Lithology Distribution And Secondary Dissolution Pores Genesis Of Archean Metamorphic Buried Hills In The Typical Region Of Bohai Sea Area

In recent years,with the development of deep oil and gas exploration,the Archean metamorphic buried hill reservoir in Bohai Sea area has made a major breakthrough and has gradually become an important target for oil exploration in Bohai Sea area.Lithology and fluid play an important role in controlling the formation of metamorphic buried hill reservoirs.Therefore,clarifying the distribution and genesis of metamorphic buried hill lithology,identifying different types of diagenetic fluids,and revealing the control mechanism of lithology and fluid on Archean metamorphic reservoirs are the key to deciphering the formation mechanism of high-quality reservoirs in Archean buried hills,and are also unavoidable issues in the field of oil and gas exploration in buried hills.This paper selects two typical metamorphic buried hill oil and gas fields in Bozhong and Jinzhou,and comprehensively uses thin section,drilling and logging data to characterize the distribution pattern of Archean metamorphic rocks.Combining zircon dating,field investigations,and regional geological background,the lithology development model of Archean metamorphic rock buried hill thermal dome-structural transformation is proposed;The mineral dissolution and authigenic mineral precipitation phenomena in the Archean buried hill reservoirs in the Bozhong and Jinzhou area were summarized by comprehensive use of casting thin sections,scanning electron microscopy and X-ray diffraction;By combining tests on the main and trace elements,isotopes,and fluid inclusion homogenization temperature of authigenic minerals,the genesis of authigenic minerals in different research areas and the types of ancient fluids that cause mineral dissolution in reservoirs were clarified,and the diagenetic evolution sequence was determined;Based on reservoir properties,drilling data,and water rock experimental simulations,the controlling effects of lithology and different types of diagenetic fluids on the reservoir were clarified.Finally,different development models of the Archean buried hill reservoirs in different research areas were established by combining regional tectonic evolution.The specific understanding is as follows:1.Lithology,distribution patterns,and development model of Archean metamorphic buried hillsThe Archean metamorphic buried hill in Bohai Sea is a complex composed of Archean TTG migmatite,supracrustal rock,cataclastic rock and intrusive rock.Through drilling core description,thin section identification and logging interpretation,the plane distribution of lithology is described.The Archean metamorphic rocks in the Bozhong area are distributed in a concentric ring,and the degree of migmatization gradually decreases from the center to the edge.The metamorphic rocks in the state area are distributed in a northeast trending belt,and are significantly controlled by strike slip faults.Based on the field investigation,zircon geochronology data and the existing understanding of the Archean tectonic evolution of the North China Craton,we believe that the Archean metamorphic rocks in the Bozhong area and Jinzhou area originated from the TTG formed by the deep melting of the basic crust under the background of the mantle plume.At～2.5Ga,TTG further underwent extensive migmatization to form regularly distributed migmatites and dome structures.After the formation of the dome structure in the Bozhong area,it was mainly affected by the Indosinian,Yanshanian compression and Himalayan extension in the Mesozoic and Cenozoic.The dome structure was destroyed but still retained the plane distribution of the lithology during the formation period.Under the influence of multi-stage strike-slip transformation of Tanlu fault zone in Yanshan period and Himalayan period,the buried hill in Jinzhou area is divided and staggered and differentially denuded,which basically does not have the lithology distribution pattern of dome formation period.2.Mineral dissolution and authigenic mineral precipitation in Archean metamorphic buried hill reservoirsSecondary dissolution pores are important storage spaces for metamorphic rock buried hill reservoirs in the study area.There are significant differences in dissolution minerals and authigenic mineral precipitation phenomena between the Bozhong and Jinzhou areas,suggesting that they have undergone different types of diagenetic fluid transformation.The dissolution of alkaline feldspar in buried hill reservoirs in the Bozhong is relatively stronger than that of plagioclase,while there is little difference in the dissolution phenomenon between plagioclase and alkaline feldspar in the Jinzhou area;The dissolution of aluminosilicate minerals in the Bozhong area is stronger than that of carbonate minerals,with selective dissolution phenomena of weak/non dissolution of carbonate minerals and strong dissolution of aluminosilicate minerals.However,in the Jinzhou area,both carbonate minerals and aluminosilicate minerals undergo strong dissolution.The Archean buried hill reservoir in Bozhong area has experienced multiple stages of iron dolomite filling,among which the first stage of ankerite filling is the most intense,which is widely filled with fractures and dissolution pores.Calcite and ferrocalcite are only developed in some wells in Jinzhou area.The authigenic quartz in Bozhong area and Jinzhou area exists in the form of quartz healing fracture and euhedral microcrystalline,and the content of authigenic quartz in Bozhong area is higher than that in Jinzhou area.3.Identification of diagenetic fluid and the mechanism of secondary dissolution pores formation in Archean metamorphic buried hill reservoirsThe fluid inclusion homogenization temperature,major and trace elements and isotopes of authigenic mineral indicate that the Archean buried mountain reservoirs in the Bozhong area have undergone various types of fluid transformation,including meteoric water,deep magmatic hydrothermal fluids,organic acids and CO₂.The Archean buried hill in the Bozhong area experienced weathering and leaching during the exposed surface period,forming secondary dissolution pores.In the burial stage,during the sedimentary period of the Guantao formation,the Archean buried hill reservoir underwent strong hydrothermal activity,precipitated a large amount of ankerite and pyrite to fill early fractures and dissolution pores.The deep faults in Bozhong area are not well developed,and meteoric water is difficult to affect the reservoir.Organic acids and CO₂ formed by the thermal evolution of organic matter in the source rocks of the Shahejie Formation in the adjacent hydrocarbon generating depression enter the buried mountain reservoir along the faults and unconformity,causing dissolution of aluminosilicate minerals such as feldspar and forming large-scale secondary dissolution pores.The dissolution of organic acid and CO₂ to the reservoir caused the strong dissolution of alkaline feldspar-weak dissolution of plagioclase,strong dissolution of aluminosilicate minerals-weak dissolution of carbonate minerals,and precipitation of authigenic quartz in Bozhong area.The formation of secondary dissolution pores in the Archean buried hill reservoirs in the Jinzhou area is mainly influenced by meteoric water.During the exposed surface period,a large number of dissolution pores were formed in the Jinzhou area due to weathering and leaching.These dissolution pores were not affected by strong destructive diagenesis,and the weathered crust reservoir could be effectively preserved.Influenced by the activity of Tanlu fault zone,a large number of deep strike-slip faults developed in Jinzhou area,connecting shallow meteoric water.In the early burial stage of the buried hill,a small amount of carbonate minerals precipitated in the reservoir due to the influence of early diagenesis;During the subsequent tectonic activity and burial stages,meteoric water infiltrates into the buried hill reservoir along deep strike slip faults,causing significant dissolution of carbonate minerals and aluminosilicate minerals such as feldspar.The organic acids and CO₂ formed by the thermal evolution of source rocks also contribute to a certain amount of acidic fluids,expanding the scale of reservoir development.The dissolution effect of meteoric water infiltration on reservoirs has led to the formation of strong dissolution of carbonate minerals and plagioclase in the Jinzhou area,as well as relatively small precipitation of authigenic quartz.4.The control of lithology and different types of diagenetic fluids on reservoir developmentThere are certain differences in the types and quality of reservoirs formed by lithology.magmatic granite is relatively more prone to fracturing under stress,usually with higher permeability and is easier to form fractured and high-quality reservoirs.Gneiss is relatively easier to form porous reservoirs,and its ability to form high-quality reservoirs is poorer compared to magmatic granite.magmatic gneiss is in between the two.Plagioamphibolite,intermediate-basic intrusive rocks,and granulite are not easy to form reservoirs,and they can form tight interlayers in buried hills that hinder fluid migration.In Jinzhou area,the physical properties of buried hill are significantly enhanced under the weathering and leaching of meteoric water,and the weathering crust reservoir is formed.Although the weathering crust reservoir in the Bozhong area has experienced strong filling damage,it has improved the solid rock structure of the buried hill,which has laid a petrological foundation for the formation of effective reservoirs in the late structure and organic acid and CO₂ transformation of the buried hill in the Bozhong area.The formation of weathering crust is controlled by factors such as weathering leaching time,paleogeomorphology,fracture and fracture development.The deep magmatic hydrothermal activity mainly plays a role in filling and destroying the buried hill reservoir.The dissolution of organic acids and CO₂ significantly increases the porosity of buried hill reservoirs,which is mainly controlled by the contact relationship between source rocks and buried hill reservoirs.Faults and unconformity surfaces are the dominant migration channels of organic acids and CO₂.The simulation of organic acid-metamorphic rock dissolution experiment shows that the increase of temperature,the decrease of p H and the increase of fracture development can significantly promote the dissolution of metamorphic rock.The dissolution rate of magmatic granite is lower than that of gneiss at 80°C,higher at 140°C,and little difference at 180°C.5.Comprehensive development model of Archean metamorphic buried hill reservoirThe formation process of the Archean buried hill reservoir in the Bozhong area can be summarized as follows:Archean metamorphic rocks formed structural fractures during the Indosinian and Yanshanian extrusions.At the same time,the leaching of meteoric water led to the formation of weathering crust at the top of the buried hill,and a large number of dissolution pores were formed in the metamorphic rocks.The hydrothermal activity during the sedimentary period of the Guantao Formation led to a large amount of early ankerite precipitation to fill the pre-fractures and dissolution pores;since the late deposition of Guantao Formation,the extension has led to fracture activation,and organic acids and organic CO₂ have entered the buried hill reservoir to form dissolution pores.The formation process of Archean buried hill reservoir in Jinzhou area is summarized as follows:Archean metamorphic rocks form two stages of structural fractures under the action of Indosinian extrusion and NE-trending left-lateral compression and torsion in late Yanshan period.At the same time,the leaching of meteoric water leads to the dissolution of metamorphic rocks at the top of buried hill to form weathering crust.In the sedimentary period of the Shahejie Formation,the third stage of structural fractures were formed by the NE-trending dextral compression and torsion.After the sedimentary period of Guantao Formation,the meteoric water infiltrated along the deep strike-slip fault,and further dissolved the buried hill reservoir to expand the scale of reservoir development.