Reservoir Characterization of the Beluga Formation In South Ninilchik Gas Field, Cook Inlet, Alaska

The low gas-pressure Beluga formation in the South Ninilchik gas field within the Cook Inlet basin is considered an under-developed prospect in a complex geologic setting. The target sands in this stacked fluvial braided stream deposit are laterally discontinuous, mixed with volcanic materials and interbedded with thin coal streaks. This high degree of stratigraphic variability, complex mineralogy, and low gas pressure which affects porosity determination and presents difficulties in permeability estimation, create a challenging task for geological and petrophysical interpretation of this unconventional gas play. Logging-while-drilling (LWD) evaluation included azimuthal density images, nuclear magnetic resonance (NMR), neutron, density and acoustic logs. The azimuthal density image log provided detailed visualization of the structure of this fluvial system and was used along with the nuclear and NMR logs to identify the different lithological units within the Beluga formation. Porosity, permeability and gas saturation were computed based on the NMR and density porosity. NMR data was also used to determine grain size distribution. Five lithological units have been identified and at least three different grain size distributions were described for the Beluga formation. The permeability and saturation values computed from the NMR data enhanced the prediction capability for the production intervals. The relationship between the LWD logs and the identified lithological units, grain size distribution and pay zones show an excellent correlation to the producing intervals verified by the percentage of gas flow per zone observed on the production log. This integrated approach in characterizing the Beluga formation utilizing LWD logs, lithological units and grain size determination verified with production data, provided an improved characterization of this unconventional reservoir compared with traditional logs. The implications of this study on the future optimal reservoir drainage strategy are significant, as the results provide a framework for understanding future well evaluation within the reservoir.