Pore Structures And Reservoir Characteristics Of Volcanic Rocks In The Carboniferous Batamayineishan Formation In Shuangjingzi Area,eastern Junggar Basin

Junggar basin is one of the largest sedimentary basins in Northwest China.Carboniferous oil and gas fields have been found in different areas in the eastern part of the Junggar basin on a large scale,indicating that the Carboniferous rocks of the Junggar basin have a huge potential for oil and gas exploration.This study focuses on the Batamayineishan Formation in eastern part of the Junggar basin,which contains volcanic lava and pyroclastic rocks,aiming to investigate the reservoir characteristics and to identify the formation mechanism of Batamayineishan Formation.The majority of existing reservoir space in the volcanic rocks of Batamayineishan Formation is dominated by secondary pores and fractures.Using the methods of petrography,pressure-controlled mercury injection(PMI),and electron probe microanalysis(EPMA),the reservoir characteristics and diagenetic history of the volcanic rocks of the Batamayineishan Formation were studied.A theoretical framework is established to provide favorable guidance for exploring Carboniferous volcanic rocks in the Junggar basin.1.Lithologic identification and characteristics of volcanic rocks of Batamayi-neishan Formation in Shuangjingzi areaThe volcanic rock types in the study area include volcanic lava and pyroclastic rocks.Volcanic lava is further divided into basalt,andesite,rhyolite and dacite in proportion of39%,12%,23%and 7%,respectively.Pyroclastic rocks include basaltic breccia and rhyolitic breccia in proportion of 14%and 5%.Volcanic clastic rocks consist of variegated agglomerate and conglomeratic breccias.Based on textural structure,basalt represents overall porphyritic texture and can be further divided into vesicular basalt and massive basalt.Based on chemical composition basalt can be divided into trachybasalt and intergranular texture can be seen in trachybasalt.Andesite exhibits porphyritic texture with interwoven structures and argillization of plagioclase phenocrysts can be seen.Rhyolite presents spherulites and flow structures and divided into vesicular rhyolite and granulated rhyolite with typical rhyolitic structure.The common minerals found in this area are pyroxene,amphibole,plagioclase,orthoclase and quartz.Secondary minerals such as quartz,calcite,saponite,chlorite and zeolite are also reported in the observed thin sections.On the whole,the volcanic rocks of Batamayineishan Formation in this area have undergone strong diagenesis.Dark minerals are more likely to be chloritized and saponified,while light minerals are more likely to be clayized and calcited.2.Pore structure characterization and reservoir space types in volcanic rocks of Batamayineishan Formation in Shuangjingzi areaIn volcanic reservoirs,variable pore and throat structures are critical factors in controlling migration and accumulation of hydrocarbons which determine the reservoir potential of volcanic rocks.The results of mercury injection indicate that the average pore throat radius and porosity of volcanic rocks are 0.068?m and 6.62%,respectively.The average value of permeability is 0.424×10^-3?m²typically suggests narrow micro throats,which contribute significantly in low reservoir permeability.The displacement pressure and mercury withdrawal efficiency are considered as important parameters controlling hydrocarbon migration and evaluating reservoir quality.The displacement pressure varies from 0.466 to 68.913 MPa and mercury withdrawal efficiency varies between 14.375-93.708%with an averaged value of 50.57%.The capillary pressure curves of Carboniferous Batamayineishan Formation can be classified into 3 types.Type I capillary pressure curve is platform shaped,which shows low displacement pressure(3.79 MPa)and narrow pore throat sorting and the average pore throat radius varies between 0.016-0.143?m and pore throats are well connected.Type 1 is mainly distributed in basaltic breccia,rhyolite and vesicular basalt.Type II capillary pressure curve shows no obvious platform,with medium to high displacement pressure(34.44 MPa)and the sorting of pore throats is poor and not well connected.Type II is commonly found in basaltic breccia and rhyolite.Type III capillary pressure curve is slope shaped,with high displacement pressure(68.90 MPa)and worst pore throat sorting,mercury withdrawal efficiency is 16.95%.Type III is developed in basalt and andesite.In this study,porosity of Carboniferous Batamayineishan Formation is not positively correlated with permeability and the relationship between them is not obvious.Permeability remains stable and does not show significant change with the increase in porosity.Despite the high porosity,the permeability is relatively low reflecting isolated and non-connected primary pores.Samples with directly related changes in porosity and permeability are indicative of well sorting and development of microfracture/cracks.This shows well connection between pores and fractures.The reservoir space in Carboniferous Batamayineishan Formation is divided into four categories due to the heterogeneity of volcanic rocks.These rocks are dominated by isolated pores,connected pores,microcracks,and fractures.The connected pores formed effective migration pathways and storage space,while mutually unconnected pores do not allow oil and gas to flow and accumulate.The reservoir space of volcanic rock reservoir in Batamayineishan Formation includes primary reservoir space and secondary reservoir space.The primary reservoir space includes primary gas pore,amygdule pore,amygdaloidal shrinkage crack,inter-granular pore,shrinkage 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.3.Filling of reservoir spaces in Batamayineishan FormationFilling is an important diagenetic process that affects the physical properties of volcanic reservoirs.Filling makes the volume of primary gas reservoir space smaller,resulting in poor reservoir physical properties.Based on the study of pore filling of volcanic rocks in Batamayineishan Formation in the eastern Junggar basin,the pore-filling minerals include quartz,calcite,zeolite,chlorite,saponite,etc.Primary gas pores are filled and developed amygdales on a large scale.It can be divided into single component amygdala and complex component amygdala.The complex component amygdala is abundant,while the single component amygdala is rare.The development of microfractures provided space and material migration path for the formation of amygdala.Filling makes the quality of volcanic reservoir worse,and the majority of the original gas reservoir space is lost.After the pores were filled,the remaining pores in the amygdala such as the shrinkage pores still had storage ability.In addition,the dissolution pores developed by dissolution and alteration also compensated for the decrease of the original gas pore volume.