Abstract:
Gas hydrates are a solid, crystalline substance composed of gas (typically methane) and water which occur in the shallow subsurface of continental margins. The primary method of mapping gas hydrate occurrence is done by interpreting gas hydrates in seismic reflection data. Bottom simulating reflections (BSRs) are seismic reflections which track sub-parallel to seafloor. They roughly track the deepest point in the subsurface where gas hydrate is stable by marking the gas hydrate to free gas phase boundary. Crucially, mapping BSR occurrence allows the estimation and quantification of hydrate occurrence globally. Despite their important role in hydrate mapping, BSRs may be absent in the presence of hydrates. This shortcoming hinders the scientific community’s ability to accurately estimate global hydrate volumes. In this study, two-dimensional (2D) seismic reflection data acquired in the Pegasus and East Coast basins is used to map the occurrence of BSRs and large anomalous BSR gaps. The Pegasus and East Coast basins are known gas hydrate provinces situated east of New Zealand’s North Island. A seismic stratigraphic interpretation produced seven seismic horizons binding seven seismic units. The stratigraphic interpretation is used to test the hypothesis that: 1. Stratigraphy impacts the formation of BSR gaps; and 2. Tectonic deformation results in the development of BSR gaps. Foremost, analysis of the stratigraphic consistency of BSR gaps indicates that no clear relationship occurs between stratigraphy and the presence of BSR gaps. Seismic sections were compared in varying offsets to test the effect of amplitude versus offset (AVO) as a contributing factor in BSR gap formation. No clear AVO effect was detected around the BSR gaps and anomalous gaps are not attributed to an imaging issue. Superposition of BSR occurrence maps onto structural data shows a clear alignment of BSR gaps with synclinal structures. BSR gaps commonly occur over the hinge area of synclines and extend 500 to 2000 m in each direction from the axial trace. Furthermore, RMS amplitude maps show that weak to moderate amplitude BSRs span the limbs of most synclines. While some seismic sections show weak BSRs crossing the hinge point of synclines, the vast majority show clear BSR gaps. A tectono-sedimentary model is presented within this study which aims to explain the cause for and controls behind synclinal BSR gaps. The model highlights the following processes: 1. Sedimentation into a developing syncline; 2.The vertical shift of the BGHSZ over time; 3. Hydrate dissociation resulting from a shifting BGHSZ; 4. Dissociated free gas migrating up-dip away from the syncline’s hinge. A depletion of free gas from the hinge is thought to result over time and thus a BSR would not be maintained there. Furthermore, free gas is proposed to migrate three-dimensionally within trough-shaped synclines resulting in BSRs crossing the trough’s flanks while simultaneously appearing absent across the deepest point of the trough. These processes are interpreted to define a syncline’s impact on the HSZ and BSR occurrence over time. This study proposes that synclinal depletion of free gas from the hinge area is the antithesis to anticlinal focussing of fluids. The model presented in this study could be considered non-unique to the Pegasus Basin and tested against synclinal structures in other hydrate bearing sedimentary basins. Importantly, the model presented in this study may help improve our constraints on global hydrate occurrence by explaining the presence or absence of hydrates within some synclinal structures.