Characteristics And Control Factors Of Archean Buried-Hill Metamorphic Fractured Reservoirs In The BZ19-6 Structural Region,Bozhong Sag

In the BZ19-6 structural region in southwest Bozhong Sag of the Bohai Bay Basin,high-yield oil and gas flow were obtained from the buried-hill metamorphic reservoirs in2016,making it an important gas generation field in Bozhong Sag.The buried hills in the research area have distinct characteristics in terms of age,buried depth,lithology,reservoir type,and reservoir feature.Based on the lithology of the buried hills and recognition of the reservoir space,this paper aims to delve into the reservoir forming mechanism and control factors of the fractured reservoirs in the research area through mechanics testings and tectonic analysis.Starting from the core observation and thin-section microscopic identification of the9 boreholes of the BZ19-6 structural region,this paper recognized and introduced the lithology and reservoir space of the buried-hill metamorphic reservoirs.A total of 13 kinds of metamorphic rocks and 6 kinds of intrusive rocks were identified in the research area,among which the intrusive rocks became intrusive veins in a later stage.The research area is dominated by granite metamorphic rocks,which can be divided into regional metamorphic rocks and dynamic metamorphic rocks according to their metamorphic types.According to the rock structure,tectonics,and mineral composition,13 kinds of rocks were recognized in the research area.According to the plagioclase ratio in the rocks,the granites can be roughly divided into granodiorite and monzonite,and the two types present north-south regionalization.Through core and thin-section observation,pores and fractures,two kinds of reservoir spaces were identified.Pores include intergranular pores,intracrystalline pores,and dissolution pore;and fractures include structural fractures and dissolution fractures.Two types of structural fractures were recognized according to mechanical properties:tension fractures and shear fractures.The reservoir spaces of the research area are dominated by fractures.As the research area is located under water with few cores,the fractures are insufficient for observation.Therefore,the imaging logging data are used to count and analyze the fractures.According to the types and formation reasons of borehole reservoir spaces,the rocks from top to bottom can be classified into the weathering crust,weathering fracture zone,and inner fracture zone in buried hills.During the process of recognizing and classifying the fractures,intragranular fractures are often found in the thin-section alkali feldspar while fractures seldom appear on the plagioclase.Moreover,in a single borehole where the lithology from top to bottom is similar,a dense fractured belt exists between the weathered and fractured belt and inner fracture belt or between two inner fracture belts.Therefore,this paper aims to figure out the formation mechanisms of different fractures through mechanics testings.The testing samples were taken from the Archean metamorphic rocks and granites in Anshan and Xingcheng,Liaoning Province,the northeast margin of the Bohai Bay Basin.According to the mechanics experiments,under the same tectonic stress,the fracture formation ability is correlated to the lithology,and the formation ability is ranked as follows: metamorphic monzonite > metamorphic granodiorite > gneiss.The fracture formation difference is related to the dark-colored mineral content,plagioclase,and the relative ratio of alkali feldspar.Based on the regional tectonic origin,lithology division,reservoir zoning,and rock mechanics testing results,this paper analyzed the control factors and distribution of the fractured reservoirs in the research area.The development of high-quality buried hill reservoirs is affected by weathering,rock type,and tectonics.The top of the buried hills is transformed into a weathered and fractured belt by weathering and tectonism while the middle part into an inner fracture belt under the influence of lithology and tectonism.On the plane,the lithology of the north is dominated by monzonite while granodiorite is mostly found in the south.In addition,the fracture formation in the north is affected by the ancient landform and tectonism while that in the south by tectonism.Therefore,the fractured reservoir in the north develops better than that in the south.