Study On The Genesis And Reservoir Characteristics Of Dolomites In Buqu Formation Of Middle Jurassic In South Qiangtang Basin Of Tibet

Qiangtang Basin is located in the center of Qinghai-Xizang(Tibet) Plateau. It is a basin with the lowest degree in oil and gas exploration in China and is the most potential target for petroleum exploration in Tethys region. The Jurassic strata have a good hydrocarbon-generating potential, but the physical characteristics of reservoirs in the study area are poor on the whole, which are obviously characterized by densed reservoirs. Searching for the reservoirs of good quality is crucial to assessment of the basin. At present, oil-bearing layers found on the surface are nearly associated with dolomites. Hence, it is significance in studying dolomite oil pool that the relationship between the genesis and reservoir characteristics of dolomites.The dissertation has in detail studied the genesis and oil-gas reservoir properties of Jurassic dolomites in Shuanghu area, and made the following progress and innovation.1. Based on the research on the sedimentary facies of main sections, the facies belts of Jurassic dolomites in the study area are differentiated as open platform facies、 refined platform tidal flat and platform margin reef or shoal facies.2. According to the grain size of the dolomite and the difference of Euhedral degree of dolomite,the dolomite of Buqu Formation can be divided into six categories:very fine-to fine-crystalline (subhedral) mosaic dolomite、medium-to coarse-crystalline (euhedral) mosaic dolomite、medium-to coarse-crystalline (anhedral) dolomite、coarse-to very coarse-crystalline non-planar-(anhedral) dolomite、coarse-to very coarse-crystalline non-planar-c(cement)dolomite and polymodal mosaic dolomite。3. Diagenetic phenomena in this area are more complex and are always changing.The main diagenesis observed through the microscope are pressure solution compaction,cement, dolomitization,dedolomitisation recrystallization, limonitization, corrosion and fracturing related structure. Among them, the dissolution, dolomitization, and the major structural cracks play a constructive role, while compaction, dedolomitisation, such as the role of cement the reservoir making the damaging effects.4. The microscopic characteristics and cathodoluminescent lights of dolomite indicate that the dolomites form from strong reducibility environment.through the study of stable carbon and oxygen isotope、laser carbon and oxygen isotope、 cathodoluminescent light, fluid inclusion homogenization temperature test and Calculating salinity by ice melting temperature of fluid inclusion and isotope compositions of digenesis pore fluid imply that they are the products of deep burial local dolomitization.5. Research on Buqu Formation dolomite reservoir properties are good:12samples of dolomite reservoir porosity rangs from0.68%-7.82%, with the average porosity of3.24%; Permeability varies from a value0.01×10-3μm2-5.55×10-3μm2, the average penetration rate is0.85×10-3μm2, overall, they belong to the low porosity and medium permeability reservoir. Casting thin sections combined with microscope and mercury curve analysis, dolomite reservoir pore space are abundant, and the principal is fractured-porous, with the mainly pore structure of pores in the throat-medium pore medium throat or fine pore medium throat,the overall evaluation is Ⅰ-Ⅱ class better-good reservoir.