川东石炭系古岩溶储层锶同位素地球化学特征

Strontium isotopic geochemical behaviors of the Carboniferous palaeo-karst reservoirs in east Sichuan basin

石炭系黄龙组岩溶岩是四川盆地东部渝北—邻水地区天然气藏最重要的储层之一,依据其结构特征,将该岩溶岩系划分为弱溶蚀岩溶岩、中等溶蚀多孔状岩溶岩、强溶蚀角砾状岩溶岩和强溶蚀交代次生灰质岩溶岩等4种类型,以及淡水方解石和白云石2种胶结物类型。对各类型样品的锶同位素分析结果表明:①不同成因类型、溶蚀强度、结构和成分特征的岩溶岩与胶结物锶同位素地球化学特征各异,其形成、演化与分布规律明显受古地理背景、流体性质和来源以及锶同位素分馏效应的控制;②岩溶过程中的87Sr/86Sr比值,具有从渗流—活跃潜流带的正偏移,至静滞潜流带逐渐转化为强烈负偏移演化趋势,对应的古岩溶储层物性由差变好至重新变差;③溶蚀过程中,白云质岩溶岩和淡水白云石胶结物对87Sr有很强的正偏移同位素分馏效应,而次生灰质岩溶岩和淡水方解石胶结物缺乏锶同位素分馏效应。

The karst rocks in the Carboniferous Huanglong formation is one of the major gas reservoir types in the area between the northern Chongqing and Linshui in east Sichuan basin.According to their structural features,the karst rocks in the Huanglong formation can be divided into 4 types,including weak-solution karst rock,moderate-solution porous karst rock,strong-solution brecciated karst rock,and strong-solution metasomatic secondary lime karst rock.Two cement types,including fresh water calcite and dolomite,are identified.The following understandings are achieved through strontium isotope analysis of various rock samples.① Karst rocks and cements of different genetic types,dissolution intensities,structures and compositions are different in strontium isotopic geochemical behaviors,and their formation,evolution and distribution are strongly controlled by the palaeo-geographic setting,fluid properties and sources as well as the fractionation effects of strontium isotope.② In the process of karstification,the 87Sr/86Sr ratio shows positive migration from seepage flow zone to active underflow zone and progressively turns into strong negative migration in stagnant underflow zone.The poroperm characteristics of the palaeo-karst reservoirs change correspondingly from poor to good and back to poor again.③ In the process of dissolution,dolomitic karst rocks and fresh water dolomite cements have strong positive migration isotopic distillation effect on 87Sr,while the secondary lime karst rock and fresh water calcite cements have no such fractionation effects.