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By combining petrologic, structural and geochemical methods, the origins and characteristics of two sorts of lacustrine dolomite form are described and demonstrated in the Western Qaidam basin. The first lacustrine dolomite is the native biogenic algal dolomite, which is not abundant in the area. The analysis results of electron probe energy spectrum scan shows that the Mg element enrichment zone and the algal layer distribution are highly consistent, and have microbiological characteristics in the Western Qaidam basin, because microbial activity can overcome dynamics obstacles and have a favorable microenvironment for dolomite production. The second lacustrine dolomite is penecontemporaneous metasomatism micritic dolomite, which is widely distributed in the study area. Polarizing microscope and X-ray diffraction analysis reveal that most of the dolomicrite contain detrital grains and argillaceous components. This is because the mixed sedimentary detritus was brought from the periphery in the fan delta sediment. The electron microprobe analysis results on the rock components demonstrated that the average content of FeO is 0.942 % and that of MnO is 0.052 %, which is significantly lower than that of hydrothermal dolomite. In addition, SiO and AlO content is very rich, which clearly shows that dolomite belongs to penecontemporaneous metasomatic sediment. The results of this research have great significance for the carbonate exploration of oil and gas in the Qiadam basin. [ABSTRACT FROM AUTHOR]

Qaidam is a plateau saline lacustrine basin in China. The Eocene in western Qaidam mainly consists of lacustrine carbonate, and the sedimentary rocks mainly include micrite, bioclastic limestone, algal limestone, dolomicrite, algal dolomite and so on. Among these rocks, dolomite is the most important petroleum reservoir rock, which mainly includes algal dolomite and dolomicrite. The pores of carbonate are mainly the intercrystalline pores, and there are dissolution pores locally. The analysis based on electronic probe energy spectrum scanning instrument shows that the enrichment region of element Mg in the algal dolomite is highly consistent with the distribution of algal laminas, which means the algal dolomite is a biogenic primary dolomite, because the microbial activities can overcome the kinetic obstacles and create a favorable microenvironment for the formation of dolomite. The analysis based on X-ray diffraction instrument shows that most dolomicrite includes clastic particles and argillaceous components because of the combined sedimentation of clastic materials brought from the peripheral fan delta deposit. The rock composition analysis based on the electronic probe shows that the average content of FeO in the dolomicrite is 0.942 %, and that of MnO is 0.052 %, which are obviously lower than the average contents of Fe and Mn in the hydrothermal dolomite. Meanwhile, due to the high contents of SiO and AlO, it is considered that the dolomicrite has a penecontemporaneous metasomatic formation mechanism. This type of dolomicrite generated a lot of disseminated intercrystalline pores during the metasomatism. The pore radius is small and the pore number is enormous, which are the main cause of relatively high porosity and relatively low permeability. The type of dolomicrite is a good petroleum reservoir in the study area, so adequate attention shall be given to the dolomicrite during the oil and gas exploration in the carbonate rock reservoir of Eocene in Qaidam Basin. [ABSTRACT FROM AUTHOR]