Multi-Dimensional Evaluating Condensate Gas Reservoir in a Fractured Igneous Basement, Offshore South China

Reservoir quality is normally attributed to the matrix porosity and permeability for conventional clastic reservoir. Differently, basement reservoir quality is determined by more factors, including weathering, lithologies, diagenesis, and fracture network due to very low matrix porosity and permeability. The recently discovered H buried hill offshore South China is a proven condensate gas reserves over 50 billion cubic meters in fractured igneous basement. The complex and heterogenous lithology and minerals due to multiple phases of magmatic activity along faults leads to unevenly developed fractures with unknown fillings and openness. Mapping good reservoir and characterizing effective fracture distribution across the area is crucial to field development strategy and recovery enhancement. Lithologies were firstly classified based on drilling, logging and geophysical data. ROP, gamma ray, bulk density, neutron, Poisson ratio, Young's Modulus and Lambda et al were cross plotted to identify basement lithologies as: granite, diorite, basalt, and volcanic breccia. Porosity and permeability are analyzed based on core analysis and formation sampling, showing that porosity ranges from 0.5% to 17.1% and permeability ranges from 0.019mD to 64.3mD. Then, dynamic fracture analysis was done by in Mohr-Coulomb to geomechanically verify whether fractures are critically stressed or stable. The results indicate that fractures with dipping angle great than 58 deg and dip azimuth from 195-225/15-30 deg, and 75-105/255-285 deg are critically stressed. The overlay of effective fracture and critically stressed fracture is the best fractures contributing to reservoir quality. The production controlling factors are considered as the effective fracture development, critically stressed fractures, and matrix porosity, and secondary (vug) porosity. The best reservoir is developed near to the top weathered fracture zone with more granite, and the better reservoir holds diorite with higher effective fracture developed near open faults. The multi-dimensional evaluation of reservoir quality using geology, geophysics and geomechanics provides a solid reference for subsequent well deployment and ensure the efficient development of fractured igneous basement reservoir. The initial production of three development wells shows good correlation with reservoir quality evaluation results and validated the overall workflow.