Karst Reservoir Of The Fourth Member Of Dengying Formation In Moxi-gaoshiti Area:Development Regularity And Forecasting

With major breakthroughs made in the exploration of the fourth Member in Dengying Formation of Upper Sinian System in Sichuan Basin in recent years,the area has become a hotspot of natural gas exploration in China.Though the researchs have realized the reservoir is the product of dolomite in mud mound-beach facies modified by karstification,there is little understanding of the reservoir development and distuibution regularity caused by the short-term exploration,which finally limited the further quality and efficient development.Given this,this paper does systemic research to the characteristics of the karst reservoir in the fourh member of the Dengying Formation in the Moxi-Gaoshiti area with fully advantage of geological,logging and seismic data,guided by the theory and methodology of modern sedimentology,carbonatite diagenesis,reservoir geology and the geophysics.Then make clear of the development and distribution of the favorable karst reservoir by discussing the mechanism and controlling factors of the karstification.Based on this,with the anatomy of the region of Well Gaoshi3--Well Gaoshi7(which have complete geological-geophysical data),explore the geological-geophysical comprehensive forecasting method of the thin-reservoirs in the fourth member of Dengying Formation.The results are as follows,(1)Reservoir characteristics study shows that the reservoir rocks are mainly clotted dolomite,frame dolomite,stromatolite dolomite and grain dolomite.And the reservoir space types are(dissolved)residual intergranular pores,inter-frame(dissolved)pores,and small-scale cavern systems.Property research indicates that the reservoirs in the fourth member have strong heterogeneity,with low porosity and permeability.Based on the difference of reservoir facies,the reservoirs in the fourth member could be divided into 4 types:namely pinhole,pinhole-smaller dissolved pores and cavities,honeycombed dissolved pores and cavities and fractured pores and cavities.Which provide favorable conditions for the research of karsitification and reservoir distribution regularity.(2)Based on 3D seismic and drill-hole data in Moxi-Gaoshiti area,this paper demonstrated and selected the impression seismic thickness from the top of Longwangmiao Formation to the bottom of Cambrian System to represent the karst palaeogeomorphology of the fourth member of Dengying Formation,which could be divided into 4 geomorphic units:namely karst platform plane,slope primary ravine and superimposed slop.Through analysis the karstification intensity and reservoir quality on the different unit,pointed out that the superimposed slop and primary ravine are the most favorable karst reservoir development zone.(3)On the basis of the palaeogeomorphologic reconstruction,the detailed studis of the karstification of the fourth member is compliacted pinhole-small solution pore,which demonstrate the supergene karstification is the most important factor of the reservoir developement in the research area.According to different Identifying feature,the supergene karstification could be further divided into continenta eogenetic karstification and coastal karstification.the ogenetic karstification controlled the resorvoirs' essential features and spatial-temporal distribution,while the coastal karstification play an important role in adjusing and improving the lateral distribution.(4)Combined with the research of reservoir characteristics,the reservoir development regularity was stutied.and then the determining criterion of favorable reservoir zone of the fourth member of Dengying formation was summarized according to the storage coefficient and the selection result of favorable karst reservoir development zone.The most favorable reservoir zone is in the I1 area of the favorable karst reservoir development zone,while its storage coefficient ?.0.the secondary favorable reservoir zone is in the I2 area of the favorable karst reservoir development zone,while 3.0>storage coefficient?2.0.(5)Through the detailed study of the well area Gaoshi3-Gaoshi7 and its adjacent area.the geological-geophysical comprehensive forecasting method of thin reservoir in the fourth member of Dengying formation in the Moxi-Gaoshiti area was established:firstly,primarily screen out the favorable zone from the karst palaeogeomorphology;secondly,make clear the target scope through the 30hz low frequency and high amplitude advantage area;finally,set goals according to the frequency attenuation feature and thin-reservoir inversion technique.Comparised with the drilling achievement of newly drilled well in the later period,this method is considered to have good applicability,which could offer help to the further quality and efficient development of the fourth member of Dengying formation in the Moxi-Gaishiti area.