Geophysical characterization of gas hydrate reservoir in fine-grained sediment in Shenhu area

doi:10.13745/j.esf.sf.2024.6.50

Abstract

Abstract:

The gas hydrate reservoir in the Shenhu area of the northern South China Sea is primarily composed of fine-grained sediments with a saturation as high as 30%, which is significantly higher than other reservoirs worldwide. Studying the characteristics of fine-grained reservoirs in this area is crucial for understanding the mechanism of hydrate accumulation, resource evaluation, and the design of hydrate trial production and development strategies. In this study, we conducted detailed processing of multi-channel seismic data in the Shenhu area to obtain velocity models and stacked profiles with different offsets. Subsequently, pre-stack seismic inversion was performed to derive high-resolution elastic parameters, including P-wave and S-wave impedance, P-wave to S-wave velocity ratio, and Poisson’s ratio. These results were combined with well-logging data to provide a refined geophysical description of gas hydrates and free gas in the fine-grained sediment. Our study reveals the following: (1) The amplitude of the BSR (bottom-simulating reflector) decreases as the incident angle increases, making it easier to identify the BSR on near-offset stacked profiles; (2) Chimney structures with low-velocity anomalies develop below the BSR and may serve as pathways for vertical fluid migration; (3) Hydrate reservoirs in the study area exhibit strong horizontal heterogeneity, and free gas is generally present beneath the hydrate with high saturation, suggesting that hydrate recycling may play an important role in hydrate enrichment in fine-grained sediments.

Key words: gas hydrate, seismic imaging, pre-stack seismic inversion, reservoir characterization

CLC Number:

P618

JI Mengfei, WANG Jiliang, WANG Weiwei, ZHANG Jiecheng, LIU Xueqin, WU Shiguo. Geophysical characterization of gas hydrate reservoir in fine-grained sediment in Shenhu area[J]. Earth Science Frontiers, 2025, 32(2): 126-139.

Figures/Tables 12

Fig.1 The location of the study area

Fig.2 Flows of seismic data processing

Fig.3 Migration velocity analysis for CMP 3600

Fig.4 Interactive velocity analysis a—Velocity spectrum; b—The CMP gather without NMO; c—The CMP gather with NMO.

Fig.5 The overlap of seismic section and interval velocity section

Fig.6 Comparison of migrated seismic sections before and after fine velocity analysis in the study area (a) Migrated seismic section without high-resolution velocity model; (b) Migrated seismic section with high-resolution velocity model. The white short lines indicate the BSR, the solid red lines mark the faults, and the dashed blue lines represent the approximate extent of the gas chimney.

Fig.7 The different angle stack sections (a) 0~10° stack section; (b) 11°~20° stack section;(c) 21°~30° stack section.

Fig.8 Well-seismic tie at Site SH-02 (a) The observed seismogram near the well; (b) The synthetic seismogram; (c) The cross-correlation between the real and synthetic seismogram; (d) Extracted seismic wavelet.

Fig.9 The inversion region and result profiles (a) The target region and well location; (b) P-wave impedance section and well curve; (c) S-wave impedance section and well curve; (d) Poisson ratio section.

Fig.10 Local enlarged seismic profile and multi-elastic parameter profiles of hydrate reservoir: (a) The locally enlarged seismic profile; (b) P-wave impedance; (c) Poisson ratio; (d) S-wave impedance;(e) Lambda*Rho; (f) Mu*Rho. White arrows indicate hydrates, and black arrows indicate free gas.

Fig.11 The cross-plot of P-wave and S-wave impedance curves in SH-02. The color bar represents resistivity value.

Fig.12 Characterization of hydrate and free gas distribution:(a) P-wave impedance of hydrate reservoir and its local magnifications;(b) P-wave impedance of free gas reservoir and its local magnifications.

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