Identifying gas hydrates in the oceanic subsurface using seismic reflection data supported by the presence of a bottom simulating reflector (BSR) is not an easy task, given the wide range of geophysical methods that have been applied to do so. Though the presence of the BSR is attributed to the attenuation response, as seismic waves transition from hydrate-filled sediment within the gas hydrate stability zone (GHSZ) to free gas-bearing sediment below, few studies have applied a direct attenuation measurement. To improve the detection of gas hydrates and associated features, including the BSR and free gas accumulation beneath the gas hydrates, we apply a recently developed method known as Sparse-Spike Decomposition (SSD) that directly measures attenuation from estimating the quality factor (Q) parameter. In addition to performing attribute analyses using frequency attributes and a spectral decomposition method to improve BSR imaging, using a comprehensive analysis of the three methods, we make several key observations. These include the following: (1) low-frequency shadow zones seem to correlate with large values of attenuation; (2) there is a strong relationship between the amplitude strength of the BSR and the increase of the attenuation response; (3) the resulting interpretation of migration pathways of the free gas using the direct attenuation measurement method; and (4) for the data analyzed, the gas hydrates themselves do not give rise to either impedance or attenuation anomalies that fully differentiate them from nearby non-hydrate zones. From this last observation, we find that, although the SSD method may not directly detect in situ gas hydrates, the same gas hydrates often form an effective seal trapping and deeper free gas accumulation, which can exhibit a large attenuation response, allowing us to infer the likely presence of the overlying hydrates themselves.

In the shallower regions of the 3D Nimitz seismic survey, there exist multiple interesting bright seismic amplitude anomalies. These anomalies, or Funny Looking Things (FLTs), occur in a confined spatial and temporal region of the seismic. They have a concave up seismic appearance along cross section. Bright seismic amplitudes can be a direct hydrocarbon indicator, or representative of strong lithological contrasts, and/or acquisition artifacts. We set out to investigate misinterpreted seismic anomalies along cross sectional lines. Therefore, we apply seismic attributes to indicate these bright spot features, which we interpret to be submarine gullies looking along time slice intersections, can possibly be mistaken for hydrocarbon anomalies in cross sectional view. However, we cannot fully rule out the presence of hydrocarbons since it is common for gas sands to create similar anomalies. Previously drilled wells within the survey (Korimako-1 and Tarapunga-1) point towards a lack of hydrocarbon potential in the subsurface. While it is possible these bright spots are due to hydrocarbon presence, we present a more likely hypothesis: the lithology of the interfluve sediments is similar to the gully-margin drapes but differs from the gully sediment fill.

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There remains a limited understanding of the controls of pre-existing structures on the architecture of deep-water progradational sequences. In the Northern Taranaki Basin (NTB), New Zealand, Pliocene post-extensional sedimentary sequences overlie Miocene back-arc volcaniclastic units. We utilize seismic reflection datasets to investigate the relationships between the buried back-arc mound-shaped structures, 876 | EAGE CLAIRMONT eT AL. Highlights • Buried syn-extension volcanoes occur only in the north area. • Differential compaction of post-extension sequences are influenced by the paleovolcanoes. • Steep clinoform foreset slopes and wide incising channels in the north, but are absent in the south. • The later differential compaction influenced the basinward migration and geometry of continental slope. • Buried volcanoes in the north modulated the postextension sedimentation patterns. 878 | EAGE CLAIRMONT eT AL. F I G U R E 1 Location and the geologic setting of the study area. (a) Topographic map of northern New Zealand showing the Taranaki and Northland Basins, large faults and the Miocene volcanic fields of the Mohakatino Volcanic Belt (modified after Stagpoole & Funnell, 2001 and Johnston et al., 2010). The location of the primary study area (Nimitz 3D Seismic Survey) is shown in the white rectangle, together with a 2D transect line used to show the extent of interpreted geologic features, discussed throughout this expert. (b) Map of the Nimitz 3D seismic survey and location of the Korimako-1 well. (c) Generalized stratigraphic column of the upper fill of the Taranaki Basin (after Unkaracalar, 2018 and King & Thrasher, 1996), and generated wireline logs and synthetic model of the Korimako-1 well. The following stage abbreviations reflect the local geology of the

The Taranaki Basin is well known for studies examining the seismic stratigraphy, depositional and erosional features, and tectonic frameworks linked to the New Zealand (NZ) continent. This particular study examines a “funny looking thing” (FLT) which we associate to be consistent with that of a braided channelized system. We observe this feature within the 3D Nimitz Survey (See Figure 1), located in the Northern Taranaki Basin (NTB) off the western continental coast of North Island, NZ. The FLT occurs within Quaternary deposits of the Whenuakura Formation which are interpreted to reflect shelfal topset sediments (O’Leary et al., 2010). It is underlain by the Giant Foresets Formation (GFF) of Pliocene to Pleistocene age, which are described as large-scale progradational and aggradational continental successions that migrated west to northwest in basinward direction (Anell and Midtkandal, 2017; Clairmont et al., 2020; Hansen and Kamp, 2002; Shumaker et al., 2017) (Figure 2). It comprises a shelf-to-slope succession of claystone to siltstone with argillaceous sandstone intervals defining an overall coarsening upward succession (O’Leary et al., 2010). The FLT within the Whenuakura Formation is characterized by chaotic facies in cross section, which shares characteristics with potential mass wasting events (Figure 3a). However, further analysis using seismic attributes improved the spatial and stratigraphic architecture of the FLT, which favored a complex channelized system interpretation over a mass transport deposit complex.