青藏高原乌丽冻土区天然气水合物成藏条件

为了探讨青藏高原乌丽冻土区水合物试验孔未钻获天然气水合物的原因,对该区第一口天然气水合物试验孔的岩心样品和周边露头样品进行了测试,结果显示,该区烃源岩有机质丰度中等,有机质类型Ⅲ型,有机质已过成熟,有利于烃类气体的生成;然而,试验孔岩心顶空气测试结果却显示,天然气组分中98%为无机成因的二氧化碳(13C-CO2,-4‰~-6‰)。结合区域地质、温压场条件等综合分析认为,该区天然气水合物气源以无机成因的二氧化碳为主,有机成因的烃类气体为辅。但是,研究区构造活动强烈且一直持续至今、深大断裂发育等因素却不利于天然气(包括无机和有机)及其水合物的保存,这可能是试验孔未钻获水合物的主要原因。另外,二氧化碳特殊的升华现象以及取样技术的不完善则可能是试验孔未钻获水合物的次要原因。

DISCUSSION ON GAS HYDRATE ACCUMULATION CONDITIONS IN WULI PERMAFROST ZONE,QINGHAI-TIBETAN PLATEAU

The first trial drilling hole of ZK1 in the Wuli area did not succeed in finding gas hydrate in the permafrost zone, the Qinghai-Tibetan Plateau. In order to find out the reasons of the failure, comparative study was made between the cores of hole ZK1 and the outcrops around the hole. It is found that the source rocks in the area is featured with the type Ⅲ of kerogen and over-maturity, and favors the generation of biogenetic gases. However, the testing results of headspace samples from the hole of ZK1 show that inorganic carbon dioxide (13C-CO2,-4‰~-6‰) accounts for 96% of gas composition. According to regional geology and temperature and pressure conditions, it is concluded that the gas source is mainly composed of carbon dioxide with few biogenetic gases. Furthermore, the strong tectonic movements up to the present and deep faults in the Wuli permafrost zone are not favorable for the preservation of organic and inorganic gases as well as their gas hydrates. The second reason is maybe related to the special sublimation of carbon dioxide and incomplete sampling technology.