珠江口盆地高饱和度砂质天然气水合物储层的地震识别和钻前预测

Gas hydrate at high saturation in sand-rich reservoir is the only exploitation test target at the present technology. Gas hydrate saturation with a value of 60% was identified in the Pearl River Mouth Basin in 2015, however, the gas hydrate-bearing sediment is silt-rich reservoir with high carbonate content forams and nannofossils. How to identify and predict gas hydrate at high saturation in sand-rich reservoir has become the most important question that must be resolved for the current gas hydrate exploration. In this study, we will use the three-dimensional seismic data with different resolution and twenty-seven well log data to analyze the distribution and the internal difference of gas hydrate-bearing sediment at high saturation. We will also try to find gas hydrate at high saturation in sand-rich reservoir using the geophysical characteristic and seismic amplitude reflections in the study area. The seismic reflection characteristic and well log response anomalies at sand-rich reservoir with high saturations in the international typical basins were compared with those anomalous characteristics identified in the Pearl River Mouth basin in fine-grained sediments. The variations of seismic amplitude response and the elastic parameters in sand-rich reservoirs were analyzed using the forward modeling caused by different gas hydrate saturations, lithologic combination, thickness of gas hydrate-bearing sediments and rock physics models. The gas hydrate occurrence at high saturation in sand-rich reservoir in the Pearl River Mouth Basin will be characterized and be predicted using the simultaneous seismic inversion, multi-parameters extraction from the three-dimensional seismic data, and the seismic amplitude responses along the gas hydrate stability zone combined with geological controls on gas hydrate formation. The research results can provide important scientific significance and practical guide for gas hydrate petroleum system and resource exploration.

高饱和度砂质水合物储层是国际上在当前技术条件下能够进行试采的唯一目标。2015年我国在珠江口盆地钻探到饱和度高达60%的水合物层，但为富含有孔虫的粉砂质储层。如何识别和预测高饱和度砂质水合物储层成为我国目前水合物勘探中急需解决的问题。本项目拟对珠江口盆地三维地震和两次钻探的27口井资料进行综合研究，揭示钻探发现的高饱和度水合物层的内部差异、分布范围，并寻找高饱和度砂质水合物储层；分析国际上高饱和度砂质水合物储层的成因机制与典型异常特征，并与该区发现的细粒沉积物中高饱和度水合物层进行对比，结合不同厚度、饱和度、岩性组合及不同模型等开展正演模拟，查明高饱和度砂质水合物储层弹性参数和地震振幅响应的差异及主控因素；综合水合物稳定带厚度、多参数反演、多属性分析和成藏地质条件分析对高饱和度砂质水合物储层进行钻前识别与预测。研究成果对水合物成藏理论研究和资源勘探具有重要的科学意义和现实的作用。

以南海北部珠江口盆地为研究靶区，系统分析了不同钻探区含水合物层的储层特性，并通过与南海北部琼东南和台西南盆地裂隙充填型水合物为对比，发现了珠江口盆地泥质粉砂储层相对富集水合物层位主要赋存在稳定带上部地层，表现为速度和电阻率的增加、密度略微增加或者无变化的测井响应特征，地震响应为一个极性与海底一致局部强振幅反射。与呈脉状、块状等分布水合物层地震响应明显不同，相对富集裂隙充填型水合物地震上呈明显上拱、振幅变弱、稳定带明显变浅特征，主要是由于高通量热流体向上运移，导致稳定带变浅，两种不同类型水合物的定量评价方法存在明显差异性。通过对珠江口典型站位分析，发现珠江口盆地水合物层并不严格受控于沉积物岩性，而是与流体运移关系密切。.通过的研究区多种属性分析，发现了沟通深部热成因气与浅层水合物稳定带的断裂对流体运移较关键，而形成水合物气源具有明显的混源特征，结合生烃数值模拟，发现在饱和度大于50%水合物层，热成因气占比达80%，认为双源叠置、混源供烃和晚期超压及新构造运动导致深部热成因气长距离运移和幕式释放控制了水合物的富集，在局部高通量流体聚集区，为水合物与游离气共存。.通过井震联合反演，形成了水合物富集层储层物性的多种反演方法，针对细粒粉砂质水合物储层，提出了“水合物饱和度划分岩相” 的地质统计学反演方法，实现了多种物性参数的高分辨率反演及空间分布的精细刻画，能清晰识别4米薄水合物层，识别水合物层与非水合物层厚度、边界及横向储层物性变化。珠江口盆地揭阳凹陷三维反射地震资料揭示，海底发育三个不同规模的峡谷，海底峡谷沉积环境，有利于高饱和度砂质水合物储层发育。受东沙隆起影响，海底侵蚀与构造隆升影响水合物系统的稳定性，导致其动态成藏，表现为BSR深度迁移。通过对成藏与源渠汇系统分析，发现在白云凹陷的西部区域，也发现了砂质水合物的局部区域潜在有利区带。