珠江口盆地白云凹陷深层砂岩储层中碳酸盐胶结作用及成因探讨

摘要

通过各类薄片显微镜下鉴定与定量统计,阴极发光、扫描电镜、电子探针、碳、氧及锶同位素、流体包裹体均一温度测试等分析,开展了白云凹陷深水区珠江组和珠海组砂岩储层中碳酸盐胶结物的类型与期次、地球化学特征、成因机制研究.结果显示,存在三期碳酸盐胶结作用,早期主要为方解石,以高Ca、低Fe、低Mg为特征.但珠江组早期碳酸盐胶结物的同位素组成(δ13 CPDB:-2.43‰～0.29‰,δ18 OPDB:-9.79‰～-3.08‰,87Sr/86Sr:0.7084～0.7109)与珠海组(δ13 CPDB:-9.37‰～-8.13‰,δ18 OPDB:-7.11‰～ 7.09‰,87Sr/86Sr:0.7138～0.7142)有一定差异,前者是在浅埋藏阶段从碳酸盐过饱和碱性海水介质中沉淀出的产物;后者与碳酸盐过饱和的碱性淡水有关.中期主要为铁方解石,以高Ca、较高Fe、低Mg、碳和锶同位素组成变化范围较大(δ13 CPDB:-20.88％～-5.29‰,δ18OPDB:-11.1‰～-8.99％‰,87Sr/86 Sr:0.7093～0.7151)为特征,其部分碳源与有机酸脱羧作用产生的CO2有关,另一部分碳源(δ13CPDB:-5.38‰～-5.29‰)可能与深部物质有关.形成碳酸盐胶结物所需的Ca2+、Mg2+、Fe2+等离子来源于砂岩中长石等碎屑的溶蚀、黏土矿物的转化以及深部热液流体.晚期主要为铁白云石(δ13CPDB:-2.83％‰～-1.83％‰,δ18OPDB: 9.45‰～ 5.77‰,87Sr/86Sr:0.7101～0.7162),以高Fe、高Mg、较低Ca、碳同位素组成与同期海水基本一致为特征,87Sr/86Sr值低于正常成岩演化形成的碳酸盐,该期碳酸盐胶结物的形成与砂岩中生物碎屑以及先期碳酸盐胶结物的溶蚀再沉淀有关,部分可能受到深部热液流体的影响.%This study preformed the analysis of phases, types, geochemical characteristics and genetic mechanism of carbonate cements from the sandstone reservoirs in the deep water area of Baiyun Sag, Pearl River Mouth Basin, South China Sea, based on identification and quantitative statistics of thin sections under microscope, analysis of cathodoluminescence and scanning electron microscopy, electron microprobe, measurement of carbon, oxygen and strontium isotope and homogenization temperature of authigenic fluid inclusions.The sandstone reservoirs consist of the Miocene Zhujiang Group and the Oligocene Zhuhai Group.Research result shows that three phases of the carbonate cementation occurred both in the Zhujiang and Zhuhai sandstones.The early carbonate cement of the Zhujiang Formation consists mainly of calcite and is characterized by high Ca and low Fe and Mg.Its isotopoic compositions(δ13CPDB :-2.43‰ ～ 0.29‰ ,δ18OPDB :-9.79‰ ～ 3.08‰, 87Sr/86Sr: 0.7084 ～ 0.7109) show some differences from that (δ13 CPDB :-9.37‰ ～-8.13‰,δ18 OPDB;-7.11‰ ～-7.09‰,87 Sr/86 Sr: 0.7138 ～0.7142) of the Zhuhai Formation, with the former resulting from precipitation of the carbonate supersaturated alkaline sea water in the shallow burial stage, and the latter related to the carbonate supersaturated alkaline fresh water.The interim carbonate is mainly ferrocalcite with characteristic of higher Ca, high Fe, and low Mg, and wide ranges of C and Sr isotope(δ13CPDB:-20.88‰ ～-5.29‰,δ18 OPDB :-11.1‰ ～-8.99 ‰, 87 Sr/86 Sr: 0.7093 ～ 0.7151).Chemical and geochemical features show that part of carbon was from CO2 released by decarboxylation of organic acid, and the other part (δ13 CPDB:-5.38‰ ～-5.29‰) may be related to the substances from the deep crust.Ca2+, Mg2+, and Fee+ ions necessary for the interim carbonate cements were derived from dissolution of feldspars and other detrital minerals in sediments, transformation of clay minerals, and deep thermal fluids.The late carbonate cements consisting mainly of ankerite(δ13 CPDB :-2.83‰ ～-1.83‰, δ18 OPDB :-9.45‰ ～-5.77 ‰ , 87Sr/86Sr:0.7101～0.7162)are characterized by higher Fe and Mg contents, and relatively low Ca content,which are basically consistent with that of homochronous seawater.But their 8Sr/86 Sr values are lower than that formed in a normal diagenetic evolution.It can be inferred that the formation of the late carbonate cements may be related to the dissolution and reprecipitation of the early carbonate cements and the bioclastic limestones, and likely affected by the deep hydrothermal fluids.