CONCENTRATED ACCUMULATIONS OF NATURAL GAS AND HYDRATE IN THE ALEUTIAN BASIN OF THE BERING SEA: CASE STUDIES BASED ON INTERVAL TIME ANOMALIES

Seismic reflection images reveal abundant evidence for major accumulations of natural gas and gas hydrate within the deep water (>3500 m) Aleutian Basin of the Bering Sea. Distinctive velocity pseudostructures, evident in the otherwise horizontal and uniform sedimentary reflection sequence, are interpreted as evidence of methane chimneys overlain by concentrated hydrate caps. These velocity-amplitude anomaly structures ("vamps") characteristically include a zone of high velocity pull-up (attributed to hydrate within the sediment) directly overlying a zone of low velocity push-down (attributed to gas in the pore spaces). A prominent hydrate bottom simulating reflection (BSR) is also present throughout this basin. Within the vamps, the BSR roughly separates the pull-up from the push-down.

Case studies of several vamp structures are presented. Examples are from single channel seismic reflection profiles acquired by the USGS during the 1986-7 EEZscan GLORIA program. They are a small subset of the total heritage seismic coverage of the basin, which is being assembled in a Landmark SeisWorks 2D database for analysis. Interpreted horizons in the upper 2 km of sedimentary section have been auto-tracked and interpolated to 1 ms accuracy for this study. It is assumed that sedimentary horizons are parallel to each other and that velocity structure is laterally uniform, with the exception of gas and hydrate contributions. These premises are consistent with DSDP Leg 19 and regional sonobuoy results. Interval travel time anomalies between seismic reflecting horizons are quantitatively interpreted as gas and hydrate indicators.

Distribution of hydrate is unexpectedly complex, particularly near the BSR, with a possibility of gas and hydrate zones alternating between 350 and 500 m below sea floor (bsf). Hydrate concentrations are maximum between 200 and 450 m bsf, diminish upward from there, and are not clearly present near the seafloor. Gas is present in the section to well below 1 km bsf.

Individual large vamps (1-3 km across, ~30 ms pull-up, ~80 ms push-down) are estimated to contain gas volumes (including hydrate) similar to those of economic gas fields (e.g. >10⁹ m³). Thousands of vamp structures exist in the deep water basin. Thus the Bering Sea appears to be one of the world’s major storehouses of trapped greenhouse gasses.