西湖凹陷平湖组砂岩中碳酸盐胶结物形成机制及其对储层质量的影响

碳酸盐胶结物是东海西湖凹陷平湖组储层砂岩中最为重要的自生矿物。薄片鉴定、阴极发光、元素成分及碳氧同位素分析结果表明,西湖凹陷平湖组储层砂岩中碳酸盐胶结物的主要矿物类型是菱铁矿、铁白云石和方解石。其中菱铁矿胶结物主要形成于同生到早成岩早期,具有最高的δ^13和δ^18值,分别为0.75‰和-10.22‰;铁白云石胶结物的δ^13接近同期海水,变化于-2.45‰和-2.98‰之间,δ^18值偏负,最低达-12.39‰,其形成于相对较早的成岩时间。方解石胶结物主要以粒状和连生胶结方式产出,粒状方解石具有复杂的多种成因机制,而连生方解石具有最低的δ^13值（-6.02‰）和δ^18值（-16.94‰）,主要形成于成岩晚期,其成因与有机酸溶解长石等铝硅酸盐矿物及粘土矿物转化作用有关。在对储层质量的影响方面,早期形成的菱铁矿和铁白云石胶结物增加了岩石的机械强度和抗压实能力,对研究区储层发育的影响是正面的,而晚期形成的连生方解石胶结物普遍占据长石溶解空间,对储层的发育是不利的。

The origin of carbonate cements and the influence on reservoir quality of Pinghu Formation in Xihu Sag, East China Sea

Carbonate cements are the most abundant authigenic minerals in the reservoir sandstone of Pinghu Formation in Xihu Sag, East China Sea Basin. The analyses on thin section, cathodoluminescence, and elemental composition and carbon isotope show that siderite, ankerite and calcite are the main minerals of the carbonate cements in Pinghu Formation. The siderite cement is mainly generated from syngenesis to early diagenesis, with highest δ^13C （0.75‰） and 8180 （-10.22‰）. The δ^13C of ankerite varies from -2.45‰ to -2.98‰, which is concordant with the coeval sea water, and the δ^180 of ankerite is low, with least of -12.39‰, and occurs during the early diagenetic stage. Granular and poikilitic cementations are the main occurrences of calcite cement. Granular calcite has many origins; however, the origin of the poikilitic calcite cement is closely related to the aluminosilicate minerals and the transformation of clay minerals. The siderite and ankerite precipitated in the early diagenetic stage enhanced the mechanical strength of rock and resistance to compaction, which is favorable for the reservoir quality, while the intergrowth calcite cementation generated in the late period is disadvantageous for the reservoir quality.