Petrophysical Characterization Of Tight Reservoirs In The Mixed Sediments And Its Fluid Types Identification In Yingxi Area

With the deepening of petroleum exploration for complex reservoirs,the reservoirs in the mixed sediments have gradually become hot fields to focus on for many petroleum companies and scholars.However,the frequently changing lithology,complex pore structure,and strong heterogeneity for the reservoirs in the mixed sediments,which cause some difficult problems such as how to name and classify the rocks,recognize and study the petrophysical characteristics and identify fluid types of reservoirs,restrict the efficient exploration and development for such reservoirs.Based on cores,thin sections,scanning electron microscope(SEM),cathodoluminescence,X-ray diffraction(XRD),and quantitative evaluation of minerals by scanning electron microscopy(QEMSCAN)and other petrophysical experiments data,the purpose of this thesis is to study the nomenclature and classification of rocks and the petrophysical characteristics of the tight reservoirs in the mixed lacustrine sediments,as a case study from the Upper Xiaganchaigou Formation(E₃²)of the Yingxi field,Qaidam.Meanwhile,the reservoir classification evaluation method has been established by the combination of conventional logging and electrical image logging,expounding the cause of low contrast oil zones and studying the identification methods of fluid types.The results show that the content of carbonate minerals is the highest,followed by siliciclastic minerals,clay minerals and halosalt minerals of the E₃² Formation in Yingxi.Based on the relative contents of these four kinds of minerals,the name and lithology of rocks in the mixed lacustrine sediments can be determined with step classification.The E₃² Formation in Yingxi is mainly composed of 5 lithologies,such as gypseous mixed sediments rocks,(gypseous)shaly&sandy lime-dolomite,gypseous lime-dolomite,mud(sand)bearing lime-dolomite,and(gypseous)lime-dolomitic sandstone,with the highest development frequency of(gypseous)shaly&sandy lime-dolomite,followed by gypseous mixed sediments rocks.The mixing degree of the gypseous mixed sediments rocks is the highest,which is manifested as frequent interbedding mixing or dispersion mixing of siliciclastic minerals,clay minerals,and carbonate minerals.The mixing degree of the lime-dolomites is medium,mainly composed of carbonate minerals and siliciclastic minerals,which are dispersed or banding distribution.The mixing degree of(gypseous)lime-dolomitic sandstone and evaporite is the lowest.The porosity of mixed reservoirs in E₃² Formation mainly ranges from 0%to 6%,and the permeability is mostly lower than 1 m D.The dolomite intercrystal pore is the most common type of reservoir space.Dolomite intercrystal pores,micro-millimeter scale intergranular(dissolution)pores,and interlaminar fractures are developed in the(gypseous)shaly&sandy lime-dolomite,and the pore throat radius mainly ranges from0.004?m to 0.05?m(mostly concentrated in 0.015?m).The logging responses of rock resistivity,density and gamma ray are medium values.The weaker frequently layered and bright speckle are dominant image patterns in electrical image logging.Dolomite intercrystal pores and matrix dilated dissolution pores are developed in the gypseous lime-dolomite,and the pore throat radius mainly ranges from 0.004?m to 0.04?m.The logging responses of rock resistivity and density are larger values,and gamma ray is smaller value.The bright speckle is dominant image patterns in electrical image logging.Dolomite intercrystal pores and dissolution pores are developed in the mud(sand)bearing lime-dolomite,and the pore throat radius mainly ranges from 0.004?m to 0.15?m.The logging responses of rock resistivity and density are medium values,and gamma ray is smaller value.The dark speckle and weaker frequently layered are dominant image patterns in electrical image logging.Dolomite intercrystal pores are developed in the gypseous mixed sediments rocks,and some interlaminar fractures are visible.The pore throat radius mainly ranges from 0.003?m to 0.02?m,with poor connectivity.The logging responses of rock resistivity and density are medium or smaller values,and gamma ray is medium value.The image pattern of electrical image logging is mainly the stronger frequently layered.Intergranular(dissolution)pores and dolomite intercrystal pores are developed in the(gypseous)lime-dolomitic sandstone,and the pore throat radius mainly ranges from 0.005?m to 0.1?m.The logging responses of rock resistivity,density and gamma ray are medium or smaller values.The image pattern of electrical image logging is mainly the block shape.There are the brecciated pores within the rocks in the study area,which the diameter mostly ranges from 0.5 mm to 2 mm.The dissolution brecciated pores are mostly developed in the lime-dolomites,and the development of structural fractures and structural brecciated pores are mainly controlled by the strength of structural activities.There is different effects on pore structure by different minerals.The increase of clay mineral content leads to the more complex pore structure,while the increase of dolomite content is beneficial to the improvement of the pore structure.The influence of calcite,siliciclastic minerals,and halosalt minerals on pore structure is uncertain.Combining conventional logging and electrical image logging,the porosity characteristic index(MQI)is constructed from porosity,lithology index and fracture porosity.The permeability characteristic index(FHI)is constructed from surface vug porosity and fracture porosity.The crossplot of these two parameters can be used for the classification and evaluation of reservoirs in the mixed sediments.The low contrast among oil and water zones of E₃² Formation is mainly affected by the coupling and combination of many factors,mainly including rock matrix minerals,the reservoir porosity,the complexity of pore structure,and the intrusion of mud.Base on detection logging,combination of electrical logging and non-electrical logging,combining conventional logging and advanced technology logging,mathematical-statistical discriminances,4 identification methods of fluid types are established for distinguishing the oil and water zones of reservoirs in the mixed sediments by correcting and counteracting the influence of non-fluid factors,such as lithological correction method,Fisher discrimination method after lithological classification,improved apparent formation water resistivity(R_wa)spectrum method,combining apparent resistivity increase rate(I_a)and macroscopic capture cross-section(?)method.