用地球化学方法勘查中国南海的天然气水合物

天然气水合物是一种未来新型能源，赋存于低温高压环境下的海洋沉积物中，但也可形成于大陆永久冻土带中。天然气水合物资源量巨大，具有经济和环境上的研究意义。近年来，国际上己对天然气水合物的产况、分布和形成机理开展了大量研究，但国内这方面的工作还刚刚开展。对中国南海的调查表明该区存在天然气水合物赋存的有利地质条件、温压条件和富含有机质的沉积条件。在南海的许多海区还发现了指示天然气水合物存在的地震标志(BSR)。介绍了在南海天然气水合物勘查中的地球化学异常标志。这些地球化学异常的产生可能与天然气水合物的形成或分解过程有关。研究内容包括沉积物中气体含量(主要为甲烷和乙烷)，甲烷的碳同位素，孔隙水中阴离子(Cl^-、SO4^2-等)、阳离子(Ca^2 、Mg^2 、Ba^2 、Sr^2 ，B^3 和NH4^ 等)浓度和δ^18，δD，δ^11B，及^87Sr／^86Sr等同位素组成，此外还对海底沉积物的热释光特征和紫外、可见、近红外反射光谱特征开展了探索性研究。通过进一步加强理论和实验研究，结合地球物理和地球化学资料，在不远的将来将会在南海发现和圈定天然气水合物矿藏。

SEARCH FOR GAS HYDRATES IN THE SOUTH CHINA SEA:A GEOCHEMICAL APPROACH

Gas hydrate is a potential new energy source,which occurs both in marine sediments and in permafrost regions.Gas hydrates are in vast amount and show potential economic and environmental significance.In recent years,natural gas hydrates have increasingly attracted worldwide attention and a vast amount of research work has been conducted regarding the occurrence,distribution,and formation mechanism of marine gas hydrates in the world's ocean.However,the gas hydrate research and exploration in China is just starting.In the South China Sea,several lines of evidence have shown favorable conditions for gas hydrate existence,including the geologic and tectonic settings,the temperature-pressure conditions,and the thick organic-rich sediments.Direct evidence for gas hydrate,such as the bottom-simulating reflector (BSR) on seismic-reflection profiles,has already been observed in many regions in the South China Sea.Furthermore,initial geological,geophysical and geochemical investigations in the South China Sea have suggested great promise for the occurrences of gas hydrates. In this paper,we discuss all available geochemical parameters that have been used and proved to be diagnostic tools for the search of gas hydrates.A number of geochemical anomalies may be related to the formation and dissociation of gas hydrates.They include gaseous hydrocarbon contents (mainly methane and ethane) in the marine sediments,carbon isotopes of the methane,downward and spatial profiles of Cl-,SO2-4 and major cations (e.g.,Ca,Mg,Ba,Sr,B,and NH4) contents,and δ18O,δD,δ11B,and 87Sr/86Sr ratios of the pore water.There are also many other potential indicators for the gas hydrates,such as thermoluminescence anomaly,reflectance spectra using diffuse reflectance spectrometry,and biogeochemical evidence.It is suggested that with the progress in theoretical and experimental study and intensive geophysical and geochemical explorations,we will finally target the gas hydrate occurrences in the South China Sea in the near future.