Forward modelling and identification of shallow gas in the Bohai Bay seabed

Abstract

The accumulation of shallow gas in the seabed reduces the strength of strata or forms a high-pressure air sac, endangering ocean engineering construction. Therefore, it is important to identify the distribution of shallow gas in the seabed within the study area. Shallow gas increases the soil mass porosity and reduces the acoustic wave velocity, causing attenuation by absorbing to high-frequency components in the acoustic waves. Based on the geological drilling data in the area surrounding an oil platform in Bohai Bay, a stratigraphic model was established for forward analysis, and the results suggest the presence of the phase inversion of reflective waves at the interface between shallow gas and strata and sunken events for the lower shallow gas. According to a survey of stratigraphic profiles surrounding the platform, a seismic attribute analysis of acoustic stratigraphic profile data concerning amplitude, instantaneous phase, and instantaneous frequency was carried out, and characteristics such as disordered weak amplitude reflection, phase inversion, sunken events and indicators, including high-frequency loss and shallow gas reflection, were identified. Given that the shallow gas reflection is columnar and ended at the top clay strata of the seabed, the shallow gas was probably produced from deep depths.