Integrated core-log facies analysis and depositional model of the gas hydrate-bearing sediments in the northeastern continental slope,South China Sea

Host sediments may exert a significant influence on the formation of gas hydrate reservoirs. However, this issue has been largely neglected in the literature. In this study, we investigated the types, characteristics and the depositional model of the fine-grained gas hydrate-bearing sediments in the northeastern margin of the South China Sea by integrating core visual observations and logging-while-drilling downhole logs. The gas hydrate-bearing sediments consist dominantly of muddy sediments formed in the inter-canyon ridges of the upper continental slope, including hemipelagites, debrites (mud with breccia) and fine-grained turbidites. Cold-seep carbonates and associated slumping talus, muddy breccia debrites, as well as coarse-grained turbidites, may locally occur. Four classes and six sub-classes of log facies were defined by cluster analysis. Core-log correlation indicates that gas hydrates are majorly distributed in fine-grained sediments with high resistivity and low acoustic transit time (AC) log responses, which are easily differentiated from the fine-grained background sediments of high gamma-ray (GR), high AC, and low resistivity log values, and the seep carbonates characterized by low GR, high resistivity, high density, low AC and low porosity log values. The primary host sediments consist of fine-grained hemipelagic sediments formed by deposition from the nepheloid layers of river material and from the microfossils in seawater column. Most of the hemipelagic sediments, however, might have been extensively modified by slumping and associated gravity flow processes and were re-deposited in the forms of debrites and turbidites. Locally developed seep carbonates associated with gas hydrate dissociation and leakage provided additional sources for the gravity flow sediments.