ABSTRACT

There are two basic distribution morphologies of gas hydrates in nature: pore filling and fracture filling. Identification of gas hydrate morphologies is essential for improved resource evaluation and exploitation. Improper exploitation may cause seafloor instability, lead to atmospheric venting, and exacerbate the greenhouse effect. To identify gas hydrate morphologies, we combine rock-physics modeling and amplitude variation with angle (AVA) analysis to study the theoretical AVA patterns of the bottom simulating reflector (BSR) beneath the pore-filling and fracture-filling gas hydrate bearing sediments (GHBS), respectively. The theoretical results indicate completely different AVA patterns between these two morphologies. The AVA of the BSR beneath the pore-filling GHBS shows a class III anomaly with negative intercept and gradient. However, the AVA of the BSR beneath the fracture-filling GHBS exhibits a class IV anomaly with a negative intercept and positive gradient. In addition, the theoretical AVA intercept and gradient are affected by the fracture orientation, manifesting an anisotropic signature varying with the fracture dip and azimuth. The processed prestack data at well sites 08 and 16 in the northern South China Sea are selected for testing our method. The AVA analysis of the actual BSRs beneath the pore-filling and fracture-filling GHBS at these sites shows class III and IV responses, respectively, in agreement with our theoretical results. The gas hydrate saturations are also estimated by comparing the theoretical AVA with the actual AVA patterns. The estimations at site 08 are close to those estimated from the pore-water freshening.

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