Summary

This paper analyzes amplitude behavior of gas hydrate from stacked seismic data, based on rock physics properties of hydrate-saturated sediments, gas saturated sediments, and water saturated sediments. Gas hydrate is likely to occur in suitable gas hydrate occurrence environment. Velocity of hydrate saturated sediments increases as gas hydrate replaces water in pore space of the sediment. In contrast, velocity of gas filling sediment decreases. The different patterns that consist of low concentrated gas hydrate, high concentrated gas hydrate, water and/or gas produce Dim out and Bright spots. The two gas hydrate indicators are illustrated in a 3D seismic data from the Green Canyon area of the Gulf of Mexico.

Introduction

Gas hydrates are ice-like solid composed of gas molecules enclosed in cages of water molecules. Under favorable conditions, they occur in marine sediments in deep water. Gas hydrates can be detected from seismic data by observations of Bottom-Simulating Reflectors (BSR). A BSR is parallel to the seafloor reflector and has the opposite polarity. In sediments, gas hydrate usually grows in the pore space. Pure gas hydrate has a P-wave velocity of 3.65-3.75 km/s (Helgerud et al. 1999). Because gas hydrate has higher velocity than those of pore-filling fluids, gas hydrate saturated sediment exhibits relatively high velocity compared to water filling sediment. Below the BSR, a low velocity layer about 1.2-1.5 km/s is often observed, which is caused by that gas displaces water in the pore space. However, Numerous expeditions have shown that the presence of a BSR does not correlate to high concentrations of gas hydrate above the BSR (Lu and McMechan, 2002; Tréhu et al., 2003). On the other hand, gas hydrate has been detected in some areas without BSR such as Gulf of Mexico. In the paper, we interpret a 3D seismic data on Gulf of Mexico by within a suitable gas hydrate occurrence environment and physical properties of sediments. Our results show two gas hydrate indicators in sands: dim out and bright spots.

Geologic setting and Data set

The study area is in the eastern Green Canyon area on the upper continental slope off Texas and Louisiana where near-surface geology is dominated by active salt tectonics and rapid sea-level-driven sedimentation. The mobile salt has extensively fractured the overlying sediments with regional growth faults and associated fault types. These faults act as conduits for the migration of hydrocarbons from deep layers to the shallow section. The terrigenous coarse-grained sediments in the upper continental slope are mainly deposited during sea level lowstands. The data used for this study is a poststack Kirchhoff time-migrated 3D seismic acquired by WesterGeco.

Suitable gas hydrate occurrence environment

Recent studies suggest a concept of gas hydrate petroleum systems to identify the occurrence of gas hydrate in coarse grained sandy deposits (Hutchinson et al., 2008; Jones et al., 2008). The concept emphasizes the sandy deposits within gas hydrate stability zone, a gas source close to the deposits; and a migration pathway which can transport gas into the deposits (Ruppel et al., 2008).

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