南海北部神狐海域天然气水合物成藏演化分析研究

Analytical research on evolution of methane hydrate deposits at Shenhu Area, northern South China Sea

南海北部陆坡具备天然气水合物成藏的基本地质条件,神狐海域天然气水合物是当前我国海洋天然气水合物勘探开发研究的重点靶区.然而,神狐海域水合物集中分布在水合物稳定带的底部薄层中,饱和度高,其水合物特征与典型的低甲烷通量控制的水合物分布有很大差异,对其成藏机理和控制因素尚不明确.本文构建了针对神狐水合物成藏过程的一维动力学模型,模型包括沉积压实作用、甲烷溶解度、以及水合物生成和沉积体渗透率,模拟计算的主控参量为海底沉积速率和水流通量,在孔隙水中甲烷浓度一定的情况下,水流速率决定了溶解甲烷的迁移速率和稳定带中甲烷的供给速率,并以此模型计算了神狐海域水合物聚集成藏的动力学过程.模型讨论了特定沉积速率和水流通量条件下水合物成藏与分布特征,并与实际观测数据进行比较研究.研究发现,基于当前沉积速率和水流通量条件模拟的水合物形成演化过程,与神狐海域实际水合物分布特征存在很大差异,但在假定系统中水合物饱和度初值达16～20%时,模拟的水合物饱和度分布特征与观测数据吻合,并因此推测在早期地质历史上,神狐海域存在更加丰富的甲烷水合物,当前的水合物分布特征是在对早期水合物继承基础上发展而成的,而且神狐海域水合物含量正逐渐减少.

Geological conditions for gas hydrate formation have been naturally prepared on thenorthern continental slope of the South China Sea. Shenhu Area is currently regarded as one of the promising targets for gas hydrate exploration and exploitation. However, the methane hydrate at Shenhu Area concentrates at base of hydrate stability zone （HSZ） and is of high saturation. The feature differs from that of the typical hydrates dominated by low methane flux. The reservoir-forming mechanism and controlling factors are not clearly understood. In this paper a one-dimensional kinetic model is constructed for investigating the evolution of special hydrate. The model includes sedimentation compaction, methane solubility, and hydrate formation and sediment permeability, and has two controlling parameters of sedimentation rate at seafloor and water flux. At a constant methane concentration in pore water, water flux determines transfer rate of dissolved methane gas and gas supply into the HSZ. The model is applied to compute the process of hydrate accumulation in the sediment at Shenhu Area. We discuss the hydrate reservoir and distribution under the given conditions of sedimentation rate and water flux, and compare the model results with observed data. The modeled results of hydrate formation based on the current sedimentation rates and water flow rates are much different from the measured values to some extent. Surprisingly the simulation matches the hydrate distribution well at an initial condition of 16-20%hydrate existing in pores of the old sediment. The results imply that a large amount of gas hydrates existed in the early sediment at Shenhu Area, the sediment evolved by inheriting the old hydrates and herewith produced the current feature of hydrate distribution, and that the hydrate content in the area is decreasing.