深部热流体活动对储层成岩作用及孔隙演化的影响:以莺歌海盆地LDX区中新统黄流组为例

Deep thermal fluid activity and its influence on the diagenesis and pore evolution of reservoirs:A case study from the Miocene Huangliu Formation reservoir in the LDX area,Yinggehai Basin,northern South China Sea

为了解莺歌海盆地深部热流体主要类型,研究热流体特征,判断热流体活动范围,分析热流体对储层成岩-孔隙演化的影响,通过铸体薄片、扫描电镜、黏土矿物X射线衍射、物性、电子探针、包裹体均一温度、稳定同位素等分析测试手段,结合前人研究成果,对盆内LDX区中新统黄流组储层进行分析。结果表明,受构造热事件影响及深大断裂控制,在超压驱动下,热流体活动范围主要为3900 m以下的黄流组中下部,以CO_(2)热流体为主,H_(2)S热流体次之。热流体影响下储层具有自生黏土矿物转化速率加快、镜质体反射率突变、地层水矿化度降低、包裹体均一温度大于正常地层最高温度、热液成因矿物发育等特点。将黄流组储层分为超压储层(黄流组中上部)及热超压储层(黄流组中下部)两类。其中,超压储层成岩作用阶段达中成岩A_(2)亚期,压实及溶蚀作用较弱,胶结作用较强,孔隙演化经历了压实减孔、压实与胶结减孔、胶结减孔与有机酸溶蚀增孔3个过程,孔隙度从38.8%减少到现今的7.6%;热超压储层成岩作用阶段达中成岩B期,压实及溶蚀作用较强,胶结作用较弱,孔隙演化经历了压实减孔、压实与胶结减孔、胶结减孔与有机酸溶蚀增孔、胶结减孔与有机酸及无机酸溶蚀增孔4个过程,孔隙度从38.1%减少到现今的9.2%。有利储层主要发育在黄流组中下部黄二段的热超压储层中。

In order to understand the main types of deep-seated thermal fluid in the Yinggehai Basin,We studied the thermal fluid characteristics,determined the range of thermal fluid activity,and analyzed the thermal fluid influence on reservoir diagenesis and pore evolution.Combined with previous research results,we investigated the Miocene Huangliu Formation reservoir in the LDX area by blue epoxy resin-impregnated thin section,scanning electron microscope,clay mineral quantification by XRD,physical property,electron probe,fluid inclusion homogenization temperature,and stable isotope analyses.Results showed that the deep-seated thermal fluid activity—which is affected by tectonic thermal events,controlled by an abyssal fault,driven by overpressure,and dominated by CO_(2)thermal fluid and to a lesser extent by H_(2)S thermal fluid—mainly influenced the middle and lower parts of the Huangliu Formation below 3900 m.The reservoir is characterized by the accelerated transformation of authigenic clay minerals,vitrinite reflectance mutation,reduced formation water salinity,inclusion homogenization temperature above the normal formation maximum temperature,and hydrothermal mineral development under the influence of deep therrmal fluids.The Huangliu Formation involves two types of reservoirs:overpressure reservoir(middle-upper parts of the Huangliu Formation)and thermal overpressure reservoir(lower-middle parts of the Huangliu Formation).The diagenetic stage of the overpressure reservoir reached substage A_(2) of mesodiagenesis,with relatively weak compaction,weak dissolution and strong cementation,and experienced pore evolution in three stages:porosity decrease by compaction,porosity decrease by compaction and cementation,and porosity decrease by cementation combined with porosity increase by organic acid dissolution.Overall,porosity decreased from 38.8% to 7.6%.The diagenetic stage in the thermal overpressure reservoir reached stage B of mesodiagenesis,with relatively strong compaction,strong dissolution and weak cementation,and experienced pore evolution in four stages:stages 1-3 are the same as in substage A_(2),and stage 4 is similar to stage 3 except the porosity increase is caused by both organic acid and inorganic acid dissolutions.Overall,porosity decreased from 38.1% to 9.2%.Favorable reservoirs mainly develope in the thermal overpressure reservoirs of the second member of the lower-middle parts of the Huangliu Formation.