Abstract:

In recent years, important breakthrough has been made in exploring petroleum from dense carbonate reservoirs of Eocene Epoch in Western Qaidam Basin. Among the carbonate rocks in the reservoirs, dolomite proves to be the high quality reservoir rock type. By the comprehensive petrologic, textural and geochemical studies, we describe and demonstrate the origin and characteristics of the two sorts of lacustrine dolomites from western Qaidam Basin. One is the protistic algal dolomite, which is rare in this area. The analytical results of electron probe energy spectrum scan showed that the Mgenriched zone and the distribution of algal lamination are highly consistent,which, therefore,strongly indicates its microbiological characteristics because the microbial activity can overcome dynamic obstacles and create a favorable microenvironment for the formation of dolomite. Another is the penecontemporaneous metasomatic micritic dolomite, which are widely distributed in the study area. The results of the observation with polarizing microscope and Xray diffraction analysis showed that most of the dolomicrites contain detrital grains and argillaceous components, which is due to the mixed deposition of detritus that brought from the peripheral braided delta. The results of electron microprobe analyses on rock components showed that the average content of FeO is 0.942% and that of MnO is 0.052%. These data are significantly lower than those of hydrothermal dolomites. And the SiO2 and Al2O3 contents are also higher, which fully show a penecontemporaneous metasomatic origin. The discrete rare elements spider diagram shows the diversity of the source material of this mixedly deposited lacustrine dolomite. The ∑REE average value is less than that of the upper crust ∑REE(146.37×10-6), but much larger than the value of chondrite ∑REE(2.56×10-6) and without Eu negative anomaly. Carbon isotope negative offset is primarily impacted by the CO2 produced from the oxidation of reservoir organic matter and the surface water provided by the peripheral braided delta, which increased its light carbon components. The oxygen isotope positive offset is mainly impacted by the saline lacustrine basin depositional environment. Evaporation makes δ18O increase in lake water. Though the surrounding river system could bring forth some light oxygen components, it caused little success. It obviously reveals that penecontemporaneous metasomatic dolomite is opposite to the hydrothermal ones which caused the oxygen isotope positive offset. During its formation process dolomicrite generated a lot of disseminated intergranular pores, on account of intergranular diffusion process. And the pore radii are small while the number of them are numerous, which is the major cause for the forming of “medium porosityextralow permeability” type reservoir. This type of pore has a strong ability to resist compaction. It is the best oil and gas reservoirs in study area. The research results have great significance for the exploration of oil and gas in the deepseated dense carbonate rocks in Qaidam Basin.

Key words: Qaidam Basin, saline lacustrine basin, Eocene, penecontemporaneous metasomatic, dolomite, intergranular pore