Characterization Of Volcanostratigraphy,Volcanic Reservoirs And Natural Gas Accumulation In The Huoshiling Foramtion In The Wangfu Fault Depression Of Songliao Basin

Reasonable stratigraphic division and correlation is the foundation for understanding characteristics and distribution of reservoirs,and is the guarantee for making reservoirs prediction.In this paper,characteristics of volcanostratigraphy,volcanic reservoirs and volcanic gas reservoirs are studied by using a method combining geology,logging and seismic,and the example comes from the Huoshiling Formation of the Wangfu fault depression in the Songliao Basin.Firstly,the volcanostratigraphic framework of the Huoshiling Formation is established.Then,characteristics of lithology and lithofacies,volcanic reservoir formation mechanisms,and reservoirs prediction are discussed within the volcanostratigraphic framework.Finally,characteristics and enrichment patterns of volcanic gas reservoirs in the Wangfu fault depression are discussed.Based on this research,a method employed for dividing and correlating volcanic stratigraphy and a workflow employed for characterizing the volcanic reservoir are summarized.After the exact and careful research,this paper comes to the following conclusions: 1.Volcanic sequences division and correlationThe volcanic sequences are divided,from large to small,into the volcanic cycle,cooling unit set and cooling unit.Cooling units,which are classified into 3 types of lava,pyroclastic and pyroclastic + lava,are the basic volcanogenic stratigraphic units and correspond to depositional systems of sequence stratigraphy.The cooling unit set is a superposition of cooling units and corresponds to a fourth-order sequence or parasequence of sequence stratigraphy.Cooling unit sets could be recognized on seismic profiles and therefore can be the basic units of seismic volcanostratigraphy.The volcanic cycle is a superposition of cooling unit sets and corresponds to a third-order sequence of sequence stratigraphy.Through the workflow of establishing identification marks(including geology,logging and seismic)of volcanic sequences at different levels,dividing the sequences in wells,and correlating the sequences on seismic profiles according to well-seismic combinations,a set of methods for volcanostratigraphic division and correlation which have a similar exploration accuracy as sand group in sandstone reservoirs are proposed.In this paper,two volcanic cycles and one volcanic-sedimentary cycle,thirteen cooling unit sets and several cooling units are divided in the Huoshiling Formation in the Wangfu fault depression.2.Characteristics of lithology and lithofaciesVolcanic rocks of the Huoshiling Formaiton are intermediate and acidic rocks,including lavas,pyroclastic lavas,pyroclastic rocks and sedimentary volcaniclastic rocks.Based on samples of drilling cores and cuttings,24 types of lithology,such as andesite,trachyandesite,rhyolite,breccia lava and breccia,are recognized,among which the proportion of trachyandesite is the largest,the next is breccia.Volcanic crater facies,volcanic slope facies and basin facies are distinguished in the study area,and 13 types of subfacies and 25 types of microfacies are further identified.Based on the research of relationship between volcanism and cooling units,models of lithology and lithofacies of different types of cooling units that formed by effusive eruption,fallout,base surges-pyroclastic flow and extrusive eruption are established.The volcanic crater,volcanic slope and basin facies are availably recognized by combining well and seismic data,and the seismic-volcanic facies can be mapped in the unit of cooling unit set.3.Characteristics of volcanic reservoirsThe volcanic reservoir spaces in the Huoshiling Formation in the Wangfu fault depression consist of primary and secondary spaces and are further divided into primary pores,primary fractures,secondary pores and secondary fractures,among which secondary pores and secondary fractures are in dominant.The main types of volcanic reservoir spaces are vesicles,intergranular pores,intercrystal pores,cooling fractures,explosive fractures,dissolved pores,structural fractures and structural-dissolved fractures.The reservoirs mainly have medium to relative high porosity and but low to low permeability.The porosity is mainly between 5~10%,whereas the permeability is almost less than 0.1×10-3?m2.The physical property of the trachyandesite member is the best and the rhyolite member is the second,wheras the volcaniclastic member is the worst.Among the four types of rocks,physical properties of pyroclastic lavas are the best,followed by lavas,and that of sedimentary volcaniclastic rocks are the worst.4.Formation mechanisms of volcanic reservoirsThe formation mechanisms of volcanic reservoirs include three processes based on the strata's evolution.First,vesicles,cooling fractures and explosive fractures formed during the formation of cooling units and concentrated near the cooling surface due to volcanism and an eruption environment,which were the foundation for the development of volcanic reservoirs.Second,parasyngenetic and secondary pores and fractures developed during the formation of cooling unit sets and volcanic cycles and concentrated in the volcanic crater,especially near the weathering crust because of collapsing,syn-faulting and weathering,which were the significant development of volcanic reservoirs.Lastly,numerous dissolved pores and structural fractures formed during burial and concentrated around fault belts because of diagenesis and tectonism,which determined the properties of volcanic reservoirs.5.Geological and seismic comprehensive prediction of volcanic reservoirsCombined geological,logging and seismic data,qualitative to semi quantitative geological and seismic comprehensive prediction of volcanic reservoirs are performed from many aspects of lithofacies,reservoir formation mechanisms,fracture distribution,reservoir inversion,seismic attribute analysis and distribution of reservoir porosity,permeability and gas saturation.The predicted reservoirs are classified into four types: reservoirs with relative-high to high porosity,medium to high permeability,or high gas saturation are defined as type ?;reservoirs with medium porosity,low permeability,or medium gas saturation are defined as type ?;type ? refers to probable reservoirs;and type ? refers to probable non-reservoirs.Seven reservoir areas of type ?,four reservoir areas of type ? and three reservoir areas of type ? are delineated in the cooling unit set 13.6.Characteristics and enrichment patterns of volcanic gas reservoirsFor the volcanic gas reservoir of the Huoshiling formation in the Wangfu depression,the cooling unit is the basic reservoir forming unit,and lava and pyroclastic+lava cooling unit are favorable for forming reservoirs.The cooling unit set is the basic natural gas accumulation unit,and a volcanic edifice is the accumulation center.The volcanic cycle is a natural gas accumulation system.The Huoshiling Formation contains two natural gas reservoir systems,the bottom one distributed in the upper part of the volcanic cycle 1,and the upper one mainly distributed in the volcanic cycle 3.Rhyolite is the most favorable type for natural gas accumulation,while trachyandensite is the main part for natural gas accumulation.Natural gas enriches in areas of volcanic crater facies,and next in areas of volcanic slope facies.Gas accumulation in the Wangfu fault depression is controlled by fault,stratigraphy and volcanic edifice together.The Shandongtun structural belt is the main area for natural gas accumulation,and in most cases,there only exist the bottom gas reservoir system in volcanic uplift zones,while the two gas reservoir systems are formed in zones from the brim area of volcanic uplift to depression.Volcanic gas reservoirs in the Wangfu fault depression are divided into three accumulation areas from north to south,i.e.the northern area,the central area and the southern area.Four kinds of gas accumulation models are summarized,including reversed anticline-stratigraphy accumulation model,fault block accumulation model,lithology-fault block accumulation model,and fault block-reversed anticline-stratigraphy accumulation model.Fault block accumulation model and lithology-fault block accumulation model are the main types.