Weak and segmented bottom simulating reflections on the Hikurangi Margin, New Zealand — Implications for gas hydrate reservoir rocks

摘要

The quality of reservoir rocks, in particular their permeability, is likely to be a key factor for the economic viability of future gas production from gas hydrates. As for conventional gas resources, high-permeability sands are considered the economically most promising gas hydrate reservoirs. Studies of subsurface lithology however, are difficult without calibration from boreholes. We investigated seismic data from the Hikurangi Margin, a subduction zone east of New Zealand and New Zealand's largest gas hydrate province. We suggest that the strength of bottom simulating reflections (BSRs) from the base of the gas hydrate stability zone may support lithologic interpretations on this margin. BSRs along large parts of this margin are exceptionally weak. Absolute reflection coefficients of a weak BSR on Puke Ridge, a thrust ridge in the accretionary wedge, are roughly between 0.01 and 0.02, an order of magnitude lower than those observed for many BSRs globally. A combination of rock physics modelling and seismic amplirude-versus-offset analysis leads to the conclusion that these weak BSRs are primarily caused by low saturation of gas with patchy distribution, i.e., gas that is only present in pores or fractures of some mesoscopic (i.e., larger than pore sizes but smaller than seismic wavelengths) sediment patches while other patches are fully water saturated. This type of distribution, combined with observed high seismic velocities, is compatible with lithified fine-grained reservoir rocks, similar to indurated mudstones dredged from a submarine outcrop close to the study area. We therefore suggest that weak BSRs may mark fine-grained reservoir rocks with usually low primary permeability. Even though these reservoir rocks may exhibit enhanced secondary permeability from fracturing, they would currently not be considered prime candidates for potential gas production from hydrates. We identified several high amplitude bright spots along weak BSRs. Two possible lithologic explanation for this reflection pattern are that (1) the bright spots mark higher saturation of gas in high-permeability, probably sand-dominated layers, as found in the Gulf of Mexico, and (2) evenly distributed networks of pores may result in gas to be distributed more homogenously in the sediments even though permeability may still be relatively low, as suggested for highly reflective layers beneath the Blake Ridge. Elevated gas hydrate saturations above layers with high saturations of gas would be expected to lead to highly reflective layers in the gas hydrate stability zone and thus, reflection patterns above the BSR may allow distinguishing between both causes.