Unravelling Heterogeneous and Fractured Igneous Basement Reservoir Quality by Multidisciplinary Integration, Offshore South China

Reservoir quality is normally attributed to the matrix porosity and permeability for conventional clastic reservoir. Differently, igneous 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 lithology and minerals lead 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 characterized based on sidewall cores, borehole image log and thin sections. Abundant geological features including lithology structure, minerals, and micro-fractures are described in detail. Conventional logs and elemental mud logs are used to identify granite and diorite. Petrophysical properties 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. The cross plot of different lithologies shows that granite holds the best porosity and permeability, the secondary is diorite, and then diabase. Regional fracture prediction was based on seismic RMS and lithology thin slicing, structure high strongly promotes high density fracture development. LWD ultrasonic image was deployed for fracture identification, three types of effective fractures and two types of closed fractures are identified, which highly reduces the uncertainty for the fracture identification by micro-resistivity image. The production controlling factors are considered as the effective fracture development, and matrix porosity, the included angle between the effective fracture strike and maximum horizontal in-situ stress (S1). The best reservoir is developed near to the top weatheredfracture zone with more granite, and the better reservoir holds diorite with higher effective fracture developed near open faults. The multidisciplinaryintegration 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.