Fracture Characteristics Of Mesozoic Volcanic Rocks In Guxi Area

In recent years,with the deepening exploration of oil and gas fields in Bohai Bay Basin,industrial oil flow has been found in Mesozoic volcanic rocks in Zhanhua Sag,Jiyang Depression.Tests show that the exploration potential of this layer is huge,and it is expected to become the next succession area for oil and gas exploration in this area.The existing volcanic rock research in Jiyang Depression is mainly concentrated in the Cenozoic volcanic rocks,while the study of Mesozoic volcanic rocks has not been systematically carried out.Based on the previous studies,this paper analyses the sequence of fracture development and filling time of veins from macro and micro perspectives by means of core data,thin section identification,cathodoluminescence technology,carbon and oxygen isotope testing and fluid inclusion technology.The paper draws the following conclusions and understandings:(1)According to core observation and thin section identification,there are six main types of fractures in Guxi volcanic rocks: autoclastic fracture,structural fracture,weathering leaching fracture,reticulated fracture,filling residual structural fracture and structural-filling-dissolution fracture.Among them,the filling minerals are calcite,clay mineral,quartz,chert and feldspar,and the main mineral is calcite.(2)The order of fractures development are: self-crushing fractures,reticular fractures,weathering fractures,structural fractures.(3)On the basis of the description of the core,by means of microscopic observation and cathodoluminescence,it is found that the crystal morphology is mainly divided into three types: elongated block texture,stretch texture and fiber texture;according to the crystal growth direction and the growth interface: syntaxial veins,antitaxial veins,and streching veins.The calcite CL in the Guxi area is mainly bright orange,medium orange and reddish brown;some of them can also observe the metasomatism and dissolution phenomena,such as calcite replac of clay minerals and quartz dissolution.Some volcanic rocks in the Guxi area develop fracture-complex veins(quartz + vermiculite + calcite or quartz + calcite),in which the siliceous minerals do not emit light under the setting of calcite CL.The fracture of the GG19 well buried at 2248.85 m develops four different CL calcite veins.It can be inferred that there are at least four phases of fluid,and combined with the thermal evolution history of the study area,the time corresponding to different calcite CL can be inferred: The first phase of reddish-brown light(shallow burial),the second phase of medium orange light(early rise and elevation),the third phase of bright orange light(deep burial)and the fourth phase of reddish-brown light(deep burial).(4)Through the carbon and oxygen isotope test analysis of calcite veins,it is possible to identify five sources of venous fluid: formation water in the upper or lower sedimentary rocks of the surrounding rock,endogenous fluids in the surrounding rock,hydrocarbon source rock hydrocarbons,atmospheric fresh water and mantle hydrothermal fluid;As the depth increases,the oxygen isotope also increases,while the carbon isotope decreases first and then increases.Electron probe analysis verified that the oxygen isotope increased with increasing depth by analyzing the iron content.The uniform temperature range of calcite veins is between 54? and 146.6 ?,which can be roughly divided into four different temperature ranges.The main peaks range from 100? to 120?,and the secondary peaks are between 80? and 100?.