LU Zheng-quan¹, WU Bi-hao¹, WANG Zai-min², ZHANG Fu-yuan^3
1Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China, luzhq@vip.sina.com
²Institute of Geophysical and Geochemical Exploration, Langfang, Hebei, China
³The Second Institute of Oceanography, State Bureau of Oceanography, Hangzhou 310012, China

　　Gas hydrate, mainly composed of hydrocarbon and water under low temperature and high-pressure conditions, is taken as the 21th clean energy. In the South China Sea (SCS), the continental or island slopes are suitable for gas hydrate to grow in consideration of temperature, pressure and hydrocarbon gases As a matter of fact, in recent years, many indicators such as BSRs which are thought to be related to gas hydrate are found in the SCS. In the eastern island slope of the SCS, less work is carried out on gas hydrate exploration than that in the northern slope. However, the eastern island slope of the SCS is a convergent island margin that the Pacific plate is subducted by the SCS plate, which may result in a large volume of quick sedimentations abundant in organic matter in a short period and might produce a lot of fluids containing hydrocarbon gases, so it is preferable for gas hydrate to occur in the sediment. Such many cases are reported abroad as gas hydrates lie in the convergent margins in the world. For instance, gas hydrate samples are dug out in Japanese Nankai trough and Hydrate ridge offshore Canada that are both situated in convergent margins. Over the past few years, with the support of the relevant units and partners, we get a large quantity of sediment samples of the eastern island slope, and we carry out research into acid degassing hydrocarbon geochemistry and sediment radioactive thermoluminescence.
　　Through our work, the contents of hydrocarbon gas acid-degassed from subsurface sediments change from 7.2 to 393 μL methane per kilogram sediment. According to the study of geochemical statistics on the data, the set of the data basically shows logarithmic normal distribution pattern. The geometric average is 60.53 μL/kg and the standard deviation is 91.97 μL/kg. So the lowest anomalous value is 152.5 μL/kg on the basis of the geometric average plus the standard deviation. Hence the points above the lowest anomalous value amount to twelve, accounting for 25.5 percent of the total.
The high radioactive thermoluminescence value of subsurface sediment varies from 108 to 31752 units. By and large, the set of the data comforms to the logarithmic normal distribution pattern. The geometric average is 2688.9 units and the standard deviation is 6795.8 units, so the lowest anomalous value is 9484.7 units on the basis of the geometric average plus the standard deviation. Hence the points above the lowest anomalous value amount to eight, accounting for 17 percent of the total.
　　It is interesting that most of the points of the high hydrocarbon gas content are coincident with those of high thermoluminescence value These high contents of hydrocarbon gas and high thermoluminescence value may be associated with oil & gas seepage or disassociation of gas hydrate Thus the points of data above the anomalous value may be indicative of existence of oil & gas or gas hydrate. What is more exciting is that among the points there are three sites where the methane contents are up to 243, 268 and 359 μl/kg and their radioactive thermoluminescence values are 8430, 9537 and 20826 unit respectively, and in the very vicinity of the three sites there are lots of BSR locations to the southwest of Taiwan identified by Chi (1998), furthermore there also exists one of the highest confident BSRs. The area is just situated in the conjunction of the northern divergent continental margin and the eastern convergent island slope whose geological settings are much preferable for gas hydrate to occur. Maybe the three sites with high contents of hydrocarbon gas and high thermoluminescence value are potential locations of gas hydrate.
　　Hereby it is inferred that all the points with high values are possibly indicating the occurrence of gas hydrate in the eastern island slope. These points are mainly distributed in the conjunction area of the northern continental margin and the eastern island slope and in the verge area of Bijianan basin.