The Volcanic Rock Reservoirs Classification Evaluation Of Yingcheng Formation In The Songnan Gas Field,changling Fault Depression, Songliao Basin

With the rapid development of the global economy, which results in a large demandfor oil and gas resources, regular sedimentary rock reserves of oil and gas resources soonfailed to meet the needs of people, so that it has been very necessary to look for a newkind of oil and gas reservoir. Accompanied by more and more large volcanic rockreservoirs are constantly discovered, the volcanic rock reservoir has become a new fieldof global oil and gas exploration now.The Songnan gas field is a volcanic gas reservoir in Cretaceous Yingcheng Fm,which develops in Yaoyingtai deeply faulted nose structure of Laoyingtai-Darhan faultedsalient which lays in the center of Changling fault depression, Songliao basin.Based on the results of previous studies on Yingcheng Fm volcanic reservoirs ofSongnan gas field in Changling fault depression, Songliao Basin, we studied the drilling,well logging and testing and analytical data thoroughly in study area, so that we couldstudy lithology, lithofacies, reservoir properties, and belts of volcanic mechanismrespectively, and then analyze to determine the affecting factors of the volcanic reservoirquality in the study area. Finally, we established the volcanic reservoirs evaluation criteriain Songnan gas field, and finished the classification evaluation of volcanic reservoirs inthe study area.1,Lithology and lithofacies characteristics of volcanic rock in study areaLithology: The lithology determines the " inherent advantages " of the volcanicreservoirs, as a result, the reservoir properties of lava are better than those of pyroclasticrocks and pyroclastic lava in Songnan gas field. Primary porosity in fumarolic rhyolite ofLava is well developed, and if communicated by primary fractures, tectonic fracturesand dissolution pore in matrix, its permeability would also be best; Pellets rhyolite is oftenassociated with fumarolic rhyolite in Songnan gas field, as glassy in matrix could improveits porosity under devitrification, it is also of good reservoir properties; massive lava isdense, even if modified later,the increases of its porosity and permeability are still limited.Pyroclastic lava has some primary pores, and if modified by dissolution, can form moresecondary porosity, resulting in favorable reservoirs. Compared with tuff lava,the increase of porosity in tuff is less after being modified. Therefore, the lithology can beused as one of the fundamental factors that reflect quality of the volcanic reservoirs.Lithofacies: The physical characteristics and reservoir space types of the volcanicreservoirs are mainly controlled by the volcanic subfacies. Because there are bigdifferences in texture and structure between various subfacies, and the texture andstructure can affect the development degree of pores and fractures, therefore differenttypes of volcanic rock subfacies have different reservoir properties. Volcanic rockreservoir properties of different subfacies and of the same lithoface are very different fromeach other. By analyzing the relationships of volcanic rock lithofacies and reservoirproperties in Songnan gas field,we discovered that the upper subfacies, the pyroclasticflow subfacies and volcanic neck subfacies are three favorable subfacies developed inSongnan gas field. So lithofacies is also one of the key factors determining the volcanicreservoir development.2Influencing factors of the volcanic reservoir quality in study areaDetermining the Influencing factors of the volcanic reservoir quality: first, weanalyzed the relationship between lithology and reservoir quality and the relationshipbetween lithofacies and reservoir quality in study area. And by making a statisticalanalysis of the logging and core data etc, we determined the Influencing factors of thevolcanic reservoir quality, those are lithology, lithofacies, favorable lithofacies' thickness,porosity, permeability, facies zone of volcanic edifice and reservoir types.Favorable lithofacies' thickness: The thickness of the favorable volcanic lithofaciesdirectly determine the thickness of the regional reservoir, and the thicker, the better.Porosity: A important parameter reflecting the volcanic rock reservoir quality. Bymaking a statistical analysis of the porosity data in the study area,it suggest that theporosity of volcanic rock has a linear relationship with its permeability in Songnan gasfield. As the porosity increases, the corresponding permeability also increases, and highporosity layers often correspond to the high-yield gas reservoirs.Permeability: The permeability is also one of the important parameters reflecting thevolcanic rock reservoir,and has a strong correlation with its corresponding porosity, inother words, high permeability corresponds to higher porosity. However, as thedevelopment of exploiting technology, permeability could be improved by fracturingtechnology.Reservoir space types: Volcanic reservoir space can be roughly divided into the pores,the pores-fractures and fractures in the study area. the permeability of fractured volcanic rock reservoir may be high, but the area of such reservoir is limited, and only developedlocally in the study area. Compared with fractured reservoir, the others are much better inreservoir quality and stability. Reservoir space type should be considered in the reservoirclassification evaluation in Songnan gas field.The volcanic edifice belts: The reservoir quality of different parts of volcanicmechanism is very different from each ather in Songnan gas field. Crater-near craterfacies are best in reservoir properties, proximal facies next, distal facies are the worst.Therefore, the development degree of the volcanic reservoirs in the the study area iscontrolled by the volcanic mechanism belt, thus the volcanic edifice belt can be used as aparameter in the classification evaluation of the volcanic rock reservoir.3,Classification and evaluation method of volcanic reservoirs and prediction ofreservoir plane distribution in the study areaFirst,we selected the key parameters for volcanic rock reservoir evaluation, andscored each reservoir,then, by analyzing the effect of each parameter to reservoirs, wedetermined the ratio of each parameter, and get a comprehensive score for each reservoirat last. By these steps,we initially established the technical route for classification andevaluation of volcanic rock reservoirs in study area. We take a cycle as a unit forclassification and evaluation of reservoirs in a single well, and then based on the results,classify and evaluate the regional reservoir under the control of plane lithofacies. Theresults show that as controlled by different cycle and different volcanic edifice, thedistribution of class1reservoir is wider in cycle three, and the reservoirs in cycle four aremainly class2and3. The class1reservoir lays mainly in the center of volcanic edifice.