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

The quality of reservoirrocks, in particular their permeability, is likely to be a key factor for the economic viability of future gas production from gashydrates. As for conventional gas resources, high-permeability sands are considered the economically most promising gashydratereservoirs. Studies of subsurface lithology however, are difficult without calibration from boreholes. We investigated seismic data from the HikurangiMargin, a subduction zone east of NewZealand and NewZealand's largest gashydrate province. We suggest that the strength of bottomsimulatingreflections (BSRs) from the base of the gashydrate 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 amplitude-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 reservoirrocks, similar to indurated mudstones dredged from a submarine outcrop close to the study area. We therefore suggest that weak BSRs may mark fine-grained reservoirrocks with usually low primary permeability. Even though these reservoirrocks 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 gashydrate saturations above layers with high saturations of gas would be expected to lead to highly reflective layers in the gashydrate stability zone and thus, reflection patterns above the BSR may allow distinguishing between both causes.