Diagenesis And Reservoir Significance Of Pyroclastic Rocks Of Huoshiling Formation In Southern Songliao Basin

Volcanic reservoirs exploration in Songliao Basin has been carried out for more than 20 years and achieved good exploration and development results.Previous studies on volcanic reservoirs mainly focused on volcanic lava,which was considered that the evolution law of reservoir physical properties and reservoir space of pyroclastic rocks is similar to that of the normal sedimentary rocks.Many stages of pyroclastic rocks developed as the Mesozoic,early Cretaceous volcanic activities are frequent in Songliao Basin,among them,several drillings in Longfengshan area of Changling fault depression and Sujiatun area of Lishu fault depression in southern Songliao Basin had obtained gas,and the area had good accumulation potential for oil and gas with good geological conditions for further exploration.Based on typical sections of pyroclastic rocks of twelve wells from Huoshiling formation in the study area,with the data of drilling core observation,blue-dye thin section analysis,scanning electron microscope(SEM),porosity and permeability test,and energy spectrum analysis,the author described and characterized the pyroclastic reservoirs,identified the characteristics and laws of reservoir development in different lithologic facies zones,and concluded the development of regional high quality reservoir to studied the evolution law of reservoir physical property and reservoir space of pyroclastic rock,and provided reference basis for further oil and gas exploration and development.The results show that: The pyroclastic rocks of Huoshiling Formation in the study area mainly consist of seven lithologies,including pyroclastic rocks and submerged pyroclastic rocks,and two types of cementation are developed.There are11 types of reservoir space in pyroclastic rocks of Huoshiling Formation in the study area,and the existing effective reservoir space is mainly secondary reservoir space which mainly existed as the combination of reticular microfractures and dissolution micro-holes.There are 9 kinds of diagenesis in pyroclastic rocks of Huoshiling Formation in the study area,and the pore evolution of pyroclastic rocks mainly experienced four stage: early compaction consolidation ? fluid hydrothermal material filling? tectonic action? the dissolution of matrix and phenocrystals and the filling destroyed the primary pores,and the dissolution pores and structural fractures are formed by tectonics and dissolution.With the porosity and permeability test data,it was concluded that the reservoir performance of the pyroclastic reservoirs in the study area is volcanic breccia > volcanic breccia lava > tuff.The influencing factors of pyroclastic reservoir included pyroclastic grain size,cementation mode and diagenetic transformation.According to the relationship between porosity and permeability,pyroclastic rocks can be divided into three types: positive correlation,no correlation and V-shaped correlation,which meant that in the middle and low porosity interval,they were negatively correlated,while in the middle and high porosity interval,they were positively correlated.The porosity and permeability of volcanic breccia are mainly positively correlated,which meant that the permeability increased with the increase of porosity.This was mainly due to the role of intergranular pores.Because the intergranular pores not only contributed to the porosity,but also contributed to the permeability at the same time because of the connectivity between them.There is no correlation between the porosity and permeability of tuff,which show a phenomenon that the permeability was generally low and did not vary with porosity.The primary intergranular pores were difficult to be preserved during the compaction,and the increase of porosity was mainly due to the secondary dissolution pores produced by secondary dissolution.However,the clastic tuff particles are dominated by volcanic ash,and secondary dissolution was usually accompanied by clay mineral formation and local filling,resulting in the reduce of permeability,which meant that the porosity was increased duo to the secondary dissolution but the permeability didn't increase.The correlation between the porosity and permeability of volcanic breccia lava group was usually negative in the middle-low porosity range,and positive in the middle-high porosity range.The reservoir space of pyroclastic lava mainly included vesicles,intergranular pores,dissolution pores and fractures.The main reservoir spaces that contributed to porosity were vesicles,intergranular pores and dissolution pores,and the main reservoir spaces that contribute to permeability were vesicles with certain connectivity and fractures.Their porosity-permeability relationship and reservoir space development characteristics can be divided into three end-member cases:(1)low porosity-high permeability: the primary vesicles were not developed and,dissolution was less developed,while fractures increased permeability;(2)mid porosity-low permeability: The filling resulted in the decrease of primary porosity,the increase of dissolution pores and the non-development of fractures;(3)high porosity-high permeability: the filling effect was weak while the connectivity of vesicles was good.