Cenozoic Volcanic Rocks In Huizhou Sag Of The Pearl River Mouth Basin:from Petrogenesis To Volcanic Reservoir

The South China Sea is located at the intersection of Eurasian plate,Indoaustralian plate and Pacific plate.Its tectonic location determines its important position in the reconstruction of Mesozoic and Cenozoic plate tectonics.The Pearl River Mouth Basin is a marginal sea basin developed in the northern margin of the south China sea during the Cenozoic.Its formation and evolution went through a complete rift evolution stage.There is a large amount of volcanic rock in the stratum.These volcanic rocks not only completely record the volcanism process in the initial evolution stage of basin formation,but also are one of the most promising reservoirs for oil and gas exploration in this area.However,due to the limitation of geological evidence for a long time,the petrogenesis,spatial and temporal distribution and volcanic reservoir development characteristics of deep buried volcanic rocks are still unclear.In view of this,the volcanic rocks encountered by drilling in Huizhou sag of Pearl River Mouth Basin were selected as the research object,based on the data of drilling,well logging,U-Pb zircon dating,geochemistry and seismic data,the study on lithologic identification,lithofacies-logging facies-seismic facies division of volcanic rocks in this area is carried out.The petrogenesis and geodynamics background are also discussed.Based on that,the external interface of the volcanic apparatus were identified and the lithofacies of the internal volcanic rocks were identificatized by seismic.The spatial distribution of volcanic rock is summarized.The relationships among diagenesis,reservoir space and reservoir physical properties were analyzed.In order to provide a theoretical basis for oil and gas exploration of volcanic rocks in the northern continental margin of the South China Sea.1.Lithologic development of volcanic rocks in Huizhou sag of the Pearl River Mouth BasinThe volcanic rocks in Huizhou sag of the Pearl River Mouth Basin are mainly composed of five types of volcanic rocks: trachyte,dacite,rhyolite,trachytic breccia and dacitic tuff.There are mainly 6 volcanic facies: the volcanic neck subfacies,the inner extrusive subfacies,the middle extrusive subfacies,the outer extrusive subfacies,the air-drop subfacies and the exoclastic pyroclastic sedimentary subfacies.2.Geochronology,geochemical characteristics,petrogenesis and geodynamics of volcanic rocks in Huizhou sag of the Pearl River Mouth BasinThe age of magma crystallization in Cenozoic volcanic rocks is early Paleogene,the eruption lasted from 59.15±0.77 Ma to 57.53±0.72 Ma later.The Cenozoic volcanic rocks in Huizhou sag of the Pearl River Mouth Basin have high silicon Si O2 = 60.3% ~ 70.44%,high potassium 2.39% ~ 5.71%.K2O/Na2O=0.48~1.34,Ti O2=0.43%~0.72%,the alkali content is medium and low Na2O+K2O=5.59%~9.57%.The S-I-M-A classification scheme of granite was used to map in the relevant geochemical diagram.The results showed that the volcanic rock samples in the study area generally belong to I-type granite.In the R1-R2 construction discriminant diagram,the Cenozoic volcanic rocks in the study area in Huizhou sag of the Pearl River Mouth Basin mainly fall within the range of late orogenic granites.The diagram of Yb+Ta-Rb,Y+Nb-Rb,Yb-Ta and Y-Nb indicates that the volcanic rocks in the study area were formed in the volcanic arc environment associated with subduction of oceanic crust to continental crust.By comparing the contents of major elements such as Si O2,Al2O3 and Mg O,as well as the contents of Y,Sr,(La/Yb)N and Sr/Y in Cenozoic volcanic rocks in Huizhou sag of the Pearl River Mouth Basin.The results show that there is a slight positive Eu anomaly in Chondrite-normalized rare-earth element(REE)distributional diagram.Enrichment of light rare earth,loss of heavy rare earth characteristics,these are showed that the Cenozoic volcanic rocks in the study area belong to adakite.On that basis,the content of compatible elements(such as Cr and Ni)in the volcanic samples in the study area is low,the K2 O content was high and some samples showed slight anomaly of normal europium(Eu=1.33~1.88).It can be seen from the Mg O-Si O2 and Ni-Mg# diagrams that the adakites in the study area belong to the lower crustal genesis.3.Cenozoic volcanic rock reservoir spatial development in Huizhou sag of the Pearl River Mouth BasinFour types of reservoir Spaces,primary pore,secondary pore,primary fracture and secondary fracture,were identified in the study area.Primary pores are intergranular pores and primary stomata.The secondary pores are phenocrysts dissolution pores and matrix dissolution pores.The primary fractures include phenocrysts blast fractures and matrix shrinkage joints.The secondary fractures are structural fractures and structural dissolution fractures.Intergranular pores and structural dissolution joints contribute the most to effective pores.Most structural fractures are filled by late hydrothermal action.The porosity of most samples of volcanic rock reservoirs in the study area is more than 4%,and the distribution of porosity is between 4.2-5.9%.The average porosity was 5.3%.The permeability distribution was relatively centralized,0.0016-0.0024 m D.The average permeability was 0.0021 m D.The correlation between porosity and permeability is good,both increased slightly with depth.There are 5 types of capillary pressure curves.The corresponding volcanic reservoirs are also divided into five types of pore structure.They are unimodal fine state,unimodal coarse state,bimodal fine state,bimodal fine state and unimodal fine state.4.Seismic identification and distribution of the Cenozoic volcanic facies in Huizhou sag of the Pearl River Mouth BasinCombined with seismic profile interpretation and seismic attribute analysis,the volcanic rocks in the study area were identified by plane and profile.The results show that a set of volcanic rocks of the syn-rift stage composed of volcanic lava and pyroclastic rocks developed under Tg,which was originally thought to be the top of the basement.The Eocene-Pliocene sedimentary sequence developed in the north continental margin of the South China Sea is a syn-rift and post-rift basin filling formed on the top surface of the volcanic rock(Tg reflector).The subsurface of the volcanic rock can be traced throughout the basin using 3D seismic data.This article interprets the underside of the layer as Tgv.The basement lithology is dominated by late Cretaceous granite.The base top surface composed of the pre-rift erosion surface,truncated section and detached fault is interpreted as Tgb.The planar facies sequence of volcanic rocks in the study area is: volcanic conduit facies ? extrusive facies ? explosive facies ? volcanic sedimentary facies.Volcanic rock is thickest near the proximal facies.Thinning away from the near-source zone towards the periphery.The maximum thickness is 1272 m,and the general thickness is 800~1000m.