珠江口和琼东南盆地天然气水合物形成和稳定分布的地球化学边界条件及其分布区

GEOCHEMICAL CONSTRAINTS AND POTENTIAL DISTRIBUTIONS OF GAS HYDRATES IN PEARL RIVER MOUTH BASIN AND QIONGDONGNAN BASIN IN THE NORTHERN MARGIN OF THE SOUTH CHINA SEA

通过对南海北部陆缘珠江口和琼东南盆地气田的天然气形成水合物的地球化学计算模拟及地质地球化学条件分析 ,对珠江口和琼东南盆地天然气形成水合物的地球化学边界条件及分布区进行了研究。认识到南海北部陆缘琼东南和珠江口盆地内的断裂构造是天然气向海底渗漏的通道 ,为天然气水合物在海底的形成提供了物源 ;盆地内巨厚的第四纪富有机质沉积也为天然气水合物形成提供了充足的细菌成因生物气源。在海底温度 2～ 16℃范围内 ,琼东南盆地气田 10种天然气和珠江口盆地气田 18种天然气形成水合物的压力有比较大的范围 ,随温度增高 ,天然气水合物形成的压力增高 ;盆地间和各天然气样品之间形成水合物的压力均是不一致的。在南海海水平均盐度 3.4%条件下 ,结合海底温度与水深变化资料 ,珠江口和琼东南盆地天然气水合物形成和稳定分布的海区是不同的 ,珠江口盆地小于 2 30 m水深的海区没有天然气水合物的形成 ,在 2 30～ 76 0 m水深的海区可能有天然气水合物的存在 ,天然气水合物的稳定分布区应该在大于 86 0 m水深的深水区 ;在琼东南盆地水深小于 32 0 m的海区不可能有天然气水合物的形成 ,在 32 0～ 6 5 0 m水深的海区可能有天然气水合物的存在 ,大于 6 5 0

We predicted the pressure ranges and potential distributions of gas hydrates for natural gas in Pearl River Mouth basin and Qiongdongnan basin in the northern margin of South China Sea under several fixed temperature conditions. Numerous faults in the basins are good pathways for gas seepages from deep to seafloor, and huge thick sediments containing high content of organic carbon may have produced a lot of biogenic gases, which are gas sources for hydrates. 10 gas samples from Qiongdongnan basin and 18 ones from Pearl River Mouth basin have hydrates formed in large pressure ranges under temperatures from 2 ℃ to 16 ℃ with sea water of 3 4% salinity and the pressure increases with temperature getting higher, and there are differences between basins and samples.The formation and stable areas of gas hydrates are different in the two basins,decided by the relations between seafloor temperature and water depths. The hydrates could not be formed in an area with a water depth less than 230 m, but occur in areas where water depths are from 230 m to 760 m, and stable areas are in areas with water depths larger than 860 m in Pearl River Mouth basin. But hydrates are not found in areas with water depths less than 320 m,formation may be limited from 320 m to 650 m in water depth and stable area lies at water depth larger than 650 m in Qiongdongnan basin.