STUDY ON THE SEDIMENTARY ENVIRONMENT AND DIAGENESIS OF UPPER CARBONIFEROUS CARBONATES IN BACHU-KEPING REGION, XINJIANG

The Late Carboniferous in Western Tarim Basin is the typical sedimentation of epicontinental carbonate platform. It is main sediments are micrite, sparite, biomicrosparite and dolomite. The study on sedimentary environment, features of diagenesis and evolution characteristic of pores of Upper Carboniferous carbonates was carried out by Cathodoluminescence Microscope, SEM, X-diffraction analysis, TAS-plus Image Analysis System and Reflectance of Vitrinite (R ° ), based on field work and a large quantity of thin section observation. Through research, the following main conclusions are proposed: 1.The sedimentary environment of Upper Carboniferous carbonates in Bachu-Keping region can be divided into four facies from west to east. (1) Open Sea Basin Facies It Is distributed north of Kalatieke Mt. and west of Atushi- Shache County area. The most of sediments are terrigenous fine clastic rock and marl. (2) Platform Border Shoal Facies It is distributed Maigaiti County area. The most of sediments are sparite and dolomite. (3) Open Platform Facies It is distributed Bachu County-Subashi village area. The most of sediments are biomicrite and marl. (4) Coastal Shoal Facies It is distributed east of Keping County and north of Shajinzi Town area. The most of sediments are quartzose sandstone, siltstone, intramicsparite and biomicrosparite. 2. Upper Carboniferous carbonates have undergone deep compaction, Presoiution, cementation, dolomitizatiori and solution. Measurement of vitrinite reflection (R ° ) and illite crystallinity in mudstone demenstrate that diagenetic temperature is 120℃ to 160℃ Therefore in author' s opinion, this sequence is in the middle-late postdiagenetic stage, that is organic mature stage. 3. There are five kinds of cement which are formed in different period in. Upper Carboniferous carbonates: (1) . Needle isopachous ring cement. (2) . Co-axial ring cement. ( 3) . Allotriomorphic to hypautomorphic granular sparrycalcite cement. ( 4) . Sparry cyclopean ferrocalcite cement. (5) Hypautomorphic to idiomorphic dolomite cement. 4.The original sedimentary environment of carbonate is the important condition that influences the diagenesis of carbonate. The good reservoir rocks are often evolved from sparry granular carbonate and biosparite. Especially sparry granular carbonate of Xiaohaizi formation in platform border fades, because the widespread dolomitization, formed dolomhic limestone which has residual structure, that is helpful for the formation of secondary pores. Mean porosity of dolomite is 10%, So the authors suggest that this sort of rocks is the hopeful resvoir of oil.