相碳酸盐岩的阴极发光性、微量元素、碳氧同位素和锶同位素组成，并在岩石学特征、成岩作用特征、阴极发光性、地球化学特征以及白云化流体的来源机制等方面作了总结，建立了江油地区下三叠统飞仙关组白云化作用模式。飞仙关组海相碳酸盐岩中亮晶鲕粒灰岩、残余鲕粒白云岩、结晶白云岩是其主要优质储集岩类型；飞仙关组白云岩的主要成岩作用类型包括：白云化作用、早期溶蚀作用、晚期溶蚀作用、晚期胶结作用；飞仙关组多数样品主要分布在弱—中、中等的阴极发光强度，仅部分样品分布在相对强的阴极发光强度；飞仙关组中方解石和白云石均具有低Mn和Fe含量、较高Sr含量、小Mn/Sr比值、稀土元素不富集、高δ13C值、较高δ18O值、较大87Sr/86Sr比值等地球化学特征，大气淡水并不是白云化流体的主要来源，贫Mn和Fe、富Sr、高13C和18O的近同期或非同期海源流体才是飞仙关组白云化流体的主要来源，早期白云化流体很可能是准同生或较浅埋藏条件下浅部潜流环境中渗透回流的近同期蒸发海水，晚期白云化流体很可能是埋藏条件下深部渗流环境中循环的非同期海源流体（卤水）。