Combining low frequency spectral decomposition and post-stack seismic inversion to identify Middle Miocene gas bearing sands at Hai Thach field
Abstract
Located in the Nam Con Son basin, offshore Vietnam, Middle Miocene gas bearing turbidite sands at Hai Thach field are very difficult to identify on conventional seismic data due to their complex architectures and geometries [1]. The purpose of this study is mapping this reservoir in terms of its geometry and distribution by applying an integration of rock physics analysis, seismic attributes and post-stack seismic inversion. In the study areas, the P-impedance of sand is the same values with shale and the presence of gas in sand causes a decrease of acoustic impedance, which can be used to differentiate gas saturated sandstone from water saturated sandstone and shale. Seismic attributes applied based on the rock physics analysis result can also be used to define gas sand distributions. Low-frequency amplitude anomalies from 17 - 19Hz, obtained from spectral decomposition, have a high gas content where penetrated by wells, whereas low gas saturation zones in the wells do not correlate with the observed high amplitudes. Moreover, the gas sand distribution was successfully identified in the extracted P-impedance slice along horizon from an inverted P-impedance volume. Thus, the combination of low frequency spectral decomposition and seismic inversion may be used to successfully map gas distribution. Proposed workflow for mapping sand and gas sand can be used for future drilling programmes.
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