Seismic Detection for Hydrocarbon in Mud Volcano-Rich Area Southwest to the Dongsha Island
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摘要: 近年来通过多道地震、浅剖及海底采样调查在东沙岛西南海域发现了大量喷溢甲烷的泥火山，通过研究泥火山可能会发现含气层，具有重大的油气资源指示意义.一条多道地震剖面显示，在该海域一泥火山MV3附近存在一个具有极性反转的埋藏背斜构造，其振幅绝对值随偏移距增大而增大.为探究该背斜是否含有烃类气体，开展了速度分析、AVO反演和频率吸收衰减分析.结果表明：该极性反转下方存在纵波低速层；该带在截距P及梯度G剖面表现为负值，在P、G交会图中落入第三象限，在横波反射率剖面中表现为正值，在泊松比变化率剖面中表现为高负值；该带高频成分快速衰减，低频成分相对增加.以上多属性特征预示着该带含气，可能与深部甲烷泄露有关.Abstract: In recent years,a number of methane-venting mud volcanoes have been found in the southwest area of the Dongsha waters from multi-channel seismic,sub-bottom profiling and dredging survey. Studying mud volcanoes has great significance for oil and gas reserves. In this paper,a buried anticline was found near a mud volcano,MV3,with reversed polarity on a multi-channel seismic profile,where the absolute amplitude increases with offset. To detect whether it contains natural gas,velocity analysis,AVO inversion and frequency absorption attenuation analysis were carried out. There is a low P-wave velocity layer under the reflector of reversed polarity. AVO inversion shows that the polarity reversal zone features negative values in intercept and gradient sections,and falls in the third quadrant in P and G cross-plot. The polarity reversal zone features positive in S-wave reflectivity section,and is highly negative in Poisson ratio change rate section. Frequency absorption and attenuation analysis shows rapid energy loss of high frequency and relative increase of low frequency. Multiple attributes point to gas accumulation there,likely associated with the methane seepage from deep.
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Key words:
- southwest to the Dongsha Island /
- mud volcano /
- multi-channel seismic /
- polarity reversal /
- AVO /
- petroleum geology
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图 2 测线MCS2504偏移剖面(a)及局部放大图(b)
蓝色方框为后续速度分析、AVO反演和频率吸收衰减分析实际范围
Fig. 2. The migrated profile of MCS2504 (a) and the snapshot (b)
图 5 速度分析实例和层速度剖面(CDP 8900-15000)
图a1、b1为速度分析双曲线拟合与反射界面同相轴的关系；图a2、b2为速度谱上能量团的拾取，折线拐点(叉号)旁边的每组数据分别为该能量团对应的双程旅行时(ms)、叠加速度(m/s)和层速度(m/s)
Fig. 5. Examples of velocity analysis and interval velocity profile(CDP 8900-15000)
图 6 P剖面(a)和G剖面(b)(CDP 8900-15000)
Fig. 6. P profile (a) and G profile (b) (CDP 8900-15000)
图 7 P、G交会图(CDP 9200-9900)
黑色高亮点大部分落在极性反转带处
Fig. 7. P, G crossplot of polarity reversal zone(CDP 9200-9900)
图 10 低频增加剖面(a)和高频衰减剖面(b)
Fig. 10. Low frequency raise profile (a) and high frequency attenuation profile (b)
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