南大巴山前缘黄龙组古喀斯特储层稳定同位素地球化学特征

Geochemical characteristics of stable isotopes of paleokarst reservoirs in Huanglong Formation in the front of southern Daba Mountain,China

南大巴山前缘上石炭统黄龙组古喀斯特储层可划分为弱溶蚀喀斯特岩、中等溶蚀多孔状喀斯特岩、强溶蚀角砾状喀斯特岩和强溶蚀交代的次生灰质喀斯特岩4种类型。以非溶蚀岩为对比参照物,系统分析了各类喀斯特岩的稳定同位素组成。结果表明,不同成因类型和溶蚀强度的喀斯特岩具有贫δ18O和高87Sr/86Sr比值,以及δ13C分布较宽的地球化学特征,明显受控于喀斯特过程中古水文条件、流体性质、同位素来源及其分馏效应;δ13C-δ18O值和87Sr/86Sr比值分别出现由渗流-活跃潜流带的弱正偏移和正偏移,至静滞潜流带均转化为强烈负偏移的演化趋势;大气源的CO32-(sq)对喀斯特流体中δ13C和δ18O组成特征及其负偏移具有重要影响,高87Sr/86Sr比值特征主要与参与古喀斯特作用水-岩反应的古陆地下水富87Sr壳源锶的注入,以及白云质基质岩对87Sr具有强富集作用有关。

The karst rock in the Upper Carboniferous Huanglong Formation of the margin of southern Daba Mountain in Southwest China is divided into four types, including slight corrosion of karst rock, medium corrosion porous of karst rock, strong corrosion-breccia of karst rock and strong corrosion and alternation of secondary lime-karst rock. Compared with the un-karstified rock, this paper systematically analyses the carbon, oxygen and strontium isotope composition of various karst rocks. The result indicates that the karst rocks with different genetic types and corrosion intensities have the geochemistry characteristics with a low δ^8O, high ^87 Sr/^86 Sr ratio, and a wide range of the δ^13 C. They are distinctly controlled by ancient hydrological conditions, fluid property, isotope sources and their fractionation effect in the process of karst. The δ^13C-δ^18 O value and ^87Sr/^86 Sr ratio respectively have the evolving trends of slight positive and positive deviations from infiltrating fluid belt to active underflow belt, and they all gradually transform into a strong negative deviation in the inactive underflow belt. The CO3^2- （sq） of atmosphere source has an important impact on the composition of δ^13C and δ^18O in the karst fluid and its negative deviation. The characteristics of high ^87 Sr/^86 Sr ratio are related to the injection of groundwater with rich ^87 Sr in the water-rock interaction process of the paleokarst, and also with the strong ^87 Sr enrichment of the matrix dolomite. The characteristics of carbon, oxygen and strontium of different karst reservoirs can provide geochemical information for forecasting and evaluating the conditions of development and distribution rule of time and space of Huanglong Formation on the margin of southern Daba Mountain.