准噶尔盆地哈山地区石炭系-二叠系裂缝充填方解石的成因机制及石油地质意义

摘要

准噶尔盆地西北缘哈山地区经历了多期强烈构造活动,石炭系、二叠系裂缝发育且普遍充填方解石脉.通过对充填方解石脉的碳、氧、锶同位素和流体包裹体等地球化学分析,探讨了该区石炭系、二叠系成缝期后的地层流体活动及其与油气成藏的关系.结果表明,充填的方解石普遍受火山热液与油源流体的双重影响,δ13C特征反映形成方解石的碳元素存在两类来源:地幔和大气CO2,δ18O偏负特征主要反映高温流体介质的影响,s7Sr/86Sr比值说明其受幔源锶和壳源锶的混合叠加影响;综合分析认为,区内裂缝充填的方解石脉存在三种成因机制:火山热液成因、油源流体改造成因和大气水改造成因,分别对应于三类流体活动,第一类,二叠纪火山热液的活动,第二类,晚二叠世以来的油源流体,第三类,燕山期喜马拉雅期的大气水.与油源流体有关的方解石脉中次生含烃盐水包裹体的均一温度分布在70～80℃,100～110℃,130～160℃三个温度区间,分别对应二叠纪末、三叠纪末、白垩纪末三期油源流体充注,油源流体活动对区内石炭系、二叠系的油气成藏具有重要贡献.%The Hashan area in the NW margin of the Junggar Basin experienced multiple phases of intense tectonic movements,with fissures well developed in Carboniferous and Permian systems and most filled with calcite.This study discussed formation fluid activity and its relationship with hydrocarbon accumulation using analyzing the geochemical features of carbon,oxygen and strontium isotopes,and fluid inclusion of fillings from calcite veins.The results indicate that calcite fillings are generally influenced by both volcanic hydrothermal fluid and oil source fluids.δ13C feature reflects that mantle and atmosphere were two sources for the carbon elements in calcite,and negative characteristics of δ18O mainly reflects the influence of high temperature fluid medium,and the ratios of 87Sr/86Sr means that it was affected by both the mantle-derived strontium and crust-derived strontium.Comprehensive analysis suggests that there were three genetic mechanisms for formation of calcite veins:volcanic hydrothermal,petroleum-derived fluid,and meteoric water,each of which corresponded to Carboniferous and Permian volcanic hydrothermal fluid activity,oil-sourced fluid since late Permian,and meteoric water in late period.The homogenization temperatures of the secondary hydrocarbon-bearing brine inclusions developed in calcite fillings are distributed in three temperature ranges:70～80℃,100～110℃ and 130～160℃,corresponding to three phases (Late Permian,Late Triassic,Late Cretaceous) of oil-source fluid charging,respectively.Among them,the Permian oil-sourced fluid made important contribution to Carboniferous and Permian hydrocarbon accumulation.