南海北部陆坡峡谷脊部精细结构、沉积成因及储层发育规律

Fine-scale architecture, sediment genesis, and reservoir formation mechanisms of canyon ridges in the northern South China Sea

  • 摘要: 海底峡谷脊部是海域天然气水合物的重要富集带。相比研究程度较高的海底峡谷,脊部的沉积特征和建造过程目前尚未充分揭示。本文选取南海北部白云陆坡峡谷为研究对象,基于高精度三维地震数据开展峡谷脊部层序地层和地震地貌学系统性研究。通过地震解释识别出5个关键地质界面,进而将10.5 Ma至现今海底划分为4个演化期次(SU1—SU4)。地震相分析和厚度图表明研究区峡谷脊部演化呈现两阶段特征:建造期(10.5~2.6 Ma)以形成初始地貌形态为主,生长期(2.6 Ma至现今)则表现为持续的垂向加积作用,其中中更新世以来(0.6 Ma至现今)沉积速率显著提升,可能与中更新世气候转型引发的陆源碎屑通量剧增密切相关。三维地震地貌学特征发现,研究区自10.5 Ma以来发育大规模波状地层(波长0.5~4.0 km,波高6.3~12.2 m)。结合其上坡迁移特征,平面的不规则圆坑状形态,同时与国内外研究对比,可排除正断层、滑坡和蠕变等解释。这些不对称的波状起伏可解释为超临界浊流水跃作用下形成的沉积物波。模型计算和地震属性提取发现,相比于波峰或底形高点,沉积物粒度在波谷处较粗,进而具有更高的水合物富集潜力。

     

    Abstract: Submarine canyon ridges are critical enrichment zones of gas hydrates in the South China Sea (SCS) and other places of the world. Compared with well-studied submarine canyons, the characteristics of sedimentation and formation processes of canyon ridges remain insufficiently revealed. By focusing on the Baiyun slope canyon in the northern SCS, we studied systematically the sequence stratigraphy and seismic geomorphology of the canyon ridge based on high-resolution 3D seismic data. Through seismic interpretation, five key geological interfaces were identified and dividing the stratigraphic sequence from 10.5 Ma to the present into four evolutionary stages (SU1~SU4). Seismic facies analysis and thickness maps showed two periods of evolution of the canyon ridge: the construction period (10.5~2.6 Ma) and the growth period (2.6 Ma-present). The construction period was mainly characterized by the formation of initial landforms, while the growth period was characterized by continuous vertical sedimentary accumulation. Notably, since the Middle Pleistocene (0.6 Ma present), the sedimentation rate was significantly increased, which may be closely related to the dramatic increase in terrestrial debris flux caused by the climate transition in the Middle Pleistocene. In addition, 3D seismic geomorphological analysis demonstrated that the canyon ridge has developed large-scale undulating bedforms (wavelength 0.5 ~ 4.0 km, wave height 6.3 ~ 12.2 m) since 10.5 Ma. Combined with upslope migration characteristics, irregular circular pit-shaped planar features, and comparative studies among domestic and international research outcomes, interpretations including normal faults, landslides, and creep processes could be excluded. These asymmetric undulations were interpreted as sediment waves formed by supercritical turbidity current hydraulic jumps. Model calculations and seismic attribute extraction revealed coarser sediment grain sizes in trough regions compared to crests or bedform highs, indicating a higher gas hydrate enrichment potential in these trough areas.

     

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