Physical Parameters Controlling Gas Hydrate Stability and Distribution on the North Slope, Alaska

Timothy S. Collett

The natural gas hydrate stability field and its controlling factors on the North Slope have been evaluated as part of the U.S. Geological Survey-U.S. Department of Energy worldwide gas hydrate assessment program. Gas hydrates exist under relatively limited conditions of pressure and temperature, the hydrate stability field, which must be evaluated along with other factors prior to volumetric assessment. Our North Slope studies suggest that the gas hydrate stability field is controlled primarily by subsurface temperatures and gas chemistry. Other factors, such as pore-pressure variations, pore-fluid salinity, and reservoir-rock grain size, appear to have little effect on the gas hydrate stability field.

Data necessary to determine the distribution of the gas hydrate stability field are difficult to obtain. Subsurface-temperature data come from 46 high-resolution equilibrated well-bore surveys and from estimates based on identification of the base of ice-bearing permafrost in 98 other wells. Geothermal gradients in ice-bearing permafrost range from 1.55°C/100 m in the East Bay State-1 well to 4.46°C/100 m in Fish Creek-1. Methane is the dominant species of gas in the near-surface (0-1500 m) sediment, on the basis of mud log gas-chromatograph data. However, because nitrogen was detected in several analyses of recovered gas hydrate and drill cuttings, several different gas mixtures other than pure methane were assumed in the stability calculations. Formation water samples and ell log calculations indicate low pore-fluid salinities, ranging from 0.5 to 18.1 ppt. The maximum recorded salinity would suppress methane hydrate stability temperatures by approximately 1.0°C. Pressure data, obtained from drill-stem testing and well log calculations, indicate a hydrostatic pore-pressure gradient (9.795 kPa/m or 0.433 psi/ft) within the near-surface sediments, thus having no abnormal effect on gas hydrate stability. The effect of sediment grain size on gas hydrate stability is very difficult to evaluate. In the near-surface coarse sand and gravel of the Prudhoe Bay area, grain size has little or no effect. However, in the more clay-rich sedimentary rocks near Harrison Bay, gas hydrate stability conditions are depressed by several degrees Celsius.

Our analysis indicates that the methane hydrate stability field is areally extensive beneath the coastal plain province; thicknesses greater than 1,000 m have been calculated in the Prudhoe Bay area. Limiting thermal conditions, however, preclude the occurrence of gas hydrates in the north-central part of the National Petroleum Reserve in Alaska and in the foothills east of Umiat.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.