Abstract Research on the formation and distribution of submarine channel systems and associated gas-bearing fluids is of great significance for gas hydrate exploration. Disseminated gas hydrates with high saturation up to 65% were recovered from a submarine ridge, equivalent to the levee of the channel–levee system in the Shenhu area, northern South China Sea. Sedimentary deposits in the submarine ridge were dominated by fine-grained silt and clay-rich silt; gas hydrates with relatively high saturation preferentially accumulated in coarser sediments with less clay content. Although abundant foraminifera fossils may have increased reservoir pore space, their presence was not a necessary condition for high-saturation hydrates. Higher levels of pyrite appeared in the reservoirs corresponding to high-saturation hydrates, which suggests that the reducing environment caused by sufficient methane provided adequate gas to form higher-saturation hydrates. Because of the migration of the channel–levee system, different channels formed their respective depositional systems composed of channel-filling, buried channel-filling, erosion grooves, and slumped turbidities. Relatively coarse-grained deposits were identified in the channel fillings and levees, and the accumulation of hydrates was affected by the lithological features of the sediments and their spatial coupling with the gas hydrate stability zone (GHSZ). GHSZ modeling based on in situ measurements indicated that erosion and sedimentation, as well as variations of the geothermal gradient, resulted in the upward/downward migration of bottom simulating reflectors (BSRs). On the erosion flank of the channel, the strata thinned, and rapid erosion was likely to destroy the shallower BSR, causing gas hydrate decomposition and methane release, and may have caused turbidite slumping and seepage, whereas the strata thickened on the deposition flank of the channel. The BSR in the channel–levee system would gradually move toward the new GHSZ, eventually forming a new BSR; parts of the BSR that formed under the original P–T conditions have remained, and double BSRs occurred in the seismic profile. The thermal fluid that moved upward through a gas chimney may also have caused the migration of the GHSZ, resulting in the emergence of double BSRs. During the lateral migration of the channel and the vertical migration of the gas-bearing fluid, there was a dynamic adjustment relationship between the GHSZ and the erosion–deposition process of the channel, resulting in the dynamic accumulation of hydrates in the Shenhu area. A model to demonstrate the relationship between channel migration and variation of the BSR was established, which is of great significance for understanding the formation and accumulation mechanisms of gas hydrates.

中文翻译：

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南海北部神狐海域与河道堤系统伴生的天然气水合物动态成藏

摘要 海底通道系统及伴生含气流体的形成和分布研究对天然气水合物勘探具有重要意义。从海底海脊中回收了饱和度高达65%的弥散天然气水合物，相当于南海北部神狐地区的河道-堤防系统。海底海脊沉积以细粒粉砂和富粘土粉砂为主；饱和度相对较高的天然气水合物优先聚集在粘土含量较少的较粗的沉积物中。尽管丰富的有孔虫化石可能增加了储层孔隙空间，但它们的存在并不是高饱和水合物的必要条件。高饱和水合物对应的储层中出现了较高含量的黄铁矿，这表明由足够的甲烷引起的还原环境提供了足够的气体来形成更高饱和度的水合物。由于河道-堤防系统的迁移，不同的河道形成了各自的由河道充填、埋藏河道充填、冲蚀槽和塌陷浊度组成的沉积体系。在河道充填物和堤坝中发现了较粗粒的沉积物，水合物的聚集受沉积物岩性特征及其与天然气水合物稳定带（GHSZ）空间耦合的影响。基于原位测量的 GHSZ 建模表明，侵蚀和沉积以及地温梯度的变化导致底部模拟反射器 (BSR) 向上/向下迁移。在通道的侵蚀侧翼，地层变薄，快速侵蚀可能会破坏较浅的BSR，导致天然气水合物分解和甲烷释放，并可能导致浊流塌陷和渗流，而地层在河道沉积侧翼增厚。河堤体系中的BSR将逐渐向新GHSZ移动，最终形成新的BSR；在原始 P-T 条件下形成的 BSR 部分保留了下来，并且在地震剖面中出现了双 BSR。通过气烟囱向上移动的热流体也可能引起了 GHSZ 的迁移，从而导致了双 BSR 的出现。在河道横向运移和含气流体垂向运移过程中，GHSZ与河道侵蚀沉积过程之间存在动态调整关系，导致神狐地区水合物动态聚集。建立了通道运移与BSR变化关系的模型，对于理解天然气水合物的形成和聚集机制具有重要意义。