Typical characteristics of fracture-filling hydrate-charged reservoirs caused by heterogeneous fluid flow in the Qiongdongnan Basin, northern south China sea

• Published in: Marine and petroleum geology 2021-02, Vol.124, p.104810, Article 104810
• Main Authors: Wei, Deng, Jinqiang, Liang, Wei, Zhang, Zenggui, Kuang, Tong, Zhong, Yulin, He
• Format: Article
• Language: English
• Subjects: Fracture parameters; Fracture-filling hydrates; Heterogeneous fluid flow; Qiongdongnan basin; Rock physics model
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2020.104810

Fracture-filling hydrates were recovered above the gas chimneys originating from the Songnan Low Uplift in the northern Qiongdongnan Basin, South China Sea. The variable distributions of gas hydrate and free gas at different drilling sites indicate that fluid flows varied significantly in space. The ambiguous and indistinct migrations of gas-bearing fluid and associated flows are potential geohazards, but limited data hampered the evaluation of the fluid flow before drilling. Nearly no study focused on the typical characteristics of heterogeneous fluid flows in the study area yet. Several wells were mainly drilled through the seepage pathways on three gas chimneys where the fluid flows exhibited three different levels from weak to strong. Fractures in these seepage pathways are the direct evidences of up-going fluid flow. To quantify these fractures, we established a new rock-physics model for unconsolidated and anisotropic hydrate-bearing sediments based on the experimental tests and pressure cores, where high-angle fractures developed within the unconsolidated deposits. The model parameters unrelated to the fractures were optimized using a reference well without gas or hydrate occurrence. Then, the fracture parameters including the fracture density and the aspect ratio of the fracture, were obtained through the Monte Carlo inversion. We found that the evolution of the fracture characterized the heterogeneous fluid flows. Weak flows only made few fractures in the sediments, but sufficient gas supply and long-term seepage produced more fractures. They gradually changed the shape of the fractures in the sediments with fewer fractures, where the isolated fractures merged to form longer fractures, and provided fracture-induced porosity similar to that of sediments with more fractures when the fractures were almost completely filled with hydrates, and nearly all of the free gas beneath the bottom simulating reflectors had been released at this time. These insights are consistent with the drilling. We analyzed the related seismic attributes, submarine geomorphology, and geochemical index, which significantly supported the proposed model. The model provides a direct and quantitative insight of the fracture-filling hydrates and explains the variations distribution of underlying free gas. •We analyzed the typical characteristics of heterogeneous fluid flow in the QDNB.•We built a new rock physics model for fracture-filling gas hydrates.•We characterized variable f