塔里木盆地塔中Ⅰ号带西北部上奥陶统碳酸盐岩同生期岩溶--以顺2井为例

The meteoric diagenesis of the Upper Ordovician carbonate rocks occurred in syn-sedimentary karstification in the No.1 belt of Northwestern Tazhong,Tarim basin——taking the Well Shun-2 as an example

在对塔中Ⅰ构造带西北部的顺2井奥陶系碳酸盐岩详细岩心观察和室内研究基础上,通过对岩溶作用中的泥质和方解石充填物的矿物组成、粘土成分、常量与微量元素和碳氧同位素的综合研究认为:顺2井良里塔格组碳酸盐岩取心井段显示出小型的潜流-缓流带及渗潜带岩溶结构,发育厚度为几十米(20～60m);具有块(粒)状结构、不发光泥微晶颗粒和裂隙充填方解石等大气水成岩特征及等粒镶嵌状结构、环带结构等埋藏或混合成岩作用特征;泥质充填物含稳定石英、钾长石、斜长石及代表埋藏早期未压实的标型矿物菱铁矿,相对高的CaO、K2O、Sr/Ba、B、F和相对低的Sr具负铈异常等;泥岩充填物中碳酸盐岩δ13C>0,中低负值的δ18O和Z>120;反映其形成于海相或处于海水-淡水混合带,代表了与沉积间断面有关的同生期表生岩溶的特征,后经历了成岩埋藏阶段的热液(水)作用改造。

This paper presents an updated review of meteoric diagenesis of the Upper Ordovician carbonate rocks occurred in syn-sedimentary karstification in the No. 1 belt of northwestern Tazhong, Tarim basin, based on the detailed observation of core and comprehensive studies in regard to the mineral types, clay compositions, the general and trace elements as well as oxygen and carbon isotopic compositions of argillaceous cavesedimental fillings or calcite cements and calcites in cleavages and cavities or vugs of Lialitaga formation. The core of Lialitaga formation of the Upper Ordovician from the Well Shun-2 shows that （1） there are small-scale structures of syn-sedimentary karstification developed in the vadose zone and phreatic zone with their thickness in the range of 20-60 m; that （2） there are typical features of meteoric diagenesis, such as the coarse and grain texture, dull-luminescent micritic grains, and calcite filling in the fractures, and there are equant, mosaic texture and ring texture showing the buried diagenesis or mixed diagenesis; that （3） the argillaceous cave-sedimental fillings of Lialitaga formation of the Upper Ordovician contain the stable minerals such as quartz, feldspar, orthoclase, marcasite, and siderite that is the critical fingerprint indicating the early stage of diagenesis, i. e. , not yet compacted; also have relative high CaO, K2O, Sr/Ba, B, F and low content of Sr as well as the negative cerium anomaly; that （4） carbonate cements in the argillaceous cave-sedimental fillings have medium and low negative value of δ^18 O, Z〉120, and δ^13C〉0‰, which can be interpreted as the indication of nonmarine environment or the mixed zone of fresh and sea water. It would be reasonably deduced that the subaerial meteoric diagenesis was overlapped by the late hydrothermal-related buried diagenesis in the Upper Ordovician carbonate in the northwestern Tazhong area,Tarim basin.