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.