Lithofacies characteristics of volcanic gas reservoirs and their influence on reservoir physical properties:a case study of Member 1 of Cretaceous Yingcheng Formation in Xudong area of the Xushen gas field

Taking the volcanic reservoir of Member 1 of Yingcheng Formation in Xudong area of the Xushen gas field as an example,the authors studied lithofacies characteristics of the volcanic gas reservoir and their influences on reservoir physical properties with the purpose of effectively forecasting and exploiting the volcanic reservoir.On the basis of identification of volcanic rock types and recognition of volcanic rock bodies by combining geological data with logging data and seismic data,the authors analyzed volcanic lithofacies characteristics and their relationship with reservoir physical properties and forecast the favorable prospecting areas by using data from wells,sections and plane lithofacies.The results indicate that the volcanic reservoir of the Member 1 of Yingcheng Formation in Xudong area of the Xushen gas field was formed by repeated volcanic eruptions,with the lithologies consisting of 10 types.The rhyolite,rhyolite tuff and sedimentary volcanic breccia played the preponderant role.The eruption type in the study area was cranny-center eruption,and crater and volcanic lithofacies were obviously controlled by ruptures.The volcanic bodies were dominated by craters,and different volcanic lithofacies were developed in different positions of volcanic bodies.The volcanic facies inside the volcanic bodies in the study area can be divided into five types and sixteen sub-facies.The results indicate that the volcanic lithology and its combination constitute the basis of the division of volcanic facies,and the spreading of volcanic facies controls the distribution of gas reservoirs.The reservoir development areas include the lower part and upper part and top overflow sub-facies of overflow facies,splash down sub-facies and air falling sub-facies,middle extrusive sub-facies,exterior sub-facies of extrusive facies and volcanic neck sub-facies of volcanic channels facies.The sub-facies of the volcanic reservoir include volcanic sub-lithfacies of volcanic neck sub-facies,dikes,sills,hydrothermal breccias,inner extrusive sub-facies,middle extrusive sub-facies,exterior sub-facies,splash down sub-facies,pyroclastic flow deposit,base surge deposit,air falling sub-facies,top part overflow facies,upper part overflow facies,middle part overflow facies,lower part overflow facies,epiclasts-bearing volcanogenic sediments and reworked volcanogenic sediments.The explosive facies and overflow facies are most developed facies,and the physical property of the volcanic sub-facies of splash down is the best.The average effective porosity is 9.00% and the average whole permeability is 4.793 mD.Volcanic lithology and its combination form the basis of the division of volcanic lithofacies.The distribution of volcanic lithofacies control the development characteristics of volcanic reservoirs.The reservoir development areas are lower part and upper part and top overflow sub-facies of overflow facies,splash down sub-facies,air falling sub-facies,middle extrusive sub-facie,exterior sub-facies of extrusive facies and volcanic neck sub-facies of volcanic channels facies.