Formation Mechanism And Distribution Of The Mesozoic Volcanic Reservoir In Bozhong Sag

In recent years,with the deep exploration,significant breakthroughs have been made in the exploration of buried hill reservoirs of the Mesozoic and Paleozoic in Bohai Bay Basin.Bozhong Sag is an important hydrocarbon-rich center in the eastern part of Bohai Bay Basin.The Mesozoic volcanic rocks are widely distributed and thick in the structural belt surrounding the Bozhong Sag.The exploration for buried hills will inevitably drill into Mesozoic volcanic formations.However,the exploration degree of Mesozoic volcanic rocks in the Bohai Sea is relatively low.The geological and geophysical characteristics of the Mesozoic volcanic rocks in the Bozhong Depression should be studied,for characterizing the distribution of lithology and facies,clarifying the characteristics of volcanic reservoirs and studying reservoir formation mechanism.On the basis of the above the main controlling factors of volcanic reservoirs should be analyzed and favorable target prediction will be achieved.In this paper,the distribution of Mesozoic volcanic rocks in Bozhong Sag is described on the basis of drilling data,and combining with the geological and geophysical characteristics of volcanic rocks.The systematically study of the volcanic reservoir space type,diagenesis,reservoir characteristics and reservoir formation mechanism in the study area has been carried out systematically through the techniques of casting thin section,scanning electron microscopy,surface porosity analysis,gas porosity,X diffraction,major element analysis,nitrogen adsorption,high-pressure mercury injection and nuclear magnetic resonance,and a series of findings has been obtained as follows.1.Volcanic lithology,facies geological-geophysical characteristics and volcanic architecture development patterns.Completed the planar distribution of Mesozoic volcanic rocks in Bozhong Sag for the first time.The Mesozoic volcanic rocks in Bozhong Sag are rich in rock types,ranging from mafic to acidic,and from lava to volcanic pyroclastic rocks.The lithological identification was completed combining drilling core description,microscopic thin section analysis and logging interpretation,and the geological-logging interpretation plates of different volcanic facies were established.Combining with seismic reflect characteristics,6 facies and 10 subfacies seismic identification and geological models were obtained.On this basis,5 types of volcanic edifices were identified,including acidic dome volcanic edifice,intermediate-acidic mound volcanic edifice,intermediate-acidic strato volcanic edifice,mafic shield volcanic edifice and clastic wedge volcanic edifice.The Mesozoic volcanic rocks in Bozhong Sag can be divided into three eruptive cycles,which generally have a basic-intermediate-acidic evolutionary feature.The Mesozoic discontinuous volcanic geological framwork of Bozhong Sag was interpreted by the fine depiction of the seismic profiles of the connecting wells.2.Volcanic reservoir space types and diagenesesThe reservoir space is dominated by pores,dissolution pores and fractures,which can be divided into primary space and secondary space according to the period of digenesis.Primary reservoir spaces include primary pores,amygdales,blast fractures,cleavage cracks and shrinkage fissures,and secondary reservoir spaces include matrix dissolution pores,intracrystalline dissolution pores,devitrification pores,cryptoexplosive juice dissolution pores,clay intergranular pores,tectonic fissures and dissolution fissures.The volcanic diageneses in the study area are divided into early diageneses and late diageneses.The early diageneses includes volatile dispersal,explosion,condensation,shrinkage,and devitrification,while the late diagenese includes filling,dissolution,alteration,tectonics,and compaction.3.Physical characteristics and mineral compositions of volcanic reservoirsThe physical properties of the volcanic reservoirs from good to bad are trachyte and trachytic cryptoexplosive breccia,rhyolite and rhyolitic cryptoexplosive breccia,dacite,basalt,and tuff,among which the effective reservoirs are all developed in the intermediate-acidic volcanic rock section.The alkaline feldspar content is higher in trachytic volcanic rocks and rhyolitic volcanic rocks.While the clay content is generally much higher in dacite,basalt and tuff.Through the comparison of the whole rock analysis and the physical properties test results,it can be seen that feldspar content is positively correlated with porosity and permeability,while quartz and clay mineral content are on the contrary.4.Quantitative characterization of pore structure and reservoir formation mechanism of volcanic reservoirsIn this study,the pore structure of volcanic reservoirs was characterized by the pore size obtained from nitrogen adsorption,high-pressure mercury injection and nuclear magnetic resonance(NMR).The NMR T₂ spectrum was converted into pore size distribution,by comparing the peak of pore size distribution of high-pressure mercury experiment with the NMR T₂ spectr?m,and combined with the pore size distribution of nitrogen adsorption method,the micro pore structure quantitative characterization of volcanic rocks was achieved.Initially,the pore size distribution of volcanic rocks in the study area can be divided into three intervals with the boundaries of 50 nm and 1?m,corresponding to micropores(clay intergranular pores and devitrification pores),mesopores(dissolution pores),and macropores(fractures and intra-amygdaloidal pores),and the storage capacity of volcanic rocks in the study area is determined by mesopores.Based on the resolution mechanism,combined with the factors of storage space types,diagenesis and pore structures of different lithologies,and the storage formation mechanism and pore structures of basalt,dacite,trachytoid volcanic rocks,rhyolitic volcanic rock and tuff are explored respectively,in which dacite and trachytoid volcanic rocks are rich in alkali feldspar and dominated by dissolution pores,while basalt and tuff are influenced by clay minerals,dominated by micropores,and mesopores and macropores are locally developed.Based on the analysis of different lithological reservoirs formation mechanism,and combined with the geological characteristics of different types of volcanic architecture,five types of high-quality reservoir geological models have been established.5.Main controlling factors and distribution characteristics of volcanic reservoirBy comparing the production capacity,physical properties and reservoir space types,it is found that the main controlling factors of volcanic reservoirs in the study area are volcanic stratigraphic cycle interface,volcanic lithology,volcanic facies and volcanic edifice.On the whole,the effective volcanic reservoirs are all developed near the third cycle interface,and the favorable lithologies are dacite,rhyolites and rhyolitic cryptoexplosive breccia,trachyte and trachytic cryptoexplosive breccia.The favorable volcanic facies are mainly intermediate-acid overflow facies,volcanic vent faices and intrusive facies.The proximal zone of the volcanic edifice is favorable target area,followed by the medial zone,and the distal zone is the worst.On this basis,combining seismic profile analysis and the seismic attribute methods such as root mean square amplitude,waveform classification,and coherent body to study the planar spreading characteristics of volcanic bodies,volcanic facies and fractures in the study area,and classify the volcanic reservoirs into four types according to the above factors.Finally it is determined that the high-quality volcanic reservoirs are mainly developed near the fault zones.