AVO Attribute Analysis for Gas Hydrate in Shenhu Area

Yang, Rui; Nengyou, Wu; Zhibin, Sha; Jinqiang, Liang

The variation in seismic energy with change in distance between shot point and receiver reflects differences in lithology and fluid content in strata. AVO (Amplitude Versus Offset) analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity and fluid content of strata. AVO analysis has gained lots of success both onshore and offshore oil and gas exploration, however, it is still in the initial stage in offshore gas hydrate exploration. Firstly, the abnormal AVO attributes are difficult to distinguish when the free gas saturation of sediment is high or low enough. Secondly, the relationship among gas hydrate, free gas saturation and elastic parameters are not clear. Thirdly, the AVO results fall under the influence of thin-layer, even show multiple solutions. But AVO analysis showed its broad prospects because the free gas can be distinguished effectively from the results.

In 2003, Guangzhou Marine Geological Survey (GMGS) of China Geological Survey carried out the high-resolution seismic exploration on the continental slope of Northern South China Sea. And in 2007, GMGS carried out the gas hydrate drilling expedition. Many scientific data and samples such as the wire-line logs, in-situ temperatures, properties, geochemical data and sediment cores, pore water, head space and void gas, etc. were obtained in Shenhu area of the northern slope of South China Sea.

In this research, seismic Line A crossing the drilling site SH2 at which gas hydrate-bearing sediments were recovered between 188m and 228m below seafloor, was selected to study the AVO attributes of gas hydrate in Shenhu area of northern South China Sea. For improving the Signal to Noise Ratio (SNR) of seismic data, amplitude preservation Kirchhoff pre-stack migration has been applied to the 3-D seismic data. Some representative sections were showed, and the characteristics of AVO response for gas hydrate and free gas were analyzed, as well, distribution of thin layers of free gas which were related with gas hydrate were discussed. Lastly, the results showed that intercept, gradient, fluid factor and Poisson's ratio change clearly reflect the location of free gas and BSR, and the spatial relations among blanking zone, BSR, gas hydrate and free gas.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013