Fine Characterization And Geological Modeling Of Mafic-intermediate Volcanic Reservoirs

With the deepening development of volcanic oil and gas exploration in the Songliao Basin,the study of volcanic reservoir has been transformed from the early macroscopic description to a more detailed study.Volcanic reservoirs in Dongling area of Songnan Gas field in the Changling fault depression of Songliao Basin are dominated by mafic-intermediate volcanic reservoirs of 1^st submerber of Yingcheng Formation（K₁yc₁）.Dongling area as an old area of volcanic oil and gas exploration and development,many geologists have done a lot of research work in this area.Due to the complex geological conditions in this area,there are certain difficulties in fine identification and lateral prediction of volcanic reservoirs.In order to deepen the study of volcanic reservoirs in this area,it is necessary to carry out fine description and geological modeling study of volcanic reservoirs.In this paper,by comprehensively using drilling data,logging data and seismic data,the construction of volcanic stratigraphic framework and the fine description of volcanic rock lithology,lithofacies characteristics analyses have been completed.By means of a variety of experimental methods including casting thin sections,scanning electron microscopy,high pressure mercury injection and nitrogen adsorption,the reservoir space of mafic-intermediate volcanic reservoirs in the study area was finely delineated and the microscopic pore structure was quantitatively characterized.Based on the fractal theory,the fractal characteristics of nano-scale pores in mafic-intermediate volcanic reservoir in the study area are discussed.On the basis of the fine structure interpretation of the study area,combining with drilling data and logging data,the modeling method was optimized to complete the establishment of the tectonic model,lithofacies model and related attribute model of the study area.The distribution characteristics of dominant volcanic reservoir were defined,and the reserve recalculation was completed.The volcanic rocks of the K₁yc₁ can be divided into four periods based on sedimentary intercalation,weathering crust and lithologic abrupt change surface.The lithology logging identification plate was completed by drilling core description,thin section analysis and logging interpretation data.Based on logging interpretation data and seismic reflection characteristics,a geology-seismic identification template of volcanic facies was established,in which identified 3 volcanic facies and 6 subfacies in the study area.On this basis,four types of volcanic architectures were identified.According to core observation,thin section analysis,scanning electron analysis and other methods,the reservoir space types of volcanic reservoirs in the study area were divided into primary and secondary reservoir spaces.And then according to the formation mechanism and characteristics of different reservoir spaces,the reservoir spaces were futher subdivided into 14 types,including primary pore,amygdala internal pore,intergranular/grain pore,cleavage fracture,blast fracture,lava blast fracture,intragranular dissolution pore,amygdala solution pore,devitrified pore,matrix solution pore,clay mineral intergranular pore,structural fracture and dissolution fracture.It is found that the reservoir spaces in the study area have a wide range of scales,including macropores（centimeter-millimeter scale）and micropores（micron scale and nano scale）.On the basis of drilling core data,thin section analysis and microscopic reservoir space classification and comparison,the diagenesis and reservoir space evolution model of volcanic reservoirs in the study area were discussed.According to the diagenesis period,the diagenesis of Yingcheng Formation volcanic reservoir in the study area were divided into two stages:early diagenesis stage and late diagenesis stage.There are many types of diageneses in Yingcheng Formation volvanic reservoirs in the study area.The early diageneses mainly include dispersion of volatiles,blasting,condensation contraction,hydrothermal precipitation and crystallization of penecontemporaneous rocks.The late diageneses include devitrification,filling,alteration,dissolution and tectonism.Based on the analysis of diageneses,the ideal evolution model of reservoir space was established according to the diagenesis stage and the evolution characteristics of reservoir spaces,and the evolution mechanism of reservoir space was analyzed.It is concluded that the development of primary reservoir space（pores and fractures）and later dissolution are of great significance to the formation of dominant volcanic reservoirs.The primary reservoir spaces（pores and fractures）in the volcanic reservoirs provide the"evolutionary basis"for the subsequent evolution of the reservoir space.The fractures developed in the early stage provide a channel for the later fluid to enter the volcanic rock,which provides the possibility for the improvement of the volcanic reservoir by dissolution.Through comprehensive study,the main controlling factors of volcanic reservoirs in the study area were discussed and it is concluded that the dominant volcanic reservoirs in the study area are mainly controlled by stratigraphic interface,lithology,lithofacies and tectonism.In this study,the micro pore structures of volcanic reservoirs were quantitatively characterized by means of high-pressure mercury injection and nitrogen adsorption.Mercury injection results show that the pore size distribution of volcanic reservoir samples have multi-peak characteristics,indicating the complexity of pore structure.The results of nitrogen adsorption experiment show that the nanoscale pores in the sample are relatively developed,and the pore morphology is characterized by irregular ink-shape type pores,V-type pores and parallel narrow wall slit pores.Combined with the results of high-pressure mercury injection and nitrogen adsorption experiments,three distribution models of nanoscale pores of volcanic reservoirs in the study area were proposed.The lower limit of effective pore size and effective porosity of mafic-intermediate volcanic reservoirs in the study area have been calculated,and the calculated result are 12 nm and3.82%,respectively.Based on high-pressure mercury injection and nitrogen adsorption experiments,4 fractal dimensions were calculated in total.Through the study of the relationship between fractal dimensions and the parameters characterizing reservoir pore structure,it is shown that all four fractal dimensions can characterize the pore structure to a certain extent,but in general,fractal dimension D1 and fractal dimension D4 are highly correlated with the parameters characterizing reservoir structures,which can better characterize reservoir nanopore structures.The three-dimensional（3D）modeling work in this study relied on Petrel software,which was based on the geological and logging data of wells,and completed the related modeling work with seismic data as constraints between wells.The fault model,surface model,lithofacies model and related attribute model of Dongling area are established respectively.According to the 3D modeling results,the main volcanic facies developed in the K₁yc₁ in Dongling area is effusion facies.The overall formations show the monoclinal structure characteristics tilted from west to east.And buried depth becomes shallow gradually from west to east,and each structural layer show the same characteristics,having a good inheritance.Two groups of faults are mainly developed in NE and NW directions,with parallel and interlacing fracture morphology.The porosity of volcanic reservoir is discontinuous in transverse direction,and the porosity of volcanic rock near the top or formation interface is higher lengthways.The permeability high values are mainly distributed near deep and large faults in plane,while the high values are mainly distributed along the top of volcanic rock mass in longitudinal direction.In the reserve recalculation,the plane is divided into two calculation units according to fault blocks,and the vertical is divided into two units of upper and lower layers,which are altogether divided into four calculation units.The volume method and 3D model methods were respectively used to calculate the volcanic gas reserves in the study area.The calculation result of volume method shows that the geological reserves of volcanic gas in K₁yc₁ of Dongling area are 23.19×10⁸ m³.The 3D model reserves calculation results show that the geological reserves of volcanic gas in K₁yc₁ of Dongling area are24.25×10⁸ m³.The reserve calculation results of the 3D model are similar to those of the volume method,and the error is within a reasonable range,indicating that the reserve calculation results are more reliable and the 3D models are reasonable.