Volcanic Characteristics,eruptive Environment And Reservoir Development Model Of The Cretaceous Huoshiling Formation In The Southern Songliao Basin

In recent years,the volcanic reservoirs of Huoshiling Formation in the southern Songliao Basin have been proved to have large gas thickness,high output per well and large reserves,which shows the great potential of Huoshiling Formation as a new field of oil and gas exploration.However,the volcanic eruption environment of Changling fault depression and Lishu fault depression Huoshiling Formation in the study area is complicated,and the lithology and lithofacies are variable,which leads to the unknown characteristics of reservoir formation mechanism of volcanic rocks,and thus highly difficult to identify favorable reservoirs.In this paper,based on the geological and geophysical characteristics of volcanic rocks,the distribution of volcanic rocks of Huoshiling Formation in Songnan area is defined by identifying volcanic eruption environment and establishing volcanic facies model.Based on the analysis of geochemical data,the origin of volcanic rocks of Huoshiling Formation is revealed.By means of core observation and description,cast thin sections,scanning electron microscopy,conventional gas porosity measurement,high-pressure mercury injection,nitrogen adsorption,nuclear magnetic resonance,X-ray diffraction and micron CT scanning,the space type,micro-pore structure,controlling factors and formation mechanism of volcanic reservoirs in the study area were systematically studied.Finally,the geological model of volcanic reservoir development was established to predict favorable areas.The following achievements are obtained:1.Petrological characteristics and petrogenesis of volcanic rocksThe volcanic rock types are mainly intermediate,and their eruption environment can be divided into subaerial eruption and underwater eruption.The types of rocks in the subaerial environment are complex,including both volcanic lava and pyroclastic rocks,while the types of underwater eruption rocks are mainly dense tuff and caisson tuff.The typical structures identified are tuff structure,breccia structure,vesicle structure,block structure and bedding structure.The volcanic rocks of Huoshiling Formation in Songnan area have high SiO₂ content,ranging from 81.44 to 51.05 wt.%,and the Harker diagram shows an obvious trend of fractal crystallization.The distribution patterns of rare earth elements in volcanic rocks from different eruptive environments are roughly consistent,showing negative anomalies of Nb and Ta,low(⁸⁷Sr/⁸⁶Sr)i values andεNd（t）,and similar Pb isotopic compositions.It is inferred that they originate from the partial melting of the lithospheric mantle containing subduction components,and then underwent crystallization in the middle and upper crust.2.The characteristics of volcanic facies and edifices and the model establishment of underwater eruptionAccording to petrology and mineralogy characteristics,the subaerial eruption environment and underwater eruption environment in the study area are distinguished.the volcanic ash spheres and other reducing environment landmark minerals such as pyrite and siderite are identified in the underwater eruption volcanic facies.The oxidation coefficient exhibits the weak reduction-reduction environment,and the seismic reflection characteristics are roughly.The underwater explosive facies can be further divided into three sub-phases:gas-carried pyroclastic flow,water-carried high-density flow and underwater fall deposition.Gas-carried pyroclastic flow sub-facies are distributed within1km near the crater,and the lithology is mainly coarse tuff,which is the favorable facies.The water-carried volcanic density flow subphase is composed of fine tuff and caisson tuff.In the stopping stage of underwater eruption,the suspended volcanic ash gradually falls down to form the sedimentary sub-facies of underwater fall,and the lithology is the thick layered tuff.The complete longitudinal sequence of a single accumulation unit of underwater eruption from bottom to top is the gas-carrying pyroclastic flow subphase,the water-carrying volcanic density flow subphase and the underwater fall subphase.Based on the identification of the eruptive lithofacies on land,6 facies and 11 sub-facies are identified.On this basis,4 types of volcanic edifices are identified as the basic eruptive shield volcanic edifice,the composite layer volcanic edifice,the pyroclastic sheets and underwater erupting tuff mound edifice.3.Reservoir space types and micro-pore structure characteristics of volcanic rocksThe main reservoir space types of volcanic rocks on land are primary pores,intergranular pores,dissolved pores and fractures,mainly meso-macropores.The primary reservoir space of underwater eruptive volcanic rocks is poorly developed,mainly characterized by devitrification pores,corrosion pores and fractures,which belong to micropores and medium pores.The physical properties of onshore volcanic rocks are better than those of underwater volcanic rocks.The volcanic rocks on land have high maximum mercury intake saturation,large nitrogen adsorption capacity,large pore volume,and hysteresis ring morphology between H2 and H3 types,indicating that there are not only spherical pores such as solution pores and devitrified pores,but also slit-plate pores such as cracks and clay mineral pores.Underwater volcanic rocks are mainly microporous and mesoporous,and the mercury injection curve is characterized by fine distortion,poor sorting,small throat,large displacement pressure,low maximum mercury intake saturation,small nitrogen adsorption,small pore volume,hysteresis ring morphology mainly between H3 and H4,some of which can be attributed to H4 type,the NMR peak value is skewed to the right,and the pore size is small.Porous andesite and volcanic breccia are the dominant lithology in the volcanic rocks on land,while the coarse-grained tuff in the volcanic rocks underwater has good physical properties and developed pores and fractures.In general,the reservoir properties of volcanic rocks in the study area are dominated by mesopore.4.Main controlling factors and reservoir formation mechanismBy comparing the physical characteristics,micro-pore structure characteristics and reservoir space evolution of volcanic rocks in the onshore and underwater eruptive environments,the results show that the main factors of the onshore eruptive volcanic rocks reservoir are lithology,lithofacies,eruptive environment,dissolution and tectonic processes,and the underwater eruptive volcanic rocks are also affected by devitrification.The eruptive environment affects the structure and the degree of secondary transformation of volcanic rocks.Affected by weathering leaching,andesite and volcanic breccia are prone to form weathered fractures and connecting pores,which are conducive to forming high-quality reservoirs.Coarse-grained tuff within underwater eruption is the dominant reservoir lithology,and the space is mainly vitrified pore and solution pore.Coarse-grained tuff with strong brittleness is more prone to fracture than tuff with weak brittleness,forming a porous and fractured double-medium reservoir.5.Development model and favorable prediction of volcanic rock reservoirsThe homologous calc-alkaline intermadiate-acid volcanic rocks of the Huoshiling Formation form different types of volcanic rocks under different eruptive environments.In the volcanic eruption and situation stage,the volcanic lava volatilized and dispersed to form pores,volcanic breccia and formation of intergranular pores;underwater eruption coarse tuff can form a small number of intergranular pores;fine tuff and caisson tuff are poor in the development of primary pores.In the supergene stage,the volcanic rocks from underwater eruption are not exposed to the surface to receive weathering leaching,while the volcanic rocks in land are affected by weathering leaching with the reservoir physical properties improved.In the early stage of burial diagenesis,the compressive compaction of lava is less,but pyroclastic rocks is more,and most of the primary pores are lost.In the middle burial stage,both underwater and onshore volcanic rocks are affected by dissolution and tectonic processes,and their reservoir properties are improved.In the late burial stage,mineral recrystallization and other processes make the pore development worse.The distribution of high-quality volcanic rocks on land is"near the crater,the thickness of weathering crust is large,and the fracture advantage development area",while the high-quality volcanic rocks from underwater are"near the crater（within 1km）,near the source rock,and the fracture advantage development area".