Use of Seismic Attributes in Identifying and Interpreting Onshore Gas-Hydrate Occurrences, North Slope, Alaska

Gas hydrates occur in shallow subsurface environments where both water and natural gasses are present within low-temperature and moderate-pressure regimes. Although much hydrate research focuses on gas hydrates in marine environments, significant accumulations of gas hydrate occur onshore in arctic regions, such as the North Slope of Alaska, within and below permafrost. The shallow subsurface geology of the North Slope is structurally dominated by the Barrow Arch and is riddled with a myriad of high-angle normal faults generally trending north-south and east-west. In addition to the complex shallow structure, the hydrate-bearing units are also stratigraphically complicated; the depositional environment is predominantly a fluvial, northward-prograding deltaic system. The complexity of the local geology and generally lower quality of onshore seismic data, coupled with a lack of consistent amplitude anomalies associated with these gas-hydrate occurrences, make interpretation of gas-hydrate-bearing zones more difficult than is typical in marine environments. In order to gain a better understanding of the gas-hydrate distribution as well as a clearer picture of the local geology, seismic attributes have been used in conjunction with traditional seismic interpretation and well-correlation techniques. Several of the attributes can yield information about the physical properties of the hydrate-bearing units. For example, hydrate layers, which have relatively high acoustic velocities and low densities, may have anomalous frequency responses. Local polarity reversals and amplitude and frequency anomalies along hydrate-bearing layers indicate the presence of free gas down-dip from the hydrates below the local hydrate-stability field. Delineation of high-angle faults and possible channel deposits are aided by the use of edge-detection volumes and additional post-stack processing. Such analyses show that shallow faults cause significant compartmentalization of the hydrate-bearing zones. The use of these and other attributes yields a more accurate structural model as well as a clearer picture of hydrate distribution.