Boundaries & Units Of Volcanostratigraphy And Their Constraints On The Description Of Seismic Facies And Geological Interpretation Of Volcanogenic Successions

Volcano stratigraphy have distinguishing features: short formation time,temporal attributes of ephemeral construction and long erosion.The formation and construction process of volcano strata is different from that of sedimentary strata.Predecessors have formed a certain understanding of recognition and characterization of volcanic stratigraphy and their boundary.However,lacking basic volcanic stratigraphic units in basin have led to random and diverse describtion of seismic facies in the interpretation of seismic volcanostratigraphy.Taking Songke 2(SK2)well and typical drilling in the Changling Depression as examples,based on data from cores,well logs and seismic,with the study of boundary and units of volcanostratigraphy in the basin,further analysis of seismic facies units is carried out to explore the relationship between different volcanic strata and seismic facies units,especially their relationships with reservoirs,in order to obtain some universal laws to provide basis for oil and gas exploration.The boundary of volcanostratigraphy can be divided into eruptive conformity boundary(ECB),eruptive unconformity boundary(EUB)and eruptive interval unconformity boundary(EIUB).There are no long-term weathered surfaces or sedimentary interlayers and other sedimentary discontinuities at ECB and EUB.The upper and lower strata occurrences of the ECB are coordinate and can be identified on the cores,While the upper and lower strata occurrences of the EUB are incoordinate and can be identified on the cores and logging curves.Due to the discontinuities such as weathering surface or sedimentary interlayer on the boundary,the EIUB is easier to identify in core,logging curves and seismic profiles.The study of volcanic stratigraphic boundary is the basis for identifying volcanic strata and characterizing seismic facies units,as well as for decomposing and describing volcanogenic successions.Volcanic stratigraphic units refer to the volcanic strata that erupts along the same volcano,has a certain shape and geometric internal architecture and restricted by the stratigraphic boundary.It is the smallest unit in the volcanic strata and can be divided into simple lava flow,braided lava flow,lava dome,pyroclastic flows,base surge,and re-worked volcanic clastics.The top surface slope of volcanic rock is analyzed through the structural map of the Yingcheng Formation and the average slope gradient of the volcanic rock surface is calculated to describe the volcanic strata through the seismic data.Three seismic facies units can be divided,namely mound shaped,plate shaped and plate/mound shaped.Drilling revealed that there are differcences in the volcanic stratigraphic units filled by seismic facies units.Mound shaped seismic facies units are dominated by lava flow,of which lava domes are the main ones;plate shaped seismic facies units are also dominated by lava flow,of which simple lava flows have a higher proportion;the plate/mound shaped seismic facies units are mainly composed of lava flow and pyroclasts,of which braided lava flows and base surge volcanic stratigraphic units are the most,followed by pyroclastic flows.The geological interpretation of volcanogenic successions shows that the characteristics of the seismic facies units are the external manifestations of volcanism and the accumulation of volcanic strata,which are related to the nature of magma,the way of volcanic eruption,and the topography.Lava domes with low porosity are the main component of the mound shaped seismic facies unit,which formed by magma with low volatile content and high viscosity.Due to the poor fluidity of these magmas during the ejection process,they are prone to form relatively stratigraphic units with large aspect ratio,which can form mound-shaped seismic facies units through the superimposition of volcanoes;the plate shaped seismic facies units are dominated by simple lava flows,and its lithology is mostly low-viscosity,low-volatile basic lava.With good fluidity during the flow process,the thickness of the lava along the flow direction changes little,prone to form plate-shaped seismic facies units owing to the superimposition of stratigraphic units;in the plate/mound shaped seismic facies,the lava flow and the pyroclasts are mainly superimposed.Reservoir distribution is affected by volcanic strata.The top boundary of volcanic strata controls weathered reservoirs,which can develop primary and secondary reseviors space;volcanic stratigraphic units influence the reservoirs inside the volcano,and most of them develop primary reservoir space.Favorable reservoir distribution is different among volcanic stratigraphic units.Simple and braided lava flow can be divided into upper porosity zone,middle tight zone and lower tubular porosity zone in the vertical direction,resulting in reservoirs in upper part are better than in the lower part of them.The upper reservoir of the lava dome is better than the lower reservoir as a whole,but overall porosity of the reservoir is low.On the whole,as the distance from the boundary of volcanostratigraphy increases,the porosity of the reservoir in lava flow decreases,indicating that its reservoir distribution is closely related to the boundary.On the contrary,the reservoir development of base surge and pyroclasic flow is less affected by the boundary,and any part of the the volcanic stratigraphic unit can be the favorable reservoir,which is more affected by primary pores,especially primary fractures.Reservoir distribution are also not the same among different seismic facies units.The probability of favorable reservoirs decreases in sequence of plate/mound shaped,mound shaped,and plate shaped seismic facies units.Plate/mound shaped and mound shaped seismic facies units have higher oil and gas accumulation efficiency.Taking the Qianshenzijing subsag in the Changling Depression of the Songliao Basin as an example,based on the volcanic stratigraphic unit and the volcanic stratigraphic seismic facies unit,under the constraints of the reservoir geological model,the volcanic edifice,the volcanic stratigraphic unit and the seismic facies unit are identified by the coherence body,the dip and the waveform analysis attributes,making them possible for plane prediction.Finally,a comprehensive multi-attribute was used to predict the favorable reservoirs,suggesting that the favorable reservoirs are mostly concentrated in the central and southern regions,which can be the next target for oil and gas exploration.