Reservoir Formation Mechanism Of Archaean Overburden Buried Hill In BZ13-2 Structural Area,Bozhong Sag

In recent years,high-yield industrial oil and gas were obtained from deep Archaean metamorphic rock reservoir in Bozhong 13-2 structural area,which has become another major breakthrough in oil and gas exploration in Bozhong Sag.Different from Bozhong19-6,which is not covered by Mesozoic,most of the study area is covered by Mesozoic,and its buried-hill formation mechanism and development model are different from that of Bozhong 19-6.This paper comprehensive utilization of the core,cuttings,ordinary slice,the casting thin sections,scanning electron microscope,XRD and methods of imaging logging,identification of buried hill lithology and reservoir space types,reservoir characterization development rule of the vertical combined with the tectonic evolution in the study area,analysis of the study area reservoir formation mechanism and influencing factors,establish the metamorphic buried hill reservoir in the study area development pattern.Based on the observation of cores and cuttings,thin section identification and scanning electron microscopy of Wells BZ13-2-A,BZ13-2-B and BZ13-2-C in Bozhong13-2 structural area,rock types and reservoir space types of overburden buried hill reservoirs are described in detail.There are four kinds of metamorphic rocks(granitic gneiss,mixed granite,granulite and cataclastic rock)and one kind of igneous rock(diorite porphyrite)in the study area.Among them,granitic gneiss and mixed granites are most widely developed in the study area.In this study,two types of reservoir spaces,namely pore and fracture,were identified in Bozhong 13-2 structural area based on core wall observation,casting thin sections and scanning electron microscopy identification.The pores are mainly dissolution pores.Fractures are mainly divided into weathering fractures,structural fractures,dissolution fractures and mineral intralystalline micro-fractures.Because the research area is located in the ocean and it is difficult to get cores,imaging logging is mainly used for fracture statistics and research of the whole well section.According to the regularity of vertical porosity and fracture development,combined with the logging response characteristics,the buried hill of Archaean can be divided into weathering crust and inner zone.The weathering crust includes weathered conglomerate zone and weathered fracture zone.The inner zone includes cataclastic zone,fracture intensive zone and regional fracture zone.According to the characteristics of reservoir zonation,the vertical development characteristics of single well in the study area are described,and the vertical development regularity of high-quality reservoirs in the study area is further summarized.The weathering crust in the study area is mainly composed of thick weathering crust and weathered glutenite belt and weathered fracture belt.In the inner zone,the inner fracture is more developed,accounting for more than 40%,and is composed of 1-2 thick layer(>30m)inner fracture zone and multiple thin layer(<30m)inner fracture zone.The formation mechanism and influencing factors of buried hill reservoir in the study area are analyzed by integrating the structural evolution process and vertical development law of reservoir,and then the development model of Archaean overburden buried hill reservoir in the study area is established.The formation mechanism of weathering crust reservoir is Indosinian-Yanshanian multi-stage uplift and weathering.The lithology and thickness of the overlying strata,the distance from the late Yanshanian active fault and the lithology are the main influencing factors.The formation mechanism of the reservoir in the inner belt is the superimposition of Indosinian compression and Yanshanian strike-slip and negative inversion,and the distance from the top of buried hill,tectonic position,intrusive body and lithology are the main influencing factors.The vertical development model,formation mechanism and influencing factors of buried hill reservoir are comprehensively analyzed,and the Archaean overburden buried hill reservoir development model of "weathering crust reservoir at the top and fracture reservoir at the bottom" is established in the study area.