海底浅层圈闭与浅层气地震反射特征对比

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

海底浅层圈闭条件是决定浅层气反射特征的重要因素,根据海底浅层圈闭条件的分析、国内外浅层气分布区反射特征与底质类型对比分析和南黄海西部地区浅部断层分析,得出结论:(1)浅层气的圈闭条件主要受海底底质类型与浅部断层控制,常见圈闭类型为岩性圈闭和断层圈闭;(2)底质类型是决定浅层气反射特征的重要因素。分析认为海底表层沉积物的泥砂含量决定了圈闭中盖层封闭能力,进而决定了浅层气聚集能力和储集量,形成不同浅层气地震反射特征;(3)某些现代三角洲地区,受潮流或河流与海洋共同作用影响,表层底质经常与整个浅层的物质组成差别较大。由于其内部一般含有较厚的细颗粒黏土质粉砂和粉砂质黏土,封闭能力较强,可封闭较多浅层气,能形成大规模的声学空白反射,但常观察到浅层气反射顶界面局部突起呈旗舰状;(4)浅部断层既可形成断层圈闭,也可充当浅层气的运移通道。断层圈闭通常较少见且难以识别,浅部断层在第四纪松散地层中更多地充当了浅层气运移的通道;(5)浅部地层存在多套含泥较多的地层且形成圈闭时,浅层气被多次圈闭,会在不同深度形成"楼层式"的多个强反射界面、声学扰动。

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 up ward. （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.