Identification Of Lithology And Lithofacies And Research On Main Controlling Factors Of The Carboniferous Volcanic Rocks Reservoir,Xiquan Area,Eastern Junggar Basin

With the discovery of volcanic reservoirs,Volcanic rocks as unconventional oil and gas reservoirs attract more and more attention from geologists and petrologists.Volcanic rocks are widely distributed in Junggar basin where rock types of Carboniferous volcanic rocks are various in xiquan area east of the basin,and the controlling factors of reservoir distribution are complex.Therefore,the studies of lithology and lithofacies identification,reservoir characteristics and main controlling factors of volcanic rocks are of great significance to guide the exploration and exploitation of oil and gas in volcanic rocks.In this paper,the research object is carboniferous volcanic rocks in xiquan area at Beisantai uplift on the western margin of the eastern uplift of Junggar basin.We identify lithology and lithofacies types and characteristics of carboniferous volcanic rocks in Xiquan area,and study deeply about characteristics and main controlling factors of volcanic rock reservoir with core observation,various experimental analysis,well logging and seismic data as the basis,using theory and analysis-research method of subjects such as the volcanic petrology and petrography,reservoir geology and petroleum geology,etc.The main achievements are as follows,The lithology types of carboniferous volcanic rock include volcanic lava,pyroclastic rocks,pyroclastic lava,sedimentary pyroclastic rocks,pyroclastic sedimentary rock and hypabyssal intrusive rock in the Xiquan area.The volcanic lava is subdivided into basalt,andesite,dacite.The pyroclastic rocks are subdivided into volcanic breccia and breccia tuff.The pyroclastic lava is mainly breccia lava.The pyroclastic sedimentary rock is mainly tuffaceous sandstone.Sedimentary pyroclastic rock is mainly tuffite.Subvolcanic rock is mainly dioritic porphyrite.Based on the accurate identification of lithology by the log response characteristics of each volcanic rock,the lithology-log identification plate of carboniferous volcanic rocks has been established in Xiquan area.According to eruption type,rock type,forms and characteristics of volcanic rock output,the types of volcanic lithofacies in the study area are be divided into four types: explosive facies,effusive facies,Volcanic vent facies and volcanic sedimentary facies.The explosive facies is the most developed in the study area,followed by the effusive facies.Referred to well logging and seismic data,the lithofacies types and distribution characteristics of volcanic rocks have been identified.The craters are mainly located near well Xiquan 027 and well Xiquan 020.Explosive facies,effusive facies,volcanic sedimentary facies are distributed in the northeast-southwest direction.According to the methods of geological-geophysical analysis and geochemical analysis,the Carboniferous volcanic rock is divided into 2 eruption cycles and 5 stages in Xiquan area,and oil and gas are mainly distributed in the eruption stages of ? and ?.According to the property and morphology of pores,the pore types of the carboniferous volcanic rocks in the study area are divided into two types: pores and fractures,and further divided into primary pores,secondary pores,primary fractures and secondary fractures according to the origin.The physical properties of the andesite and volcanic breccia are the best in the study area,the physical properties of breccia lava,breccia tuff and dacite are the medium,and the physical properties of basalt and sedimentary tuff are the worst.According to characteristics of capillary pressure curve and pore structure parameters,the pore structure types of volcanic lava and pyroclastic rock were studied respectively.Based on the analysis and research of pore types,physical properties and pore structure,referred to the electrical properity and oil-bearing properties,the classification and evaluation criteria of carboniferous volcanic reservoirs is established in xiquan area.Class ? reservoir accounted for 14%,which is the advantageous reservoir.Class ? reservoir and class ? reservoir accounted for 25% and 17% respectively,as the better and medium-poor reservoir.Class ? reservoir accounted for 44%,as dense reservoir or water layer.The reservoir properties of carboniferous volcanic rocks in the Xiquan area are mainly affected by the following factors:(1)The lithology and lithofacies of volcanic rocks are the basis to determine the reservoir properties.The layers with good oil test results are mainly volcanic breccia in explosive facies and andesite in effusive facies.(2)Secondary dissolution pores formed by weathering,denudation and leaching improved the reservoir properties of volcanic rocks.Volcanic rocks of 0-50 m below the unconformity,especially the secondary pores of the andesites and dacite in the volcanic lava are more developed,and the distribution of favorable reservoirs is obviously controlled by the weathering and leaching zone,volcanic rocks reservoir belongs to weathering crust reservoir.(3)Tectonic fractures generated by tectonic movement play a major role in controlling the formation of volcanic breccia reservoirs in the areas of 70-115 m and 130-155 m below the unconformity surface,volcanic rocks reservoir belongs to tectonic-lithologic reservoir.(4)The formation of primary reservoir space and the evolution of secondary reservoir space were controlled by diagenesis in volcanic rocks.The formation and evolution of carboniferous volcanic reservoir space in Xiquan area mainly experienced syngenetic stage,epigenetic stage and burial diagenetic stage,these stages experienced volatilization,condensation contraction,post-magmatic hydrothermal alteration,weathering and leaching,shallow burial diagenesis,tectonization,2 times weathering and leaching,and deep burial diagenesis.The oil and gas in carboniferous volcanic rocks distributed obviously different in the south and the north of the Xiquan area.The main factors causing this difference are as follows,(1)The difference of the lithology and lithofacies.The content of the volcanic breccia in the explosive facies and the andesite in the effusive facies in the south accounted for 64%,while the content of those two in the north only accounted for 29%.(2)The difference of weathering and leaching.The content and the weathering intensity of the andesite and dacite located in the weathering zone in the south are higher than that in the north,physical properties of volcanic reservoir in the south are better than that in the north.(3)The difference of tectonization.The fracture and physical properties of volcanic rocks in the south are generally more developed than those in the north.(4)The difference of reservoir types.The types of volcanic reservoirs in the south include weathering crust reservoirs and tectonic-lithologic reservoirs,while it is mainly weathering crust reservoirs in the north.