Seismic studies of the northern Cascadia accretionary prism: sediment consolidation and gas hydrates




Yuan, Tianson

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This thesis work was directed at aspects of two related problems: (1) sediment compaction and fluid expulsion processes in a subduction margin accretionary prism, and (2) the nature and concentration of gas hydrates that form bottom-simulating reflectors (BSRs) observed in the accretionary prism sediments of the northern Cascadia margin. The formation of the gas hydrate and the occurrence of BSRs in the study area are believed to be mainly a consequence of upward fluid expulsion in the accretionary prism. Therefore, the two study objectives are closely correlated. Most of this thesis work was carried out analyzing multichannel seismic data and incorporating available information including downhole and other geophysical measurements. Seismic techniques, such as velocity analysis, forward modelling, and waveform velocity inversion, were used in analyzing the data to advance our understanding of the tectonic and geophysical processes in a dynamic accretionary prism environment. The velocity structure and the inferred porosity variations across the frontal region of the accretionary prism have been quantitatively assessed by a detailed seismic velocity analysis. Within the Cascadia basin sediments approaching the deformation front, and within the frontal thrust zone of the accretionary prism, seismic velocities increase landward as a result of sediment consolidation. An important conclusion is that more than one third of the pore fluid content of the incoming sediment is lost by the time they are incorporated into the accretionary prism. In the lower slope region of the deformation front, a pronounced velocity decrease is evident. This low-velocity zone is explained by underconsolidation resulting from rapid horizontal shortening and vertical thickening of the sediment column, accommodated by displacements along thrust faults or by distributed deformation. A prominent BSR becomes visible immediately landward of the deformation front in the accreted sediment, and is developed over much of the low-to-mid continental slope. The upward pore-fluid migration is believed to play an important role in the formation of a gas hydrate BSR. From the estimated fluid loss of 35% over the 3-km-thick Cascadia Basin sediments with an average sediment porosity of 30%, the quantity of the expelled fluid reaches 315 m3/m2 over a distance of 12 km before the basin sediments are incorporated into the accretionary prism. Assuming that 100 mmol/L of methane is removed from the expelled fluid as it moves into the hydrate stability field, a 90-m-thick layer with an average hydrate saturation of 10% of the pore space can be formed by the rising fluids. A velocity-depth function in the lower slope region, representing a no-hydrate/no-gas reference profile, has been established from the detailed semblance velocity analyses and the ODP log data. The observed and measured sediment velocities near the ODP drill sites increase downward more rapidly than the reference profile above the BSR. Based on the reference profile, the velocity inversion results imply that the velocity increase due to hydrate above the BSR accounts for ~2/3 of the impedance contrast required to produce the BSR reflection amplitudes. The remainder of the impedance contrast appears to come from the velocity decrease associated with small concentrations of free gas below the BSR. The integrated analysis of the multichannel seismic and ODP downhole velocity data has allowed the velocity enhancement associated with the formation and concentration of gas hydrate to be estimated. If the BSR is overlain by a 100 m zone of sediment with a mean porosity of 50% in which the hydrate saturation increases linearly from zero at the top of the zone to 20% at the BSR, the estimated hydrate concentration-depth profiles indicate a total hydrate amount of about 5 m3/m2 of ocean floor or methane amount of 820 m3/m2 at STP. Throughout the Vancouver Island continental margin, where the clear BSR have been observed in an area of 30x200 km, the total methane gas estimated can amount to about 175 Tcf (trillion cubic feet) or 2.6 Gt of carbon.



Sediment compaction, Geology, Structural, Northwest, Pacific, Seismology