Study On Formation Mechanism And Seismic Prediction Method Of Deep Glutenite Sweet Spot Reservoir In Mahu Area,Junggar Basin,China

In order to solve the problems of unknown formation mechanism and low accuracy of seismic prediction technology encountered in deep glutenite exploration,we developed a high-precision sweet spot reservoir prediction technology based on the glutenite of Baikouquan formation in Mahu area,Junggar basin.This technology is based on the comprehensive study of basic geological analysis,seismic petrophysical test and sweet spot reservoir prediction method.It greatly improves the accuracy of seismic reservoir prediction by constraining the formation mechanism of sweet reservoirs.Firstly,the formation mechanism of sweet spot reservoir is clarified through basic geological research,and then,combined with petrophysical test,the influence of main control factors on rock elastic properties is summarized,and a seismic petrophysical model reflecting the sedimentation and diagenetic evolution process of deep glutenite is established,which can be used to guide the sweet spot reservoir prediction in the study area.This study can not only provide technical support for oil and gas exploration and favorable area optimization of deep glutenite sweet spot reservoir in Mahu area,but also provide a good reference for oil and gas exploration of deep clastic rock in other areas.Through this research,we have obtained the following4 achievements and understandings:(1)Through a large number of basic geological studies,we systematically summarized the influence of different sedimentary microfacies and diagenesis on deep glutenite reservoir capacity in Mahu area,and clarified the formation mechanism of sweet spot reservoir in the study area.The relationship between the lower limit of critical physical properties and the depth is established for the first time in the formation process of glutenite sweet spot reservoir by introducing the analysis idea of reservoir critical property.According to the idea of diagenesis system theory,the influence of organic acid dissolution pore increase and compaction pore reduction on rock physical properties is refined.Comprehensive analysis shows that the reservoir with the best physical properties in Mahu area is medium fine glutenite and coarse sandstone with quartz support and organic acid dissolution transformation in offshore or nearshore underwater distributary channel microfacies.Comprehensive analysis shows that the reservoir with the best physical properties in Mahu area is medium fine glutenite and coarse sandstone with high content of rigid particles and quartz support in the off shore or nearshore underwater distributary channel,which has been transformed by organic acid dissolution.(2)In the study area,we conducted the first high-frequency petrophysical test of deep glutenite in the Mahu area.Combined with the measurement results of rock microstructure and dynamic and static elastic characteristics,the elastic parameters and reservoir parameters of the glutenite in the study area and the relationship between them are systematically summarized.From the test results of the samples in the study area,the differences of mineral composition content,mineral occurrence mode and pore type and shape caused by sedimentation and diagenesis have great influence on the elastic parameters and physical properties of the samples in the study area.Some special phenomena include:(1)When the content of quartz is high,the porosity of rock is higher,and the velocity ratio of P-wave and S-wave is lower.This is mainly due to the fact that the early siliceous cement is more resistant to compaction than the calcareous cement,which can effectively inhibit the destruction of the original pores by compaction;(2)The content and mode of occurrence of plastic debris have a great influence on the variation of elastic parameters and porosity of samples in the study area,because plastic particles such as volcanic rock cuttings and clay are easy to be transformed and deformed by compaction to fill the pores,which will reduce the porosity of the samples and increase the velocity.However,the influence of this influence on the P-wave velocity is greater than that of S-wave velocity,so the velocity ratio of P-wave and S-wave increases with the increase of P-wave velocity.(3)According to the sweet spot reservoir formation mechanism of deep glutenite in Mahu area of Junggar basin,this paper puts forward a new idea of"classification,from coarse to fine,step by step control"combined with geological and geophysical exploration.This new idea would be used to predict the sweet spot in the study area.On the basis of considering the sedimentary and tectonic evolution background,the Paleogeomorphology restoration technology with dip correction is developed for the first time,and the high-precision sequence stratigraphic interpretation technology is used to interpret the marker bed,so as to obtain the true thickness of the stratum and improve the prediction accuracy of the favorable facies zone.Before pre stack reservoir prediction,in order to effectively extract geological information from seismic data in OVT domain,we proposed for the first time that paleogeomorphology and sedimentary microfacies are used to constrain the selection of dominant gathers,which provides the basic data of facies control for sweet spot reservoir prediction of deep glutenite.When the dominant gathers are partially stacked,the method of using the Ruger equation to build the partial stack template is tried for the first time.Finally,using this set of basic data,the sensitive parameters of sweet spot reservoir obtained by our petrophysical experiments are accurately predicted.In slope area of Mahu sag,we would use this method to predict the sweet spot reservoir and find 2100 km~2 reservoir development area in Baikouquan formation,which provides effective technical support for searching favorable exploration area and increasing reserves and production.(4)Based on the analysis of the causes of abnormal high pressure in Mahu area,the distribution law of abnormal high pressure is preliminarily defined.Then,we design a high-frequency petrophysical experiment to simulate pore overpressure.The variation law of elastic parameters of glutenite in Mahu area under different formation pressures is summarized for the first time,and a new effective stress coefficient calculation method is established based on the rock physical test results.After comparing the effective stress coefficient calculated by this method with that of Biot,it can be seen that the new effective stress coefficient is more suitable for the glutenite in Mahu area.Finally,the new effective stress coefficient is used to improve the dual-phase medium model to establish a low-permeability glutenite formation pressure prediction model,which improves the accuracy of formation pressure prediction,and has achieved good application effect in Mahu area.