Bimodal Volcanism And Its 3D Micro-Scale Pore System From The First Member Of Yingcheng Formation,Changling Depression,Songliao Basin-Implications For Hydrocarbon Reservoir

Due to its huge potential,volcanic rock reservoirs have attracted worldwide attention in recent years,and the research on them are increasingly abundant.In fact,the pore system within volcanic rocks can be divided into three development stages:the first is the primary stage,i.e,the stage of magma ascending and eruption process;the second is arerial condensation and weathering leaching;another is the burial transformation and modification stage.The latter two are also called secondary stage.At present,researches on the pore space type,physical properties,and the main controlling factors of volcanic reservoirs are relatively mature.However,very few studies appear on completed evolution of the pore system in volcanic rocks,especially the related research with the primary stage.Also,quantitative evaluation of burial modification is lacked due to unclear primary pore system.Known as huge volcanic gas potential,the seismic data,drilling data,and logging data are continuously increasing in the Changling fault depression in the Songliao Basin in recent years,and a detailed volcanic stratigraphic framework has also been established.This provides an ideal window for revealing complete evolution of pore system,from primary stage to burial modification stage,within a single volcanic eruption.Based on the volcanic stratigraphic framework already established in the Chanling area,this study supplements seismic,drilling,logging,and U-Pb zircon dating data,with more previous dating data collected.They further clarify the sequence of volcanic eruption cycles and their age range as the Huoshiling eruptive cycle（122-118Ma）,the eruptive cycle of the first member of Yingcheng Formation（115-106Ma）,and the eruptive cycle of the third member of Yingcheng Formation（105-102Ma）.The middle volcanic eruption cycle,i.e.,the first member of Yingcheng Formation（K₁yc¹）,is selected as the object,where huge volcanic gas resources are located.Detailed petrology,mineralogy,geochemistry and Sr-Nd-Pb isotope analysis are carried out,revealing the mantle source,magma evolution,petrogenesis,and eruptive assemble and background of this volcanic cycle.In addition,7 typical samples are chosed for X-ray CT scanning experiments,and Avizo software is used to perform 3D quantitative reconstruction of the micron-scale pore system,including its pore shape,pore size distribution,porosity,connectivity and etc.Connected with the volcanism,the Conflow software is introduced to simulate the degassing behavior of the different magmas from the first member of Yingcheng Formation,revealing the mechanism of the vesicles’development.Then,the secondary modifications of the pore system are explored in order to reveal its complete evolution.We find that the K₁yc¹cycle（115-106 Ma）,belonging to the Early Cretaceous,is mainly composed of trachyte basalt,rhyolite,and rhyolitic pyroclastic rocks,with lacking andesitic components,a typical bimodal distribution.The basic volcanic rocks are characterized by high Si O₂,low Mg O,Ni and Cr,with obvious fractional crystallization but not contamination by crustal material.They are enriched in light rare earth elements（LREE）,large-ion lithophile elements and high field strength elements（i.e.,Nb,Ta）,U-Th slightly depleted.And the Sr-Nd-Pb isotope show similar characteristics to OIB-type mantle source.Compared with the low Nb basalts in South China with a similar geodynamic background,it can be concluded that the source of the mantle is asthenospheric mantle enriched by subduction related fluids.Some other trace element characteristics including various La/Sm,low La/Yb and moderate Dy/Yb,indicate that it is derived from the transition zone from a garnet-bearing mantle to a spinel-bearing one.The rhyolitic members show obvious crystallization of the plagioclase,Fe-Ti oxide and apatite,and exhibit the uniform Nd and Pb isotopic characteristics with the basic ones.The basic sample SN185-2111 was selected to perform the silumation of fractional crystallization in Pele software.The results show that the rhyolitic rocks originated from crystallization of basic magma in the same eruptive cycle,with different degrees of contamination of the upper crust.Through comparing the K₁yc¹cycle with other two eruptive cycles during rifting period,i.e.the Huoshiling Formation and the third member of the Yingcheng Formaiton,it is found that the K₁yc¹cycle is a typical alkaline series and bimodal volcanic eruption.During this period,the deep mantle-derived basic volcanic rocks are most widely distributed,of which develop along faults.These signatures indicate that obvious thinning of the lithosphere,the mian stage of basin rifting,the deep major faults acting.The simulating results from Conflow software indicate that different magmas’degassing behavior in volcanic concuits are highly distinguished and are controlled by different factors.Combined with the research of previous projects,the basic lavas from this cycle in the Changling area mostly go up through deep major faults as volcanic channels.The degassing behavoir of magma in the volcanic conduits is mainly controlled by water content and pressure.The higher the water content and the lower the initial pressure,the higher the degassing efficiency but with gradual changes.However,the temperature has little effect.The basic magma comes from the asthenospheric mantle enriched by obvious subducted fluids,making it hydrous.In addition,the tectonic setting is obvious lithospheric thining and main basin rifting,which can provide the low initial pressure and thus favorable conditions for the extensive vesicles’development within the basic rocks in the study area.Different from the degassing behavoir of basic magma,the rhyolitic magma in much smaller conduits is controlled by temperature and water content,moreover slight changes in these factors will make the degassing behavior change largely,indicating its unstable character.Especially,the reduction of temperature or water content will make the degassing process complete quickly,where acidic pyroclastic rocks with explosiv phase are easy to erupt.However,explosive mode makes the vesicles hardly developed.These results show that vesicles in the rhyolite sample depends largely on the local nature of magma,and only the proper match of multiple factors can make rhyolitic lava with developed vesicles erupt to the surface.This is consistent with the fact that vesicles are widely developed in basic lavas and locally developed in acidic rocks in the study area.Connected the observation of cores,thin sections with the analysis of X-ray CT results,significant secondary modifications on 3D pore system are found,including mineral filling,dissolution,condensation and shrinkage,and tectonic effects.Among them,mineral filling mainly occurs in basic lava samples,in which the residual pores are mostly blade and prolate shape,with a size distribution of 0-500μm,and the filling pore shape tends toincrease in prolate and compact shape.Their size distribution is mainly750-1500μm,indicating that the later fluids preferentially precipitate in the larger primary pores and the smaller pores or fractures are less influenced.The filling pores’volume is as high as 12.85%,resulting in much smaller residual pore volume（0.23%-2.78%）,without connectivity.In acidic lavas and pyroclastic rocks,intracrystalline dissolution of feldspar mineral,condensation and tectonic processes are mainly found.The development of primary vesicles superimposed with dissolution and condensation shrinkage can make the sample porosity larger and interconnected,and the structural fractures greatly improve the connectivity of the pore system.The occurrence of different secondary diagenesis in basic rocks and acidic rocks largely depends on their unique petrological and mineral characteristics.The above-mentioned micron-scale pore system in the volcanic rocks in the Changling fault depression is significantly affected by both primary and secondary diagenesis,while the smaller-scale,nano-scale pore system in the volcanic rocks has significant differences in terms of pore type,origin,pore size distribution,and connectivity.The nano-scale pores experienced unique microscopic diagenesis including devitrification with general plate-slit and wedge-shaped structures.They are superimposed and accumulated on each other,making considerable porosity and connectivity.The cooperation or combination of micro-scale pore system and nano-scale pore system makes volcanic rock micro-pore system have hunge potential for oil and gas reservoir.