Characteristics And Main Controlling Factors Of Permian Volcanic Reservoirs In Western Sichuan Basin

In the past fifty years,Permian volcanic rocks have been drilled in several formations in the Sichuan Basin,but few oil and gas resources have been found.The reason for this is the lack of systematic research on the development characteristics of volcanic edifice,distribution rules of volcanic rock strata and the main controlling factors and distribution pattern of volcanic reservoir in the western Sichuan Basin.In this paper,typical outcrop profiles and typical drill wells of the Permian in the western Sichuan Basin are selected for the study.Based on the data from drilling,logging,cores and testing experiments in the region,the lithological characteristics and petrographic divisions of outcrop profiles and drill wells,the longitudinal development characteristics and planar distribution characteristics of volcanic reservoirs are analyzed,and the eruption mode of Permian volcanic rocks in western Sichuan and their relationship with the Emeishan mantle plume are elucidated based on geochemical characteristics.On the basis of this analysis,the diagenesis of volcanic reservoirs and the type of reservoir space were studied in depth,the diagenetic evolution of volcanic reservoirs was discussed,and the main controlling factors of volcanic reservoirs were revealed.The main research results are as follows.（1）The rock characteristics of Permian volcanic reservoirs in western Sichuan were analyzed by casting thin section observation,scanning electron microscopy,and logging data.The Permian volcanic rocks in the study area are classified into four categories,namely,volcanic lava,volcanic clastic lava,volcanic clastic rocks,and sub-volcanic rocks,based on structural and compositional characteristics.Among them,volcanic lava is absolutely dominant,volcanic clastic lava and volcanic clastic rocks also occur,and subvolcanic rocks develop basaltic porphyrites.Among the volcanic lavas,cryptocrystalline and microcrystalline basalts,porphyritic and porphyritic basalts,porous and amygdaloidal basalts,slaggy lavas,and porous lavas are common,and the main constituent minerals are plagioclase,pyroxene,chlorite and other alteration minerals;among the volcanic clastic lavas,the volcanic clastic materials are tuffs,plagioclase and breccias,which are formed by basaltic magma cementation;the volcanic clastic rocks are mostly tuffs and breccias;Subvolcanic rocks include basaltic porphyrites,which are homologous with basaltic lavas.The Permian volcanic rocks in western Sichuan have continuity and consistency in lateral distribution,i.e.,large sets of volcanic lava are interspersed with some volcanic clastic lava or volcanic clastic rocks.（2）The Permian volcanic facies in western Sichuan can be divided into four categories based on lithology and spatial and temporal superposition relationships:eruptive facies,effusive facies,overflow facies and subvolcanic facies.Among them,the eruptive and overflow facies are the most developed.The eruptions of volcanic rocks are multi-phase and homogenous,and can be divided into three large eruptive cycles.The overflow and effusive facies are alternately distributed,indicating that volcanic eruptions experience multiple eruptive rhythms.The volcanic facies and lithology have good correspondence,with eruptive facies corresponding to volcanic clastic rocks,overflow facies corresponding to volcanic clastic lava,effusive facies corresponding to volcanic lava,and sub-volcanic facies corresponding to basaltic porphyrites.Combined with the output state and volcanic petrographic features,the basaltic rocks of Yong Sheng 1 well are divided into three eruptive spirals and 16eruptive rhythms,and there is no obvious weathering sign or ancient weathering crust between the three spirals,implying that the interval of their magma eruption may be short.The basalt in the Tongjiezi section is divided into four overflow-facies lithologic assemblages,five eruptive-facies lithologic assemblages,one eruptive-facies lithologic assemblage and two ash-flow-facies lithologic assemblages.The lithological petrographic assemblages of the two are very similar.（3）Based on the lithological characteristics and petrographic divisions,the volcanogenesis was analyzed by geochemical methods such as inductively coupled plasma mass spectrometry and X-ray fluorescence spectroscopy.The eruption of volcanic rocks in the study area is multi-period and contemporaneous,and can be divided into three large eruptive cycles.The volcanic cyclicity is manifested in the superposition of multi-phase volcanic lava layers,and the characteristics of the three cyclic compositions reflect that the basaltic parental magma of the set was formed by the condensation of peridotite after three partial melting in the upper part of the upper mantle.The standardized distribution pattern of rare earth element spheroidal meteorites and the standardized spider web map of trace element primitive mantle indicate that the volcanic rocks in the study area are similar to the magma source area of Emeishan basalt in the region.Meanwhile,the main and trace elements indicate that the magma composition of the volcanic rocks in the study area is obviously different from the original magma and is a highly evolved magma;Trace elements show that the study area belongs to the intraplate continental overflow basalt in terrestrial phase.Combined with the above analysis and volcanic spreading characteristics,the volcanic eruption mode in the study area belongs to multi-phase rif ted overflow-eruption.（4）The characteristics of the volcanic reservoir storage were analyzed by casting thin section observation and scanning electron microscopy.The porous lava,plutonic lava and slag lava in the study area have more developed pores and many diffuse micropores exist.Pores are mostly present in volcanic lava,among which porous lava and pore-amygdaloidal basalt are the most abundant.There are a large number of filled pores and amygdaloidal bodies in the plasma chip,and the pores are mostly tear-shaped,portland-shaped and ellipsoidal,filled with sodium feldspar,chlorite,calcite and other minerals.The volcanic lava contains feldspar porphyry and other soluble minerals and unstable components that form dissolution pores under the effect of secondary dissolution.The inner wall of the dissolution pores is irregular,accompanied by the proliferation of zeolite and chlorite minerals.Intergranular pores and dissolution pores can be seen together.Intergranular pores are mostly found between basal plagioclase crystals,and dissolution pores are mostly developed along curtainstone joints.The fractures are partially filled,mostly with calcium-rich hydrothermal minerals,and most of the contraction joints are plume-like or curved.The distribution characteristics of the porosity of the volcanic reservoir in the study area were summarized by the pore structure characteristic image analysis method and rock physical properties analysis methods such as full-diameter core pore permeability test,and the porosity distribution of the volcanic reservoir was more concentrated,with the minimum value of 11.8%and the maximum value of 25.5%,and the average of 18%.The permeability of most samples of volcanic reservoirs is less than 1×10^-3μm²,and samples with permeability less than 0.2×10^-3μm²account for 46.2%of the total number of samples.The high porosity and low permeability of the reservoir are mainly due to the dispersion of pores that are not connected.（5）The main control factors of volcanic reservoirs were analyzed by fluid inclusion thermometry and cathodoluminescence.Volcanic reservoirs are mainly controlled by lithology,lithofacies and diagenesis.The lithology and lithofacies are the basis for the development of primary pores and dissolution pores,and the primary secondary pores play a decisive role in the reservoir.The physical properties of volcanic reservoirs in the study area are positively correlated with quartz and plagioclase content,and negatively correlated with rock density.The basalt is dominated by the dark minerals plagioclase,pyroxene and olivine,and alteration mostly produces chlorite,soapstone and calcite,resulting in the pore space being filled severely.The volcanic facies zone directly affects the development of storage space such as pores and primary joints,gas pores,intergranular pores and contraction joints are mostly developed in the overflow facies volcanic lava in the study area,gas pores are mostly seen in the upper part of the overflow facies,elliptical elongated pores are occasionally seen in the lower part,intergranular micropores are mostly distributed in the middle;gas pores and contraction joints are mostly developed in the effusive facies volcanic clastic lava,contraction joints and pores are mostly produced at the top and bottom of the cooling unit as a result of the contact between the top or bottom of the magma and the atmosphere or early stratigraphy;the eruptive facies volcanic clastic rocks develop more intergranular pores and fractures;the sub-volcanic facies develops more intergranular pores.diagenesis directly affects the pore evolution of the reservoir,and the volcanic reservoir in the study area is deeply buried,and the filling,cementation and clay mineralization are more destructive to the reservoir.The dissolution phenomenon is extremely common in the volcanic reservoirs in the study area,which can form intracrystalline dissolved pores,intergranular dissolved pores,intergranular dissolved pores,intra-amygdaloidal dissolved pores and dissolved microporosity,etc..It is the key factors for the formation of high-quality reservoirs..（6）A comprehensive evaluation of the volcanic reservoir was carried out.The evolution of volcanic reservoir diagenesis in the study area can be divided into four stages:condensation consolidation diagenesis,hydrothermal diagenesis,epigenetic diagenesis,and buried diagenesis.At the stage of condensation consolidation,primary pores,intergranular pores and primary condensation shrinkage joints are mainly formed;at the stage of hydrothermal diagenesis,the primary pores and fractures are drastically reduced by filling minerals;at the stage of epigenesis,secondary fractures and secondary dissolution pores are produced;at the stage of burial diagenesis,dissolution is very common,and some pores are filled by secondary minerals at the late stage.Based on the lithological and tectonic characteristics,the study area is divided into three trap formations,and the Permian volcanic reservoirs in western Sichuan are comprehensively evaluated based on three geological factors:the volcanic rock facies,the intensity of weathering and leaching,and the intensity of dissolution.The pores and fractures of various genesis are very developed.The most favorable lithologic facies for pore development is the porous lava and slag lava on the overflow flow without excessive alteration and weathering in the front.The highest quality Permian volcanic reservoirs in western Sichuan are geographically distributed in the Chengdu plain area,followed by the eastern edge of the Emeishan uplift zone,and the reservoirs in the eastern edge of the Longmenshan uplift zone have poorest physical properties.