Abstract: When biogenic methane produced in marine shallow sediments flows laterally into hydrate stable zones, there is likely to be formation of disperse stratigraphic-diffusive hydrate reservoirs that generally appear thin-bedded and disseminated. The stratigraphic-diffusive layers are also potential industrial exploitation targets because they have moderate abundance, wide distribution, and convenient mining maneuverability. After success of the second trial production in Shenhu area of South China Sea, it is urgent for us to find out the evolution dynamics of the stratigraphic-diffusive hydrate system which is widespread along oceanic continental slopes. Here, the Pearl River Mouth Basin in the northern slope area of South China Sea as a typical passive margin was selected to probe the principles of the accumulation mechanism and distribution pattern of stratigraphic-diffusive methane hydrates, determine the optimal degree of aggregation and spatial extension, and thus evaluate the potential recovery value. Based on the composite stratigraphic sequence, we theoretically described the occurrence characteristics of this type of stratigraphic-diffusive hydrate reservoirs. A two-dimensional model that coupled sedimentation process, fluid flow and reaction kinetics was designed to gauge the influence of engineering geological conditions and other controlling factors since the Pliocene (5.3 Ma) in this system. Several key parameters (including porosity, temperature, salt, and hydrate content) at three different moments, 5 Ma ago, 1.5 Ma ago and at present, were exhibited. The impacts of geothermal gradient and kinetic reaction coefficient on the evolution process were also discussed. Finally, the methane generated from local particulate organic carbons and different start time was used to verify the rationality of this theoretical approach. Our two-dimensional model clearly depicted the occurrence and accumulation characteristics of local stratigraphic-diffusive hydrates, disclosed the exploration value and exhibited the exploitation possibility.

中文翻译：

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南海北部神狐陆坡上新世以来地层扩散型甲烷水合物储层时空演化

摘要：海洋浅层沉积物中产生的生物甲烷横向流入水合物稳定带时，可能形成分散的地层扩散型水合物储集层，一般呈薄层状、播散状。地层扩散层由于丰度适中、分布广、开采操作方便，也是潜在的工业开采目标。南海神狐海域二次试采成功后，迫切需要了解沿大洋陆坡分布的地层-扩散水合物系统的演化动态。这里，以南海北部斜坡区珠江口盆地为典型被动边缘，探讨地层扩散型甲烷水合物的成藏机理和分布规律，确定最佳聚集度和空间延伸程度，从而评估潜在的回收价值。基于复合地层序列，我们从理论上描述了该类型地层扩散型水合物储层的赋存特征。设计了一个耦合沉积过程、流体流动和反应动力学的二维模型来衡量该系统自上新世（5.3 Ma）以来工程地质条件和其他控制因素的影响。几个关键参数（包括孔隙度、温度、盐度、和水合物含量）在三个不同时刻，5 Ma 前，1.5 Ma 前和现在，被展示出来。还讨论了地温梯度和动力学反应系数对演化过程的影响。最后，利用局部颗粒有机碳产生的甲烷和不同的起始时间来验证该理论方法的合理性。我们的二维模型清晰地描绘了局部地层扩散水合物的赋存和聚集特征，揭示了勘探价值，展示了开发的可能性。利用局部颗粒有机碳产生的甲烷和不同的起始时间来验证该理论方法的合理性。我们的二维模型清晰地描绘了局部地层扩散水合物的赋存和聚集特征，揭示了勘探价值，展示了开发的可能性。利用局部颗粒有机碳产生的甲烷和不同的起始时间来验证该理论方法的合理性。我们的二维模型清晰地描绘了局部地层扩散水合物的赋存和聚集特征，揭示了勘探价值，展示了开发的可能性。