Integrated Reservoir Characterization and Effective Reservoir Identification by Advanced Logging Series for Complex Volcanic Gas Field – A Case Study From Songliao Basin, China

Abstract

Volcanic reservoir is difficult to evaluate due to complex lithology and strong heterogeneity. To better characterize the reservoir and identify good reservoirs, we combined elemental spectrum logging data, electrical image data, array sonic data, two-dimensional (2D) nuclear magnetic resonance (NMR) data, as well as dielectric permittivity logging data to carry out an integrated study including lithology identification, lithofacies analysis, fracture and fault analysis, effective porosity calculation, pore structure analysis, fluid identification, and optimized saturation calculation First, lithology and lithofacies were identified based on elemental spectrum logging data, electrical image data as well as core data. Second, beddings, fractures, and faults were identified and classified, and the borehole structure was analyzed. Third, effective porosity was obtained, and pore structure was analyzed with NMR data. Reservoirs with high porosity and good pore structure were identified. Finally, the fluid type was analyzed by combining 2D NMR logging data and dielectric permittivity logging data, and gas saturation was calculated; innovatively, gas saturation of the mud intrusion intervals was compensated. The implementation of this integrated method achieved big success in the study area. Lithology identified mainly includes tuff, breccia-bearing tuff, volcanic breccia, sedimentary tuff, sedimentary volcanic breccia, and sedimentary breccia-bearing tuff. And the minerals and lithology vary a lot from well to well. Volcanic lithofacies include volcanic explosive facies and volcano-sedimentary facies. Good reservoirs are mainly distributed in breccia-bearing tuff, tuff, and volcanic breccia of explosive facies, with high albite content, high effective porosity, and more big pores. There are a lot of microfaults and fractures developed in the reservoir. They have the same strike direction as the regional fault, which are quite important for hydrocarbon migration and the development of dissolution. The best reservoirs with free-fluid porosity higher than 3% were located at the fault-fracture developed zones. Fluid types were identified by integrating 2D NMR plots and dielectric permittivity logging data. For some intervals with very high free-fluid porosity, the inverted resistivity from dielectric permittivity data are much lower than the resistivity of conventional logs because of mud invasion. In this case, gas saturation was compensated by integrating 2D NMR data and array dielectric data. The best interval with the highest effective porosity and highest gas saturation was selected for the test, and the result from the test agreed with our evaluation very well. Conventional logs can hardly evaluate pore structures and connectivity for such complex reservoirs. The accuracy of gas saturation was greatly improved by combining 2D NMR data and array dielectric data, especially for mud invasion zones. The achievements from this study have dramatically improved the geological understanding of the reservoir and provided valuable information for the deployment of new wells.