Coexistence of natural gas hydrate, free gas and water in the gas hydrate system in the Shenhu Area, South China Sea

• Published in: China Geology
• Main Authors: Xu-wen Qin, Jing-an Lu, Hai-long Lu, Hai-jun Qiu, Jin-qiang Liang, Dong-ju Kang, Lin-sen Zhan, Hong-feng Lu, Zeng-gui Kuang
• Format: Article in Journal/Newspaper
• Language: English
• Published: KeAi Communications Co., Ltd. 2020
• Subjects: Mixing layer; Gas hydrate; NMR logging; Sonic logging; Core analysis; Oil gas exploration engineering; Engineering (General). Civil engineering (General); TA1-2040; Geology; QE1-996.5; Methane hydrate
• Online Access: https://doi.org/10.31035/cg2020038; https://doaj.org/article/16129db3ed424fad84240b38da7632b7

ABSTRACT: Shenhu Area is located in the Baiyun Sag of Pearl River Mouth Basin, which is on the northern continental slope of the South China Sea. Gas hydrates in this area have been intensively investigated, achieving a wide coverage of the three-dimensional seismic survey, a large number of boreholes, and detailed data of the seismic survey, logging, and core analysis. In the beginning of 2020, China has successfully conducted the second offshore production test of gas hydrates in this area. In this paper, studies were made on the structure of the hydrate system for the production test, based on detailed logging data and core analysis of this area. As to the results of nuclear magnetic resonance (NMR) logging and sonic logging of Well GMGS6-SH02 drilled during the GMGS6 Expedition, the hydrate system on which the production well located can be divided into three layers: (1) 207.8–253.4 mbsf, 45.6 m thick, gas hydrate layer, with gas hydrate saturation of 0–54.5% (31% av.); (2) 253.4–278 mbsf, 24.6 m thick, mixing layer consisting of gas hydrates, free gas, and water, with gas hydrate saturation of 0–22% (10% av.) and free gas saturation of 0–32% (13% av.); (3) 278–297 mbsf, 19 m thick, with free gas saturation of less than 7%. Moreover, the pore water freshening identified in the sediment cores, taken from the depth below the theoretically calculated base of methane hydrate stability zone, indicates the occurrence of gas hydrate. All these data reveal that gas hydrates, free gas, and water coexist in the mixing layer from different aspects.