Seismic Imaging of Gas Hydrate Accumulations Beneath Thick Permafrost Areas of the Mackenzie Delta, NWT, Canada

Abstract

Abstract The Mackenzie Delta in Canada's Northwest Territories hosts manypermafrost-related gas hydrate accumulations that were indirectly discovered orinferred from conventional hydrocarbon exploration programs. In particular, gashydrate intervals characterized with high saturation show high resistivity andhigh P- and S-wave velocity on well-log data, and are typically found insand-rich horizons. As demonstrated at the Mallik site, the velocity contrastbetween highly saturated gas hydrate-bearing sediments and unconsolidatedwater-bearing sediments is significant and allows their detection with seismicdata. Here, we use 2D and 3D seismic reflection data acquired by industry onRichards Island to map and characterize gas hydrate accumulations beneath athick permafrost area of the Mackenzie Delta. Specifically, we show new seismicevidences of gas hydrate accumulations near YaYa, Ivik and Umiak. The presenceof gas hydrate was previously inferred from well-log data in several boreholeslocated in those areas. All seismic data were re-processed following anAVO-friendly flow that preserved relative amplitude relationships. On suchdata, the strong acoustic impedance of gas hydrate produces strong amplitudeseismic reflections. The seismic signature of gas hydrates is confirmed byseismic-to-well correlation in areas where boreholes are available. Resultsindicate that gas hydrate accumulations occur in structurally-controlled playstypical of conventional oil and gas traps found in this area, and furtherdemonstrate that gas hydrates are part of the regional petroleum system. Introduction Gas hydrates located offshore and onshore beneath thick permafrost areasconstitute one of the largest untapped natural gas resources. Most known gashydrate occurrences in the Mackenzie Delta and submarine permafrost areas ofthe Beaufort Sea were discovered or inferred indirectly in wells drilled forconventional hydrocarbon exploration. Well-log data intersecting gas hydratesshow the typical high resistivity and high P-wave velocity signature associatedwith relatively porous hydrate-bearing sandstone. Progress made in the lastdecade has shown that gas hydrates can also be mapped using seismic reflectiondata. The acoustic impedance contrast between non-hydrate and hydrate-bearingsediments usually produces strong amplitude reflections on seismic data. Such asignature was previously observed onshore at Mallik, Northwestern Territories(Collett et al., 1999), and on the North Slope of Alaska (Collett et al.,2011). In addition to mapping, gas hydrate saturations were also estimated fromseismic data where well-logging data are available for calibration (Riedel etal., 2009).