Evolution Of Oil Accumulation Dynamic Of Tight Sandstone Reservoir Of Yanchang Formation In Zhengjing Area

Tight sandstone oil is world's key areas of unconventional oil and gas exploration and development.The process and mechanism of oil accumulation is unclear,greatly affect understanding the regulation of tight oil migration and accumulation.The dynamic of tight oil migration and accumulation is the focus of the study.This paper take Yanchang Formation tight reservoir of Zhengjing Area as research object,on the base of the studying current strata temperature and pressure characteristics,analyze the causes of abnormal low pressure,research the mudstone compaction and compute the maximum depth ancient pressure,estimate the denudation thickness of Mesozoic and above strata and establish the fine geological model according to geology,seismic and geochemical datas,according to the measured pressure,mudstone ancient pressure,Temis Basin Modeling software is used to restore the evolution of abnormal pressure,comprehensive analysis the relationship of the diagenesis and the oil accumulation,and restoration the reservoir ancient property,match the oil migration dynamic and the reservoir resistance,preliminary discuss the dynamic and mechanical of tight oil accumulation.Through research,the papers obtained the following results:The pressure system of Yanchang Formation is a normal pressure system,the measured pressure is lower than the hydrostatic pressure,abnormal low pressure may not reflect actual formation pressure state,the so-called “negative” is mainly due to the low permeability of tight reservoir,and the formation water bearing surfaces under the surface pressure is the another main casue which casue the low pressure.Abnormal high sonic data of the source rocks can not objectively reflect the variance of porosity in study of mudstone compaction which is used sonic data,modify the Wyllie equation of rock volume model,the organic rich source rocks are divided into 3 parts:rock matrix,kerogen and pore fluid,put forward the correction formula of acoustic time increment caused by organic matter.After correction the compaction coefficienct decreases about 30%~55%,the uncompacted belt amplitude declines significantly,the mud overpressure of C7 oil shale calculating based on the equilibrium depth method is 0~3.5MPa,equate to the resoult of numerical basin modeling,decreases 5~8MPa compared to the pressure without correction,indicate that previous study may overestimate the abnormal pressure of the Yanchang Formation.According to the geological situation,takeing use mudstone compaction trend extrapolation to estimate Cretaceous strata denudation thickness,while the strata trend contrast method is used to recover the four stages erosion thickness.Erosion thickness restoration results show that the the 3 stages strata denudation thickness respectively is 345~465m,150~ 220 m,and160~250m,however the denudation thickness of the end of the Cretaceous is much more than the other three stages denudation thickness,vary from 980 m to 1280 m.Yanchang Formation went through 4 curtain process of overpressure developdiffusion.The late Early Cretaceous is the peak of overpressure in the process of pressure evolution.The overpressure of Chang 7 is up to 4.0MPa,and the mudstone undercompaction is the main mechanism to the overpressure,due to the low TOC,thin thickness and low Ro,the overpressure is just 0.5MPa casuded during the process of organic crack and generate hydrocarbon.Combined with the porosity and permeability evolution of reservoir,the reservoir property in crucial hydrocarbon accumulation period and the study of petroleum migration pathway,comparation the dynamic of accumulation and the resistance,we propose a mechanism of tight sandstone oil accumulation:in the first stage,the diagenesis of reservoir is weak,the porosity and permeability of reservoir is beneficial for oil migration,the sedimentary facies and fluid potential is the key factors affect the oil migration,the accumulation of oil change the reservoir wettability,reservoir become lipophilicity,the diagenesis and oil migration alternate happens,and the injection of oil restrain diagenesis,while in the process resovire becoming tight,in the last stage,due to the lipophilicity of resovire,the capillary pressure become the dynamic of oil miagration,it is advantage of oil migration and accumulation.