Reservoir Formation Mechanism Of Lacustrine Carbonate Rocks In Western Qaidam Basin

In recent years,oil and gas exploration of paleogene lacustrine carbonate strata in western Qaidam Basin has made breakthroughs continuously.Since 2018,a number of pre-exploration and evaluation Wells have been drilled in the Nanyishan area,and the results of oil test and gas test indicate that the upper member of xiaganchaigou Formation in the Xiaoliangshan-Nanyishan area has great exploration potential.Western Qaidam Basin is one of the important areas for oil and gas exploration in qaidam Basin.The oil and gas reservoirs are mixed with lacustrine carbonate and clastic rocks,and the reservoir heterogeneity is very strong.The original geological understanding can not meet the research needs of oil and gas accumulation laws.This brings great difficulties to oil and gas exploration and restricts it to a large extent.In this paper,the core observation of more than 10 Wells in the work area was carried out,and a variety of experimental methods,including thin section(cast and ordinary thin section)identification,carbon and oxygen isotope analysis,environmental scanning electron microscopy(SEM),electron probe,X-ray diffraction(XRD)whole-rock and clay mineral analysis,were used in the key Laboratory of Reservoir Characterization of CNPC.The lithologic characteristics,reservoir space types,pore structure,diagenesis and pore evolution of upper member of Lower Ganchaigou Formation in XiaoliangshanNanyishan structural belt in western Qaidam Basin were studied.Based on the above experimental results,the influencing factors of reservoir development in each formation were explored,and the evaluation criteria of reservoir in the upper member of Xiaganchaigou Formation were established,and the favorable reservoir was classified and evaluated.The results and understanding are as follows: 1.The reservoirs in xiaoliangshan-Nanyishan structural belt are generally mixed with carbonate rocks and clastic rocks.The content of carbonate rocks ranges from 17% to 64.2% with an average of 42.3%,and salt rocks,gypsum and other salt minerals are common.2.The pore types of the reservoir are mainly dissolution pores and intergranular pores,and some layers are microfractures,but they are often filled by later cements.The reservoir as a whole shows the characteristics of ultra-low porosity and ultra-low permeability,with the porosity varying from 1.0% to 13.3%,the average of which is 4.9%,and the permeability is mostly less than 0.02 m D.3.Based on reservoir characteristics,clay mineral evolution and related geochemical data,it is considered that the upper member of Xiaganchaigou Formation is in the middle diagenetic stage A.4.Sedimentary facies and diagenesis are two main factors controlling reservoir development.The general rule is that gray yunping and algal mat develop high quality reservoir,and algal mat is the best.Dissolution in the early stage and dolomization in the syngenetic stage greatly increased the reservoir space and improved the seepage capacity of the reservoir,while cementation of salt minerals in the later stage played a destructive role in the development of the reservoir.Tectonism has a dual effect on reservoirs(constructive rather than destructive).The research results of this thesis are of great significance for reservoir sweet spot prediction and reservoir reevaluation in the area.