塔里木盆地西北缘震旦系和古生代白云岩成因及其储集性

本文研究了塔里木盆地西北缘各时代白云岩的岩石矿物学和地球化学特征,区内由它形泥,微晶白云石构成的白云岩,产于震旦系和寒武系中、其有序度较低。氧碳同位素较重,这类白云岩是在盐度较高的局限台地和半闭塞泻湖环境中沉积的高镁碳盐盐沉积物,经过准同生白云化作用改造而形成的。岩石的孔渗性较差。由自形、半自形粉晶-中晶白云石构成的白云岩,主要产于奥陶系和石炭系中,具有较高的有序度,氧碳同位素较轻,是成岩早、中期混合白云岩化作用的产物,并具较好的也渗性,是研究区最有利的碳酸盐岩储层。而白云岩晶洞中自形白云石,有序度很高,氧同位素极轻,阴极发光镜下具环带构造,是成岩晚期深埋条件下形成的。

The Origin of Dolostone and Its Reservoir Characteristics of Sinian and Palaeozoic,Northwestern Margin of Tarim Basin

Massive dodlostone are widely developed in the strata of Sinian, Cambrian, Ordovician and Carboniferous in the northwestern margin of Trim Basin,three types can be divided according to the shape and size of dolomite in them. 1. Dolostone composed of automorphic rhombohedral dolomite,mainly occur in Ordovi- cian and Carboniferous system. The size of crystal dolomite are mainly 100 - 500μm, two groups of cleavages are developed. Cathodoluminescence colour of which are orange to dark red,there are ring structures shown by dark red and brown,degree of order is high(0. 885 in average) ;the content of CaCO₃ is about 51-55% and the oxygen isotope is lighter (the average of δ18O is -8. 73‰PDB). 2. Dolostone composed of hypautomorphic granular dolomite mainly found in Cambrain and Ordovician with grain size of 50-150μm. One group of cleavages always developed in dolomite,the colour of Cathodoluminescence are mainly orange,isotopic composition of oxygen is light (the mean of δ18O is - 7. 55‰PDB). In the miarolitic cavity of this kind of dolo-stone,dolomite with very high automorphic degree are commmonly developed, which has a very high order degree(0. 95 in average) and very light oxygen isotope composition ((δ18O is - 12‰PDB in average),and rings consist of dark red and brown are seen under Cathodoluminescence. 3. Dolostone composed of anhedral microcrystaline dolomite,mainly occured in Sinian and Cambrian,and always accompany by shale and gypsum argillaceous and have laminated bedding and stromatolithic structure. Under electronic microscope,unequiganular dolomite distributed unregularly and there are no cleavages. Cathodoluminescence colour is bright orange,ordering degree of dolomite is low(0. 737 in average)and oxygen isotope composition is heavier(δ18o is -5. 74‰. PDB in average),usually,the contain of CaCo₃ is higher. After analysis and study,the author proposed that the dolomite composed of xenomor-phic muddy-micritic dolomite is meta-homogenerous which formed by dolomitization caused by permeation and return flow,the porosity and permeability of which is worse. Dolostone composed of automorphic-hypautomorphic, powder-medium crystalline dolomite is formed by dolomitization of mixing water during early-middle diagenesis,this type of dolostone is the most favourable carbonate reservoir in the study area because of the higher porosity and permeability.