海洋地质与环境重点实验室知识库

天然气水合物（简称水合物）成藏受到多种因素综合影响，而储层条件是影响水合物空间展布与饱和度的一个重要因素。由于不同盆地构造环境与地质环境差异，水合物形成的地质条件复杂多变，影响水合物成藏的储层条件与流体运移方式不同，导致了水合物微观赋存形态、饱和度和空间展布特征存在差异。在强流体活动的细粒沉积物中，由于水合物形成推开沉积物颗粒或者在局部裂隙中，水合物呈脉状、块状、球状等形态分布在富泥细粒储层，呈现出不均匀分布特征，如冷泉发育区裂隙充填型水合物。在峡谷-水道发育粉砂质、砂质储层，水合物均匀充填孔隙空间，但是空间展布与储层条件、流体运移等密切相关，水合物在横向饱和度存在差异，该类型水合物与裂隙充填型水合物分布明显不同。因此，不同储层条件下水合物赋存特征不同，评价水合物饱和度与空间分布方法也不同。本文对南海北部马尼拉俯冲带前缘活动冷泉区与琼东南盆地古冷泉发育区裂隙充填型水合物和珠江口盆地峡谷发育区的粉砂质储层的孔隙充填型水合物开展储层研究，系统研究两种不同类型水合物的异常特征及其评价方法。利用2D与3D地震、测井、地质以及海底观测等数据，开展岩石物理分析、正演模拟、储层反演与储层解释，揭示两种不同类型水合物形成地质条件与储层评价方法差异。

基于俯冲带前缘台西南盆地与被动大陆边缘琼东南盆地裂隙充填型水合物系统，冷泉区通过研究海底底质特征与生态群落、地震振幅异常、地球物理属性特征等综合分析了冷泉系统演化过程及其地球物理异常，揭示冷泉由“生长”到“逐渐消亡”不同阶段异常特征变化。围绕裂隙充填型水合物仅分布在局部几百米空间，横向上连续性差、地层环境变化大等特点，通过多属性建模的方式开展宽频反演，来刻画裂隙充填型水合物空间变化。研究发现裂隙充填型水合物形成与下部局部隆起及其形成的断层强流体活动有关。

珠江口盆地峡谷发育区，含水合物层异常特征明显不同，脊部地层呈层状变化，局部区域断裂发育，深部流体向上运移，出现水合物层状分布并与游离气共存的复杂特征，分别利用叠前同时反演与地质统计学反演开展水合物精细的储层评价。利用叠前反演识别水合物与游离气界面研究，发现纵波速度对水合物层的分布刻画有效，纵波阻抗和密度进行游离气分布研究更合理，纵横波速度比开展水合物-游离气过渡层研究具有较好效果；而基于岩相和精细地质模型约束的地质统计学反演，能够较高精度揭示不同饱和度的薄厚互层水合物储层横向及垂向变化，满足试采开发阶段对储层评价要求。

The accumulation of natural gas hydrates (gas hydrates) is affected by a variety of factors, but reservoir is an important factor affecting the spatial distribution and saturation of gas hydrates. Due to the structural environment differences, the geological conditions for theformation of gas hydrate are complex. However, the reservoir and fluid migration of gas hydrate accumulation are different in different geological settings, which cause the differences in microscopic morphology, saturation and spatial distribution of gas hydrates. In the fine-grained sediments with strong active fluid flow, gas hydrates are distributed in mud-rich reservoirs in the form of veins, massive, and nodules, showing the inmohogenious distribution, such as fracture-filling gas hydrates in the cold seeps showing chimney-shaped reflection. Gas hydrate in in canyons and channel-levee system where silt or sandy reservoirs are developed, the pore-filling gas hydrates are formed in the pore spaces with homogeneity. The spatial distribution and saturation are closely related to reservoir conditions and fluid migration. Gas hydrates saturations are variable in lateral, which is different from gas hydrates in chimney-shaped zone. Therefore, the characteristics of gas hydrate occurrence are different in different reservoir conditions, and the methods for evaluating gas hydrate saturation and spatial distribution are also different.

In this study, the fracture-filling gas hydrate reservoir of the active cold seeps near the Manila subduction zone and the paleo-cold seeps in the Qiongdongnan Basin, northern slope of South China Sea (SCS) are compared to the pore-filling gas hydrate in silty reservoir of the canyon development area in the Pearl River Mouth Basin. The anomalous characteristics, geological setting for the formations of two type gas hydrates and the methods for estimating saturations are studied using two dimensional (2D) or three dimensional (3D) seismic data, logging data, and seafloor observations. The petrophysical analysis, forward modeling, reservoir inversion and reservoir interpretation are displayed to show the differences of two types of gas hydrates.
The fracture-filling gas hydrate system found in the Taixinan basin, the front of the subduction zone, and the Qiongdongnan basin in the passive continental margin of SCS, the evolution processes of the cold seep system are comprehensively analyzed by studying the characteristics of the seafloor and the chemical ecological community, seismic amplitude anomalies, and geophysical properties. The geophysical anomalies and the changes of seafloor sediments and ecological community reveal the anomalous characteristics of the cold seeps at different stages from "active" to " gradually inactive". Due to the regional, discontinuous distribution, the fracture-filling gas hydrate is about several hundreds meters in lateral andgreat change in sedimentary stratigraphic. The broadband inversion is carried out through multi-attribute modeling to characterize the spatial change. We found that the formation of this type gas hydrate is related to the strong fluid activity of faults and local uplifts of basement.

Gas hydrate in the the Pearl River Mouth Basin is quite different from those of chimey-shaped zone, where the canyons and the channel-levee system are well developed. The reservoirs of the canyon ridge change greatly. Gas hydrates with layered distribution and the coexistence of gas hydrates and free gas are found at the crest of canyon, where deep fluids migrate upwards along faults. Pre-stack simultaneous inversion and geostatistical inversion are used to reveal the thin and layered properties of gas hydrate reservoir in spatial distribution. The geophysical propereties of prestack inversion are used to identify gas hydrate and free gas interface, and the P-wave velocity is effective for the distribution of the gas hydrate layer. The P-impedance and density are useful to study the distribution of the free gas, and the ratio of P-wave veloicty and shear wave velocity (Vp/Vs) can be used to study the transition layer of gas hydrate and free gas. Geostatistical inversion based on the constraints of lithofacies and layer controlled geological models can reveal the lateral changes of thin interlayer hydrate reservoirs with different saturations, which can meet the requirements for reservoir evaluation during the trial development phase of gas hydrate.