High-altitude well log evaluation of a permafrost gas hydrate reservoir in the Muli area of Qinghai, China.

The Muli area is the only permafrost region on the Chinese mainland wherein gas hydrates have been discovered. The gas hydrates are present in the fractures and pore spaces of the host rocks with a lamellar or micro-disseminated structure. By combining conventional and image logs, we describe the thickness of the permafrost layer and the well log responses of the gas hydrate reservoir, and calculate the porosity and gas hydrate saturation. We then analyze the advantages and disadvantages of different logging methods for evaluating gas hydrate reservoirs. Our results indicate that (1) gas hydrates are present below the permafrost in the Muli area, (2) gas hydrates predominantly occur in rock fractures, (3) the apparent resistivity is sensitive to gas hydrates present in pore spaces, and both apparent resistivity and acoustic logs are sensitive to gas hydrates present in fractures, (4) a density log is more appropriate for calculating porosity, and (5) gas hydrate saturation can be effectively calculated by the Archie equation, the modified Archie equation, and the Indonesian equation.