珠江口盆地东部海域近海底天然气水合物地震识别及地质成因

Seismic identification and geological origin of gas hydrate in near seafloor sediments in the eastern part of the Pearl River Mouth Basin

2013年珠江口盆地东部海域钻探取样结果表明,天然气水合物以不同形态分布于海底之下约10m至稳定带之上的不同深度区间。在GMGS2-08井位置,天然气水合物分别以脉状、块状形态出现在海底之下9~25m和65~80m(与碳酸盐岩有关)深度范围内。地震反射剖面上,含分散状水合物沉积层的底部深度与地震反射剖面上的似海底反射深度很好对应,但没有典型地震反射特征表明近海底沉积物中存在天然气水合物。在研究区另一钻探位置(GMGS2-16),取样也获得存在于近海底处的天然气水合物样品。本文研究以多道高分辨地震数据为基础,利用含天然气水合物沉积物具有较高声波速度的特征,在正演分析近海底存在薄高速层时地震反射速度变化规律的基础上,然后采用沿层速度分析方法获取近海底处的速度,据此推测近海底处天然气水合物的分布特征。结果表明,过GMGS2-08井和GMGS2-16井的地震反射数据的海底速度相干反演表现出明显的高速异常,而在GMGS2-11和GMGS2-12井位置则没有任何速度异常,与钻探取样结果一致。将地震数据的反射特征与区域地质条件进行综合分析,进一步预测这一区域近海底天然气水合物分布模式及地质成因。本文的研究结果为近海底天然气水合物地震探测和识别提供了有效手段,对天然气水合物资源勘探以及深海油气工程安全都具有重要意义。

The results of drilling and core sampling in the eastern part of the Pearl River Mouth Basin in 2013 reveal that the gas hydrate bearing sediments could be present at different depth intervals with different morphologies between the rough 10 mbsf and base of the gas hydrate stability zone.At site GMGM2-08,the massive gas hydrates were visible at the depth interval of 65-80 mbsf in association with carbonate platforms;moreover,the gas hydrate recovered as thin veins in shallow fine-grained sediments at depths 9-25 mbsf unexpectedly.The presence of both forms of gas hydrate show no marked characteristics reflecting on the seismic profiles such as BSR,blanking zone,etc.In contrast,the layers of disseminated gas hydrate in deeply buried fine-grained sediment sequences lie just above the base of the gas hydrate stability zone with classical high-amplitude BSRs.At another site GMGS02-16,the gas hydrate bearing sediments just below the seafloor about 12 m was also recovered.Since the sediment velocities are markedly increased by the inclusion of highvelocity hydrate grains,the study focused on detailed velocity analysis for the thin layers just below the seafloor using high resolution multi-channel seismic data.The forward modeling shows that the presence of a thin layer lying below the seafloor can cause velocity values to vary on the coherency spectrum hence deviate from model parameter.Based on the results of forward modeling,the coherency inversion of horizontal velocity along the seafloor was applied to calculate the velocity deviance on real seismic data across the drilling sites,from which the presence or absence of gas hydrate lying just below the seafloor would be inferred.The horizontal velocity of seafloor appears to be increasing at sites GMGS2-08 and-16,matching well with the presence of gas hydrate in near surface sediments;however it varies little at sites GMGS2-11 and-12 comparing to that of seafloor（1500m/s）.Linking regional geological setting conditions and coincident seismic data to the discovery of drilling and coring at sites helps to further analyze the origin of gas hydrate distribution in such shallow sediments just below the seafloor.Methods developed in this paper provide a viable solution for detecting the gas hydrate in near-surface sediments with seismic reflecting data,it has great significance for resource exploration of gas hydrate as well as safety management in deepwater petroleum engineering.