Reservoir Characteristics And Controlling Factors Of Mesozoic Granite Buried-hill In Songnan Low Uplift,Qiongdongnan Basin

Qiongdongnan basin is the main area of natural gas exploration in the west of the South China Sea.In recent years,the discovery of Yongle 8-1 high-quality natural gas reservoir in Songnan low uplift in the deep-water area in the middle of Qiongdongnan Basin shows the exploration potential of pre Paleogene basement buried hill.Buried hill oil and gas reservoir has become a key exploration field of natural gas in the west of the South China Sea.The early exploration revealed that the Songnan low uplift is surrounded by depression and the supply of source rocks is good,but the exploration time and degree of basement buried hill oil and gas reservoir are short and low.The rock type of buried hill,reservoir space type,buried hill reservoir development characteristics,buried hill genetic type and control factors of reservoir formation in Songnan low uplift need to be further studied:The main lithology of the study area is metamorphic monzogranite,cataclastic metamorphic monzogranite and felsic porphyry.Through the observation of wall core and cast thin section,it is recognized that the reservoir space of buried hill in the study area is mainly developed into 2 categories and 7 species.The two categories include secondary pores and fractures.The secondary pores are divided into intracrystalline dissolution pores,intergranular dissolution pores,mold pores and matrix dissolution pores.Fractures are divided into structural fractures,structural dissolution fractures and weathering dissolution fractures.According to mechanical properties,they are divided into shear fractures and tension fractures.Select the visual field with obvious pore and fracture structure in the cast thin section for image scanning,and obtain the pore and fracture characteristics.Use the color image segmentation method to extract the pore and fracture dyed by the cast,and measure and count the face ratio of the pores and fractures respectively,so as to obtain the proportion characteristics of the pore and fracture area in the buried hill reservoir.According to the performance of the characteristics in the vertical development of the reservoir,combined with the causes of the formation of pores and fractures,The buried hill reservoir in the study area is vertically divided into three zones: weathering dissolution zone,weathering fracture zone and structural fracture zone.On the basis of reservoir zoning,the morphological characteristics of density logging,compensated neutron logging and acoustic moveout logging in conventional logging data are summarized.Using the balanced profile technology to analyze the evolution process of buried hills in the study area,that is,using the stratum leveling method,according to the causes in the evolution process,the buried hills are divided into 2 categories: tension type and tension erosion type.According to the development form of buried hills,they are further divided into 4 categories: fault Horst type buried hills,single fault uplift hill type buried hills,fault block erosion type buried hills and warping nappe unconformity buried hills.Due to the influence of the strength and duration of fault activity and the length of weathering and denudation time at the top of granite buried hill in the evolution process,the development of different types of buried hill reservoirs is different.Among them,weathering crust fracture reservoir is developed in tension buried hill,and fracture reservoir is developed in tension erosion buried hill.Based on the analysis of regional geological data,buried hill structural evolution characteristics and reservoir development characteristics,it is concluded that the development of buried hill reservoir in the study area is mainly affected by tectonic movement,paleogeomorphic characteristics and weathering and leaching.Through the rose map of imaging logging trend,the distribution of basement faults and the observation and analysis of peripheral field faults,combined with the macro description of wall core and the observation of microstructure under thin section,multi-stage tectonism makes a large number of fractures in the granite in the tight zone.These fractures can not only serve as reservoir spaces,but also provide favorable migration channels for fluid migration;The paleogeographic location of the buried hill determines the strength and length of weathering and denudation on the top of the granite.The granite in the relatively high part has a long denudation time and high intensity,and the formed weathering products are transported away.The granite in the relatively low part has a fast burial time,and the products produced by weathering and denudation are preserved earlier to form a better weathering crust type reservoir;The formation and development of dissolution pores in weathered crust type reservoir in granite buried hill are affected by weathering leaching,which also affects the formation of pores and the expansion of fracture dissolution at a certain distance in the lower part of weathered crust type reservoir in buried hill.