Diagenesis Of Carboniferous Batamayineshan Formation Volcanic Reservoir In Eastern Junggar Basin

Unlike Songliao Basin,where volcanic oil and gas is mainly stored in primary pores and fractures,the volcanic oil and gas of Batamayineishan Formation in Junggar Basin is stored in the weathering and alteration zone at the top,and the reservoir space is dominated by secondary pores and fractures.Therefore,the study of diagenesis,especially late diagenesis,is the key to reveal the formation mechanism of volcanic reservoirs in Batamayineishan Formation in the eastern Junggar Basin.The study of diagenesis is inseparable from petrology and chronology.Because the diagenesis types of different lithological types are different,the length of secondary transformation of volcanic rocks will also lead to the differences in diagenesis and reservoir characteristics.In view of this,this article takes the volcanic rocks of Batamayineishan Formation as the research object and studies their reservoir characteristics from 3 aspects of petrology,chronology and diagenesis,so as to reveal the formation mechanism of Batamayineishan Formation volcanic reservoir and provide theoretical basis for petroleum exploration of volcanic rocks in Junggar Basin.1.Lithological characteristics of the volcanic rocks from Batamayineishan FormationThrough core and field sample description,thin section identification,major element analysis and the “ diagenetic mode-chemical composition-mineral components”volcanic rock classification method,the lithological development and secondary transformation characteristics of the Batamayineishan Formation volcanic rocks in eastern Junggar Basin are determined.The volcanic rock types of the Batamayineishan Formation in study area are quite complex,including volcanic lava,pyroclastic lava,pyroclastic rock and sedimentary pyroclastic rock.Volcanic lava can be further subdivided into 10 basic rock types,such as basalt,andesite and rhyolite,etc.pyroclastic lava includes 14 basic rock types,such as basaltic agglomerate/ breccia/ tuff lava,basaltic/ andesitic/ trachytic crypto-explosive breccia,etc.pyroclastic rock include 11 basic rock types,such as basaltic agglomerate/ breccia/ tuff,andesitic agglomerate/ breccia/ tuff,etc.The pyroclastic rocks include two basic rock types: sedimentary volcanic breccia and sedimentary tuff.In addition,a small amount of hypabyssal intrusive rock can also be seen,including diabase and trachyte porphyry.In terms of rock types,the Batamayineishan Formation is dominated by volcanic lava,accounting for 68%,followed by pyroclastic lava and pyroclastic rock,accounting for 14% and 16% respectively.In addition,a small number of hypabyssal intrusive rock and sedimentary pyroclastic can be seen.In terms of rock composition,intermediate rocks and acidic rocks account for 34% respectively,while basic rocks are relatively few.At the same time,Batamayineishan Formation shows the characteristics of basic rocks at the bottom,intermediate rocks increasing upward and acidic rocks at the top.In terms of specific rock types,andesite and rhyolite accounting for 25% and 24% respectively,followed by basalt,accounting for 12%.Various rock types have undergone more or less secondary transformation,and the vesicles of intermediate and basic volcanic rocks are mostly filled to form amygdule structure.Devitrification and dissolution of intermediate and acidic rocks are common.In addition,alterations such as chloritization and calcification are also common in the volcanic rocks of Batamayineishan Formation.2.Zircon U-Pb age and its geological significance of the igneous rocks from Batamayineishan FormationBy systematically sampling to Batamayineishan Formation and the overlying Shiqiantan Formation,using LA-ICP-MS zircon U-Pb dating and conodont selection methods,the chronological characteristics of Batamayineishan Formation volcanic rocks and their geological significance are determined.Zircon U-Pb ages of eight volcanic rock samples obtained from the Batamayineishan Formation outcrop section in Shuangjingzi are 347.9±7.7Ma,336.6±2.7Ma,348.7±2.9Ma,335.2±1.9Ma,339.1±1.5Ma,327.9±1.5Ma,340.7±1.4Ma and 318.7±4.2Ma,respectively;zircon U-Pb age of one volcanic rock sample from the Batamayineishan Formation outcrop section in Baijiangou is 333.8±4.6 Ma.Four complete conodonts were selected from limestone samples of Shiqiantan Formation.They belong to two genera,namely Idiognathodus and Streptognathodus.The Idiognathodus first appeared in late Bashkirian Age,flourished in Moscovian Age to Early Permian,and Streptognathodus was a standard fossil of Late Carboniferous,common in Kasimovian Age to Gzhelian Age.This is consistent with the volcanic rock age test results.Thus it can be seen the volcanic rocks of the Batamayineishan Formation are diachronous,whose ages range from the Early Carboniferous to the Late Carboniferous.This conclusion is valid in Shuangjingzi and Baijiangou areas.Based on previous researches and regional tectonic background,the formation age of the Batamayineishan Formation can be limited to 348.7Ma ~ 300.4Ma.In addition,the age of a trachyte porphyry rock sample obtained from the Batamayineishan Formation in Shuangjingzi area is 255.9 Ma,which is the first discovery of Permian intrusive rocks in the study area and the youngest Paleozoic igneous rocks found in Junggar area at present;According to these U-Pb ages and regional tectonic,it can be determined that the closure time of the Junggar oceanic represented by the Karamaili ophiolite is between Famennian Age and 348.7Ma;A large number of zircons are captured in volcanic rock dating samples,ranging from 354.7 Ma to 971.2 Ma,which are consistent with the ages of island arc igneous rocks and ophiolitic melanges representing the oceanic lithosphere around the basin.It is presumed that the basement of the Junggar basin may be composed of island arc and oceanic crust,and there is a Precambrian basement.3.Diagenesis of volcanic reservoir in the Batamayineishan FormationBy means of sample description,thin section identification,facial porosity analysis,physical property analysis,electron probe micro-analysis(EPMA)and electron microscope,the diagenesis types and reservoir space development characteristics of the Batamayineishan Formation volcanic rocks are determined.Two types of late diagenesis,vesicle filling and alteration,are emphatically studied.Their genetic models and reservoir significance are analyzed.1)Diagenesis types and reservoir space development characteristicsThe diagenesis of volcanic reservoirs in Batamayineishan Formation includes early diagenesis and late diagenesis.The early diagenesis includes vaporization of volatile matter,condensing constriction,quenching effect,phenocryst burst effect,quasi-syngenetic hydrothermal precipitation and crystallization,welding effect,crypto-explosion effect and early compaction and cementation.The late diagenesis includes devitrification effect,filling effect,alteration effect,compaction and cementation,dissolution effect and tectonic effect.Among these diagenesis types,vaporization of volatile matter,condensing constriction,quenching effect,phenocryst burst effect,crypto-explosion effect,devitrification effect,alteration effect,dissolution effect and tectonic effect are often considered as favourable factors for volcanic reservoir.The reservoir space of volcanic rock reservoir in Batamayineishan Formation includes primary reservoir space and secondary reservoir space.The primary reservoir space includes primary vesicle,amygdule pore,lithophysa cavity hole,amygdule shrinkage pore,inter-granular pore,shrinkage fracture(net shrinkage fracture,bedding joint fracture,columnar joint fracture and spherical joint fracture),cleavage fracture and inner-phenocryst fracture.The secondary reservoir space includes devitrification pore,dissolution pore(phenocryst dissolution pore,matrix dissolution pore and amygdule dissolution pore),dissolution fracture and structural fracture.The remaining effective reservoir space is dominated by secondary reservoir space.Specifically,porous reservoir space is mostly composed of phenocryst dissolution pore and matrix dissolution pore.The primary vesicle has disappeared,mainly partly or totally filled by secondary mineral,such as chlorite,calcite and chalcedony.Fracture-like reservoir space is dominated by structural fracture,filling-residual structural fracture,dissolution fracture and cleavage fracture.Similarly,it is mainly composed of secondary fracture.2)The types,genesis model and reservoir significance of vesicle fillingThe vesicle filling minerals of the volcanic rocks in Batamayineishan Formation include silica,zeolite,celedonite,chlorite,saponite.There are 2 filling models,monoand multiple component ones.Mono-component amygdules include silica and celedonite ones.10 types of multiple component amygdules can be recognized,including quartz ? zeolite,calcite ? quartz,quartz ? saponite ? quartz,saponite ?quartz?calcite?quartz,chlorite?quartz,quartz?celedonite?saponite,celedonite?chlorite?quartz,saponite?zeolite,celedonite?chlorite?saponite,celedonite?saponite.The formation temperature of chlorite at the edge of amygdules is between 84.7-123.6?,which indicates a low temperature filling process.Based on the comprehensive analysis of the formation time,mineral composition,element composition and edge characteristics of amygdules in volcanic rocks,it is considered that the amygdules form from precipitation and crystallization of epigenetic fluid.The genetic models are as follows: 1)the formation of vesicles and microfractures provides space and material migration pathways for the formation of amygdules;2)the host rocks dissolve under the action of epigenetic fluid.Dissolution and alteration provide material basis for the formation of amygdules.3)Ion-rich epigenetic fluid precipitates and crystallizes in the vesicle to form amygdules.The quality of volcanic reservoirs becomes worse due to the vesicle filling process.70.2% of primary vesicles were completely filled,the remaining 29.2% of primary vesicles were partially filled.Taken together,94.5% of the primary vesicle space is completely lost.Although vesicle filling has such a negative influence on reservoir quality,residual amygdule inner pores and amygdule shrink pores still have some reservoir capability.In addition,the dissolution pores generate by the fluid which participate in the formation of amygdule can also compensate the reduction of vesicle volume to a certain extent.3)The types,genesis model and reservoir significance of alterationThe products of alteration of the volcanic rocks in Batamayineishan Formation include saponite,iddingsite,celedonite,biotite,magnetite,chlorite,sericite,clay mineral,calcite,cryptocrystalline and siliceous mineral.It is effective to use MFW alteration diagrams to determine the alteration degree of volcanic rocks in Batamayineishan Formation.According to MFW diagrams and W value distribution characteristics,the volcanic rocks in eastern Junggar Basin show the characteristics of low degree of alteration,the alteration degree of drilling samples in the basin is higher than that in the basin margin,and the alteration degree of acid rock is higher than that of basic rock.The alteration of volcanic rocks in Batamayineishan Formation can be divided into two types: alteration along fracture and alteration along crystalline zoning.The occurrence of both alteration types is closely related to fractures and hydrothermal fluids.The genetic model can be summarized as follows: 1)fractures are formed in volcanic rocks,which provide pathways for the migration of hydrothermal fluids;2)hydrothermal fluids rich in various ions interact with minerals at the edge of fractures and precipitates altered minerals such as calcite and chlorite along the fractures while dissolving alterable minerals;3)As alteration develops,alteration extends to the core of a mineral.Finally,the mineral is totally altered to secondary minerals.Alteration can make the dense volcanic material becomes porous,thus improving the reservoir quality of volcanic rocks.The devitrification can increase the porosity of the reservoir by 15%,and the permeability increases exponentially.The choritization can increase the areal porosity of volcanic reservoir by 9.19%-15.98%.With the increasing of alteration degree from basic to acid volcanic rocks,the physical property of the volcanic reservoir becomes better.