Characterization of a Low Gas Pressure Reservoir: Beluga Formation, South Ninilchik Gas Field, Cook Inlet, Alaska
The Beluga formation is considered an under-developed prospect located in the South Ninilchik gas field within the Cook Inlet basin. 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, coupled with low gas pressure affecting porosity and permeability quantification, and complex mineralogy creates 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 to identify the different depositional facies within the Beluga formation. Porosity, permeability and gas saturation were computed based on the NMR and density porosity. NMR data was also used to delineate grain size distribution.
Five depositional facies have been identified and at least three different grain size distributions have been modeled for the Beluga formation. Due to low-pressure gas affecting the hydrogen index, pay and high-permeability zones were identified using the crossover of density and NMR porosity, similar to density and neutron cross-over, but is more reliable because NMR porosity is independent of mineralogy. 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 depositional facies, grain size distribution and pay zones show a direct 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, depositional facies and grain size modeling proven with production data, provided a better characterization of this unconventional reservoir. 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.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California