CONTRAST BETWEEN TRAPS AT THE SHALLOW SUBBOTTOM DEPTH AND THE SEISMIC REFLECTION FEATURES OF SHALLOW GAS

Trap conditions at shallow subbottom depths are important and decisive to seismic reflection of shallow gas.Based on analysis of the trap conditions,we conclude that there are two kinds of traps normally:lithologic trap and fault trap.According to the seismic reflection features of shallow gas,subbottom sediment,and shallow faults in the western South Yellow Sea,we conclude that:(1)The traps of shallow gas at the shallow depths are mainly controlled by the bottom sediment and shallow faults,and the traps are usually lithologic and fault ones. (2)The bottom sediment is normally important and decisive to the seismic reflection features of shallow gas.(3)Under the actions of tidal current,river and ocean,bottom sediments in modern deltas are greatly different from the shallow strata beneath them.As the sediments of modern delta have thick muddy silt and silty muddy layers,their trap ability is strong,and they can trap enough shallow gas to form large scale of acoustic blanket,but projecting flag shape at its top boundary is often observed. (4)Shallow faults can form fault trap,and also can be the passway for shallow gas to migrate upward.(5)If mud cencenrtation is high in some strata, shallow gas will be trapped several times,and reflection boundary and acoustic turbidity showed as stairs can be observed.