Formation Environment and Distribution of Lacustrine Carbonate Rocks in Northwest Qaidam Basin

Abstract

AbstractA set of unique Paleogene-Neogene lacustrine carbonate rocks characterized by deep burial depth, fine grains, thin single layer thickness and high degree of mixing developed in the northwest Qaidam Basin, the main areas for oil and gas exploration. The formation environment and distribution of Paleogene-Neogene lacustrine carbonate rocks are key geological problems need to be solved in oil and gas exploration and development. The reservoir rock types and reservoir space of lacustrine carbonate rocks are studied by drilling core data, thin section and whole rock X-ray diffraction system; Combined with major and trace elements and carbon and oxygen isotopes, the formation environment of Paleogene-Neogene lacustrine carbonate rocks was analyzed; Through the study of sedimentary sequence and sedimentary model, combined with the well connected comparative analysis, the distribution pattern of Paleogene-Neogene lacustrine carbonate rocks is revealed. We identified four kinds of lacustrine carbonate rocks in Northwest Qaidam Basin, including algal dolomite, grain dolomite, sandy dolomite and laminar micritic dolomite, and recognized five types of reservoir space, including trellis pore, dissolution pore, intergranular pore, intercrystalline pore and laminar fracture. The Paleogene-Neogene lacustrine carbonate rocks in Northwest Qaidam Basin were formed in anoxic environment with high paleosalinity. Four typical sedimentary sequences were developed during the decline stage of lake level. The lacustrine carbonate rocks are vertically superimposed and migrated around the center of the lake basin, and horizontally distributed in continuous or intermittent order from the shoreline to the center of the lake. The research results promoted the development of lacustrine carbonate sedimentology, and provided guidance for oil and gas exploration and development, and created significant social and economic benefits. KeywordsLacustrine carbonate rocksRock typesReservoir spaceFormation environmentDistributionNorthwest Qaidam Basin