| Under the theoretical guidance of petrology, geochemistry, reservoir geology and modern hydrocarbon accumulation geology, through comprehensive analysis of the data including field outcrop, seismic, well logging, core and rock thin section, using analysis technologies such as polarized light microscope, whole rock X-Ray diffraction,geochemistry of major, trace and rare earth elements and etc., this thesis systematically and thoroughly studies the lithology, the architecture and reservoir characteristics of the basement rock weathering crust, the role of the basement rock weathering crust in the hydrocarbon migration and accumulation of the Dongping area in the front of Altun mountain and the North Kunlun Faults Zone in the front of Qimantage mountain in the western margin of Qaidam Basin. Based on the above, this thesis discusses the main controlling factors of the reservoir development in the basement rock weathering crust,ascertains reservoir models of basement rock weathering crusts based on different rock types, and establishes hydrocarbon accumulation models of the basement rock weathering crust in the western margin of Qaidam Basin.The lithology of the basement in the study area is mainly granitic crystalline rock.Lithologies of the basement of Donging 3 wellblock in the front of Altun Mountain are mainly monzonitic and k-feldspar granites, as in geochemistry, they are strong peraluminous, high-k calc-alkali s-type granites, derived from the melting of crustal material under the volcanic arc environment. Lithologies of the basement of Dongping 1 wellblock are mainly gneissic rocks, including granitic gneiss and plagioclase gneiss,which are complicated. Lithologies of the basement of the North Kunlun Faults Zone in the front of Qimantage Mountain are mainly granites, granodiorites and monzonitic granites, as in geochemistry, they are mainly aluminous-peraluminous, high-k calc-alkali s-type granites, derived from the melting of crustal material under the volcanic arc environment and intraplate environment.This thesis divides the crystalline basement rock weathering crust into five-layer architectures, named soil layer, weathered layer, dissolution zone of semi-weathered layer,disintegration zone of semi-weathered layer and unweathered basement rock from top to bottom. Among them, the soil layer is the earth surface for the animal and plant living which basically consists of clay minerals. It is often rich in organic matter and has a reddish-brown color after oxidation, with a thickness always less than 2 m, but in the study area, it is generally absent due to erosion. The weathered layer is an in situ melange containing weathered clay (content is more than 50%) and tiny residual breccias of the basement rock. It has a thickness always 0 m?15 m. The dissolution zone is characterized by abundant development of dissolution pores and fractures and the dissolution joints and tectonic fractures interwoven into mesh. It has a thickness approximately 30 m?200 m.The disintegration zone is characterized by dominated weakly dissoluted joint fractures,while tectonic fractures and dissolution pores are less development. It has a thickness approximately 300 m?1000 m. The unweathered basement rock only has a small amount of microfractures, which is very dense, slightly alterated even unalterated and lack of clay minerals. Architectures of the basement rock weathering crust can be well discriminated in conventional logging and imaging logging data.The reservoir space of the crystalline basement rock weathering crust mainly contains matrix micropores, dissolution pores and fractures. Among them, the matrix micropores mainly include intercrystal pores of flake minerals such as mica and micropores within ferromagnesian. The dissolution pores mainly contain dissolved pores and dissolved fractures which are derived from the dissolution of plagioclase. The fractures mainly contain primary joints, secondary tectonic fractures and exhumation fractures. Integratedly controlled by weathering and tectonic processes,the distribution of the reservoir in the basement rock weathering crust has following characteristics: in the vertical direction, high quality reservoirs are developed in the dissolution zone and the up section of the disintegration zone, among them, the dissolution zone is characterized by massive and continuous dissolution pore-fracture network system, the dissolution pores are distributed around the strongly dissolved fractures. The disintegration zone is characterized by strong heterogeneity weak dissolved fracture-dissolved pore system. In the lateral, the thickness and quality of the reservoir in the weathering crust are controlled by the paleotopography. The slope zone is the best, the highland zone takes the second place and the depression zone is the worst. In addition,the reservoir thickness and dissolution pores are more developed in the area which is influenced by the pre-existing fault and its related broken zone. Generally, the development of the reservoir in the basement rock weathering crust is controlled by a combination of many factors, such as the architecture of the basement rock weathering crust, lithology of the basement rock, ancient exposed topography, weathering and denudation history, fault tectonics and the late magmatic intrusion and its related hydrothermal fluids.Different architectures of the basement rock weathering crust have different roles in the hydrocarbon migration and accumulation. The soil layer and the weathered layer are the seal rock of the basement reservoir, their seal abilities are mainly controlled by their thicknesses. If they are too thin, their sealing capabilities will easily be damaged by faults.The semi-weathered layer has big thickness and good porosity and permeability, and it can be continuously and large-scale developed around the basement rock weathering zone,therefore, it is not only a good hydrocarbon reservoir, but it can also be the efficient pathway for long distanced migration of the hydrocarbon along the unconformity. |
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