Identification of Gas Accumulation Adjacent to the Gas Hydrate Bearing Zone by Inversion Utilizing AVO Attribute

Abstract

The presence of gas hydrate and associated gas in the Makran Accretionary Prism, offshore Iran, has been confirmed by various seismic indicators, including the bottom-simulating reflector (BSR), amplitude blanking, and bright spot. The pre-stack analysis and the study of amplitude-variation-with-offset (AVO) attributes are the most reliable tools for semi-quantitative study of gas hydrate and free gas resources. Due to the lack of well data in the most regions of gas hydrate accumulations in deep oceans, the study of AVO attributes and application of seismic inversion techniques are solutions to evaluate both gas hydrate and the free gas trapped beneath it. Conventional seismic stacked data increases the signal-to-noise ratio; however, amplitude stretching in far offsets resulted in normal move-out (NMO), and Poisson’s error in velocity analysis decreases the frequency content of stack section. Moreover a stacked seismic data do not present a real zero-offset seismic data. In this study, the AVO intercept section was used to remove far offset effects and produce a more realistic zero-offset section with a high frequency content. Consequently, the acoustic impedance produced from the inverted intercept section shows acoustic property of the sediment in a better accuracy. According to the inverted section from AVO intercept data, free gas zone, and double bottom-simulating reflector, which identified about 50 ms below the first BSR, were observed with high vertical resolution..

Keywords

• amplitude-variation-with-offset (avo)
• intercept attribute
• gas hydrate
• free gas
• double bottom-simulating reflector (dbsr)