Geophysical characteristics and identification of the coexistence of gas hydrate and free gas

doi:10.13745/j.esf.sf.2024.11.22

Abstract

Abstract:

Numerous drilling expeditions in the world’s oceans have confirmed the coexistence of gas hydrate and free gas. The geophysical anomalies and enrichment patterns of these coexistence zones vary across different regions, making their identification crucial for accurately evaluating gas hydrate resources. This study analyzes a variety of logging and seismic data from typical drilling sites in the South China Sea and other global oceans to explore the geophysical characteristics of gas hydrate and free gas coexistence zones. Four distinct types of coexistence are identified: (1) coexistence near the base of the gas hydrate stability zone (BGHSZ) in fine-grained, clay-rich reservoirs; (2) coexistence below the bottom simulating reflector (BSR) in fine-grained, clayey silt reservoirs; (3) coexistence above the BGHSZ in cold seep systems; and (4) coexistence in coarse-grained sand reservoirs in high-sedimentation zones. Crossplot analysis and intersection analysis of multiple attributes are effective methods for identifying these coexistence zones. The results indicate that both biogenic and thermogenic gases contribute to the coexistence of gas hydrate and free gas. In gas hydrate systems formed by biogenic gas, high sedimentation rates cause the BGHSZ to shift upward, leading to gas hydrate dissociation. However, this dissociation process requires time, and localized coexistence of gas hydrate and free gas may occur during specific periods. In contrast, gas hydrate systems formed by thermogenic gas often display widespread coexistence of structure II gas hydrate and free gas due to distinct controlling factors. The study highlights that crossplots of porosity and resistivity contrast versus water-saturated layers can be used to identify gas hydrate morphologies based on changing trends within coexistence zones. Additionally, geophysical property analysis reveals that S-wave velocity is a key parameter for distinguishing between gas hydrate and free gas enrichment in these zones. This research provides valuable insights into geophysical characteristics and identification methods for gas hydrate and free gas coexistence zones, contributing to the accurate evaluation of gas hydrate resources.

Key words: gas hydrate, free gas, coexistence, shear wave velocity, crossplot analysis

CLC Number:

P618.13

WANG Xiujuan, HAN Lei, LIU Junzhou, JIN Jiapeng, KUANG Zenggui, ZHOU Jilin. Geophysical characteristics and identification of the coexistence of gas hydrate and free gas[J]. Earth Science Frontiers, 2025, 32(2): 20-35.

Figures/Tables 12

Fig.1 Distribution map of gas hydrate and free gas coexistence zones in the world’s oceans

Fig.2 Conceptual models of gas hydrate and free gas coexistence under different geological conditions

Fig.3 Comparative log responses of connected wells for gas hydrate, free gas, and coexistence layers in the Pearl River Mouth Basin, South China Sea (modified from [15,19]).

Fig.4 Seismic profile across Sites W11 and W17 in the Pearl River Mouth Basin, South China Sea

Fig.5 Log response characteristics of gas hydrate and coexistence layers at Site DCE, northwest of Borneo, South China Sea (modified from [13]).

Fig.6 Seismic profile across connected wells in the northwest of Borneo, South China Sea (modified from [13]).

Fig.7 Log response characteristics at Site 892 on the continental margin of Cascadia

Fig.8 Seismic profile across Site 892 on the Cascadia margin (modified from [31])

Fig.9 Log response characteristics at Site U1519 on the continental margin of New Zealand

Fig.10 Comparison of BSR characteristics in seismic profiles on the continental margin of New Zealand

Fig.11 Conceptual diagram showing the upward shift of the base of the gas hydrate stability zone due to rapid sedimentation

Fig.12 Crossplot of porosity difference versus resistivity difference at Sites GMGS3W17 and DC-E

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