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Schnurle, P., and C.-S. Liu, 2009, Structural controls on the formation of bottom-simulating reflectors offshore southwestern Taiwan from a dense seismic reflection survey, in T. Collett, A. Johnson, C. Knapp, and R. Boswell, eds., Natural gas hydrates—Energy resource potential and associated geologic hazards: AAPG Memoir 89, p. 490–504.

DOI:10.1306/13201120M893357

Copyright copy2009 by The American Association of Petroleum Geologists.

Structural Controls on the Formation of Bottom-simulating Reflectors Offshore Southwestern Taiwan from a Dense Seismic Reflection Survey

Philippe Schnurle,1 Char-Shine Liu2

1Institute of Oceanography, National Taiwan University, Taipei, Taiwan
2Institute of Oceanography, National Taiwan University, Taipei, Taiwan

ACKNOWLEDGMENTS

We would like to thank the captains and crews of the R/V Mona Wave, the N/V L'Atalante, and particularly the R/V Ocean Researcher I for their efforts in collecting the swath bathymetric and seismic data used in this study. Long lasting discussions with S.-W. Lin, T.-Y. Yang, and J.-C. Chen at the National Taiwan University have helped improve our understanding of the geochemical processes associated with the gas hydrates. This study also benefited greatly from the collaboration with Chinese Petroleum Corporation-Exploration and Development Research Institute and particularly with T.-H. Hsiuan. Finally, AAPG editors, and in particula F.P. Whitehurst, offered valuable improvements to the final proof. We are grateful to P. Jaiswal and D. Scholl for their thorough reviews and many constructive remarks. Finally, AAPG editors, and in particular F. P. Whitehurst, offered valuable improvements to the final proof. Maps were generated with generic mapping tools (Wessel and Smith, 1995). Seismic sections and the nonstandard processing were generated with Seismic Un*x (Stockwell, 1999). This study is supported by the Central Geological Survey grants 5226902000-06-93-01 and 5226902000-05-94-01. Part of the seismic data used in this study was collected through the support of the Chinese Petroleum Company.

ABSTRACT

A dense seismic reflection survey with a 400-m (1312-ft) line spacing has been conducted in a 14- by 16-km (9- by 10-mi) area offshore southwestern Taiwan where bottom-simulating reflectors (BSRs) are highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north-south–trending thrust faults and northeast–southwest oblique ramps exist in this region, which impacts on the distribution and seismic characteristics of the BSR. A clear BSR is observed almost continuously in the southeastern half of the survey box, whereas BSRs either appear as small patches or are absent in the northwestern half. The reflection coefficient at the BSR presents abrupt lateral variations, with an average value of minus0.094 or 32% of the reflection coefficient at the sea floor. A pattern of seismic blanking beneath the ridge crests overlying a high-amplitude BSR with bright reflections below the BSR in the slope basins reveals the significance of tectonics and sedimentation of the formation of BSRs. Local heat-flow values estimated from BSR subbottom depths show different ranges across the northeast–southwest-trending Yung-An lineament (YAL), with higher heat-flow values ranging from 55 to 70 mW/m2 in the southeastern half of the survey area, whereas the heat-flow values range from 45 to 55 mW/m2 to the northwest of the YAL. In the southeastern part of the survey area, an elongated diapiric feature deforms the sedimentary layers, BSRs occur shallower, and high heat-flow values are inferred. Furthermore, local dips of BSRs and sedimentary strata that crosscut the BSR at intersections of any two seismic profiles have been computed. A dominant N308deg stratigraphic updip direction characterizes the anticlinal ridges; stratal dips fan out toward N285deg south of the ridges and toward N330deg north of the ridges. The concentric patterns of steep stratigraphic dips constitute favorable migration paths for the upward transport of fluids with significant dissolved-gas content across the BSR, which correlate well with the high gas volumes sampled in this area.

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