The Formation Mechanism Of Underpressure In The Middle-Shallow Reservoirs Of Northwestern Qaidam Basin

The pressure system in middle-shallow layer reservoir of northwest area of Qaidam Basin is complex,with the characteristics of abnormal pressure,normal pressure and subnormal pressure coexistence.In this paper,based on the reservoir pressure distribution characteristics of shallow layer in this area,combination of reservoir fluid property,Sealing mechanism,sedimentary characteristics,tectonic evolution and geological features,analysis deeply the main controlling factors that result in abnormal pressure mainly from four aspects including tectonic uplift,light hydrocarbon dissipation effect,oil and gas filling effect and differences of reservoir physical property.Studying on various geological factors controlling effect on the evolution of abnormal pressure which during basin evolution as the core of the hydrocarbon accumulation.Clarifying the formation and evolution mechanism of different pressure system,and coupling relationship associated hydrocarbon accumulations process.The obtained concrete results and recognitions are as follows:Youquanzi oil field reservoir pressure belongs to subnormal low pressure in Youshashan formation and Shizigou formation of middle-shallow layer reservoir of northwest area of Qaidam Basin.Nanyishan oil field reservoir pressure mainly belongs to subnormal pressure and normal pressure and Xiaoliangshan oil field reservoir pressure mainly belongs to abnormal pressure.Tectonic uplift is the most important mechanism that results in formation pressure distribution of the northwestern Qaidam Basin.Uplift resulting in temperature decreasing and the pore rebound effect bring about pressure reduce respectively 45.58MPa,27.36 MPa and11.26MPa in Youquanzi,Nanyishan and Xiaoliangshan oil field.The difference of reservoir properties in the study area results in the difference of rock pore volume compressibility coefficient.Because of the larger rock pore volume compressibility in Youquanzi oil field,strengthened pressure decrease caused by pore volume rebound in tectonic uplift process,therefore tend to form a low pressure reservoir.What is more,the oil hydrocarbon chromatogram showed that light hydrocarbon loss has a certain effect on the formation of low pressure in Youquanzi.The middle-shallow reservoir pressure distribution is the comprehensive-effect results include tectonic uplift difference,difference of reservoir properties and hydrocarbon diffusion effect in the northwest area of Qaidam Basin.Based on above comprehensive analysis of oil and gas reservoir,the reservoir pressure of formation and evolution of northwest area in Qaidam Basin is divided into four stages:the stage of settlement and under compaction pressure?E₃^1N₁?,hydrocarbon generation and abnormal pressure increasing stage?N₂^1N₂³?,shallow and deep pressure transfer stage?N₂³end?and the tectonic uplifting stage(N₂³end^Q₁₊₂).Tectonic uplifting stage is the northwest of Qaidam Basin reservoir pressure differentiation period,tectonic uplift in Youquanzi oil field intense that the shallow reservoir fluid pressure is greatly reduced,the light hydrocarbon loss and hydrocarbon retention effect exacerbated the pressure decreases,the continuously uplift and denudation evolution has formed low pressure reservoir in Youquanzi shallow layer.Nanyishan oilfield tectonic uplift degree is lower than Youquanzi oil field,porosity rebound and the decrease of temperature effect makes the formation pressure decreased which made the reservoir pressure in Nanyishan oil field suffered evolution from abnormal high pressure before uplift to normal pressure.Xiaoliangshan oil field suffered weak tectonic uplift,porosity rebound and the decrease of temperature have little effect on the influence of formation pressure,and in addition,providing supplementary hydrocarbon generation continued to pore fluid makes it remains high formation pressure,and finally achievements the abnormal pressure reservoir.