摘要:
海底天然气渗漏是海洋环境中广泛分布的自然现象,在世界各大洋中都有发现。海底渗漏的气体赋存于浅部地层,可以改变近表层沉积物的物理性质,使其在声学剖面上得以反映。通过对南海北部陆坡某海域研究区浅地层剖面和地震数据分析,在浅地层剖面上发现了声空白、声混浊、增强反射层、速度下拉等特征,在地震剖面上则识别出气烟囱或泥底辟、亮点、速度下拉、增强反射层等特征。以似海底反射层(BSR)作为地震剖面上明显的含气层标志,划分了2套含气系统。通过浅地层剖面与地震剖面联合解释认为,BSR之下气烟囱/泥底辟的发育导致了亮点、速度下拉、增强反射层等声学特征的发生,BSR之上水合物层的存在则可能起到封堵天然气而使其发生侧向运移的作用,在气体封堵相对薄弱的位置,天然气向上运移形成声空白、声混浊、增强反射层、速度下拉等特征。以声空白代表的天然气聚集带可能成为块状水合物的发育场所,可能成为较有潜力的勘探目标。
Abstract:
Submarine gas seepage trapped in shallow strata occurs widely in the marine environment. It may change the physical properties of the sediment and leave some prints on the acoustic profiles. The sub-bottom profiles and multi-channel seismic reflection data collected from the northern slope of the South China Sea were processed and analyzed in this paper. Acoustic blank, acoustic turbidity, enhanced reflection and velocity pulling-down are discovered on sub-bottom profiles, and gas chimney or mud diapia, bright spot, velocity pulling-down and enhanced reflection on seismic profiles. The Bottom Simulating Reflector(BSR), indicator of gas occurrence on seismic profiles, is a boundary, which separate the gas system into two parts. The bright spot, velocity pulling-down and enhanced reflection caused by gas chimney or mud diapia, occur under the BSR, and the gas hydrate above the BSR may prevent gas migration upward, and force it to move laterally along the BSR. The continuous gas migration upward at the weak zone of the seal may cause the formation of acoustic blank, acoustic turbidity, enhanced reflection and velocity pulling-down. Therefore, the acoustic blank in the shallow strata, may be regarded as a right place for accumulation of block-hydrate and thus a potential exploration target.
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