Drilling discoveries and accumulation characteristics of gas hydrate in the Northern Slope of South China Sea

doi:10.13745/j.esf.sf.2024.6.56

Abstract

Abstract:

Gas hydrates have garnered significant attention due to their potential as an energy resource. Since the 1990s, China has been conducting gas hydrate investigations in the South China Sea, with the Guangzhou Marine Geological Survey leading nine drilling expeditions that have achieved notable breakthroughs in gas hydrate exploration. These drilling results confirm the occurrence of various types of gas hydrates in the northern South China Sea. Diffusion gas hydrates are prevalent in the Shenhu area, vent and sandy gas hydrates are primarily found in the Qiongdongnan area, and complex gas hydrates are present in the Dongsha area. This paper integrates drilling results to describe the seismic reflection and logging response characteristics of these gas hydrate systems. The diffusion gas hydrate system is characterized by a prominent bottom simulating reflector (BSR) and strong positive polarity reflections above the BSR in seismic data. Logging data reveals high resistivity, high P-wave velocity, and high S-wave velocity. In the mixed layer, where gas hydrate coexists with free gas, logging data show high resistivity, low P-wave velocity, high S-wave velocity, and a neutron-density crossover. The vent gas hydrate system typically lacks a distinct BSR but exhibits bright reflections above the BSR, pull-up reflections, and columnar blanking reflections in shallow strata on seismic data. Additionally, anomalous geomorphological features such as seafloor mounds and pockmarks are commonly associated with vent gas hydrate systems. Logging data for vent gas hydrates shows extremely high resistivity, slightly increased P-wave and S-wave velocities, and significant changes in bedding dip and/or azimuth. Sandy gas hydrates are identified by very strong BSRs and strong positive polarity reflections on seismic data, indicating the presence of sandy reservoirs. Logging data for sandy gas hydrates are characterized by low gamma-ray values, very high resistivity, very high P-wave and S-wave velocities, slightly increased density, and decreased neutron porosity. The study also summarizes the controlling factors for gas hydrate formation in the Shenhu and Qiongdongnan areas. In the Shenhu area, the development of concentrated gas hydrate reservoirs is primarily controlled by deep, large faults and inclined levee deposits. In contrast, gas hydrate formation in the Qiongdongnan area is influenced by paleoburied hills and differential compaction.

Key words: gas hydrate drilling, diffusion gas hydrate, vent gas hydrate, sandy gas hydrate, northern South China Sea

CLC Number:

KUANG Zenggui, REN Jinfeng, DENG Wei, LAI Hongfei, XIE Yingfeng. Drilling discoveries and accumulation characteristics of gas hydrate in the Northern Slope of South China Sea[J]. Earth Science Frontiers, 2025, 32(2): 1-19.

Figures/Tables 15

Fig.1 Locations of gas hydrate drilling legs in the northern South China Sea. Expedition of GMGS7 was halted due to bad weather.

Fig.2 Seismic profiles across wells drilled in GMGS1

Fig.3 Seismic profiles across wells drilled in GMGS34

Fig.4 Logging response of gas hydrate and free gas in SH-W17-2015

Fig.5 Seismic profiles across wells drilled in GMGS2

Fig.6 Seismic profiles across wells drilled in GMGS5

Fig.7 Logging response of gas hydrate in QDN-W09-2018 a—62-63 mbsf (The location of the sample is indicated as a red box on the left), b—102-103 mbsf are CT images of pressure cores.

Fig.8 Seismic profiles across wells drilled in GMGS6-9

Fig.9 Logging response of gas hydrate in QDN-W20-2021

Fig.10 The characteristics of dee-prooted faults and fluids migration model in Shenhu area

Fig.11 The distribution of fault, gas chimney and BSR and the locations of the drilling wells in Shenhu area.

Fig.12 The coupling structuresedimentary control model in Shenhu gas hydrate accumulation where gas hydrate production test has been carried out in 2017 and 2020

Fig.13 RMS (root mean square) seismic amplitude attribution map of the base of gas hydrate stability zone in Qiongdongnan area

Fig.14 The model of fluid flow in W09 gas hydrate accumulation in Qiongdongnan area

Fig.15 The model of fluid flow in W08 gas hydrate accumulation in Qiongdongnan area

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