Detection and Evaluation of the In-Situ Natural Gas Hydrates in the North Slope Region, Alaska

Abstract

ABSTRACT Natural gas hydrates are snow-like mixtures of natural gas and water, in which gas molecules are trapped within the crystalline structure of frozen water. Stable at temperatures well above the freezing point of water, in-situ natural gas hydrates are found in permafrost regions and beneath the sea in outer continental margins and ocean basins. In the North Slope region of Alaska, pressure and temperature conditions suitable for formation of in-situ natural gas hydrates are widespread. The in-situ natural gas hydrates represent a potential drilling hazzard, and as such their detection is of vital interest in regions where they occur. In addition, they may in the future provide a valuable resource for natural gas. The purpose of this study is to develop techniques for the detection and evaluation of gas hydrates from well log data and determine possible geologic controls on the occurrence of hydrates. The high density of oil and gas drilling in Prudhoe Bay provides a broad data base with over five-hundred completed wells. Logs used include the dual induction laterolog, borehole compensated sonic, self potential, gamma ray, caliper, neutron-activation, mud and temperature log. Interpretation of these logs is not straightforward, and often the zones of potential hydrate occurrence are not logged or the quality of the logs may be poor. Another problem in the evaluation of hydrates from well log data is the lack of prior quantitative work. For this study, several new methods of evaluation were developed and incorporated with existing techniques. One-hundred and twenty-five wells were examined for potential hydrate occurrence, with 102 definite occurrences in 32 different wells. The resistivity of the hydrate ranged from 50 ohm meters to 1,000 ohm meters and the sonic logs indicated acoustic velocities ranging from 3.1 km/s to 4.4 km/s. For each well examined, the geothermal gradient was determined and the theoretical stability zone for methane hydrate was calculated. The detectable hydrate occurences were corelated with recognizable geological units. A structural-stratigraphic framework consisting of thirty-two key beds picked from gamma ray logs has been established to a depth of 1,000 meters. These units are characterized by alternating layers of gravel and silt with a gentle dip to the northeast. There is strong indication of three distinct coarsening-upward sequences, which may represent several deltaic centers prograding to the northeast. The distribution of gas hydrates appears to be related to the lithology, with several non-porous silt units acting as possible cap rocks for hydrate saturated gravels. The high occurrence of hydrate in the Kuparuk region, with a southwest to northeast trending structure, suggests a possible migration of free gas from lower units along the bedding plane into the zone of hydrate stability. In several wells, the occurrence of coal appears to be directly related to hydrate occurrence, representing a possible source relation for the methane needed for hydrate development.