Risk-Weighted Volumetric Analysis to Estimate Gas Reserves and Evaluate Coalbed Methane Potential of the Lower Matanuska Valley, Cook Inlet Basin, Alaska

By

D.O. Ogbe, J.B. Packer (University of Alaska, Fairbanks), and J.G. Clough (Alaska Division of Geological & Geophysical Surveys)

Cook Inlet Basin fill exceeds 25,000 feet of Tertiary sediments with the highest coalbed methane potential concentrated in the Oligocene to Miocene Tyonek and Paleocene Chickaloon formations. A risk-based methodology for estimating the gas reserves in coal beds was developed to evaluate the coalbed methane potential of a 75,000- acre unit located in the lower Matanuska Valley at the northern end of the Cook Inlet Basin.

In 1994, the first exploratory coalbed methane well (AK94-CBM- 1) was drilled by the Alaska Department of Natural Resources in the Matanuska Valley to collect information on coal seam thickness and methane gas content. Based upon limited data obtained from this well, a volumetric method was utilized to estimate the initial gas content and water in place. Probability distributions of variables that include coal seam thickness, areal extent, initial connate water saturation, effective porosity, initial adsorbed gas content, and formation volume factors were estimated using correlations developed from coal rank and depth data. A risk-based Monte Carlo Simulation was utilized to calculate gas reserves and coalbed methane potential.

Our results indicate that the lower Matanuska Valley 75,000-acre unit contains approximately 1.8 to 2.7 trillion cubic feet (tcf) of initial gas in place. Risk-weighted reserve estimates range from 1 to 1.8 tcf of gas that could be recovered from the unit. Our research approach indicates that abundant coalbed methane reserves are likely present within the study unit. This methodology can be applied to determine the coalbed methane reserves in other areas of the Cook Inlet Basin.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.