南海北部陆坡天然气水合物沉积成藏特征

指示天然气水合物存在的重要地球物理标志——BSR(似海底反射)在南海北部陆坡晚中新世以来的地层中广泛分布。根据BSR在晚中新世以来3个三级层序地层内的分布特点,结合区域热动力背景、沉积特征(沉积相、沉积速率、含砂率、岩性特征等)和典型沉积体(构造坡折带、滑塌体、沉积物波)的综合分析,系统研究了南海北部陆坡天然气水合物的沉积成藏特征。研究表明,BSR的分布在满足温压、气源的基础上更明显地受沉积体系展布和所处构造部位的控制,气源和温压条件只是影响BSR分布的必要条件而非充分条件。建立了南海北部陆坡典型沉积成藏模式,总结了在陆架、陆坡、陆基上随着水深的增加各种沉积体的分布及其与BSR和水合物稳定带的关系,展示了水合物成藏的典型要素。BSR与构造坡折、深水重力流及等深流沉积密切相关,大多数BSR分布区均位于地貌变化陡峭、地形起伏较大、长期继承性隆升与沉降交汇的黏土质粉砂和粉砂等沉积物中。在陆架三角洲前缘,BSR侧向较连续但延伸距离短,水合物成藏的关键在于深部气源的沟通条件;在陆坡海槽与底辟发育区,BSR受断层与底辟切割往往呈断续分布,沉积物的快速卸载可为水合物的赋存提供较好的沉积储层条件;在陆基至深海区海底扇、浊流、等深流发育的位置,BSR侧向连续延伸距离远,气源主要为浅部生物气,以侧向运移为主。

Depositional accumulation characteristics of gas hydrate in the northern continental slope of South China Sea

Bottom simulating reflections (BSR) are important geophysical markers of gas hydrate and widely present in the northern continental slope of South China Sea (SCS) since the late Miocene. In this study, we systematically analyze the depositional accumulation characteristics of gas hydrate in the northern continental slope of SCS in accordance with the distribution pattern of BSR in three third-order sequence stratum since the late Miocene, and combined with an integrated study of regional thermodynamic background as well as the depositional conditions (depositional facies, depositional rate, sand content, and lithology) and typical depositional bodies (structural slope, slumping block, and sediment waves). Under preferential conditions of temperature, pressure and gas source, the distribution of BSR is obviously constricted by the expansion of depositional system and the background of geological structure. That is, temperature, pressure, and gas source are necessary rather than sufficient conditions for the distribution of BSR. Following the above findings, a typical depositional accumulation model for the northern continental slope of SCS is established. The proposed model summarizes the distribution of depositional systems in continental structures (shelf, slope, and rise) with increasing water depth and their relationship with BSR and gas hydrate stability zones, displaying the typical elements of gas hydrate accumulation. The presence of BSR is closely related to structural slope break zones, deep-water gravity flow, and contourites. The majority of BSR are distributed in clay silt and silt deposits in seaward areas with steep and rough relief and long-term successive uplift and subsidence. In the frontier of continental shelf, BSR are laterally continuous but extend in a short distance, where the key of gas hydrate accumulation lies in the transport conditions for deep gas source. In the slope-trough and diapir development zones, BSR show discontinuous distribution as divided by faults and diapirs, where rapid unloading of deposits provides preferential depositional accumulation conditions for gas hydrate. In the continental rise to deep-sea areas with the development of submarine fans, turbidity currents and contour currents, BSR are laterally continuous and extends in a long distance with shallow biogas as the major source and mainly through lateral mitigation.