New discovery of the permafrost gas hydrate accumulation in Qilian Mountain, China

Zhen-qun Lu, Gng-yi Zhi, You-hi Zhu, Hui Liu, Ting Wng, Hui Fng, Zhong-jun Sun,Shi-qi Tng, Yong-qin Zhng

a Oil and Gas Survey, China Geology Survey, Beijing 100083, China

b Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences, Langfang 065000, China

c Institute of Exploration Techniques, Chinese Academy of Geological Sciences, Langfang 065000, China

1. Objectives

As the discovery of gas hydrate in Qilian Mountain permafrost in 2008, the main controlling factors and distribution of gas hydrate are not clear. This restricts the further progress in gas hydrate survey in this area. Hence,China Geological Survey erected the project "Exploration of gas hydrate resources in the Qilian Mountain and its adjacent areas" continuously to better understand the gas hydrate accumulation in terms of greater gas hydrate discovery.

2. Methods

Details in the previously discovered gas hydrate were analyzed as a typical example in Qilian Mountain permafrost.The gas source, the formation conditions, the controlling factors and the accumulation distribution of gas hydrate were studied. Combined with geophysical and geochemical survey data, the deployment of gas hydrate drilling was implemented(Fig. 1).

3. Results

The maximum burial depth of gas hydrate source rock is about 2000–3500 m below the ground surface. Local fractures provide migration paths for the hydrocarbon gases to move upwardly. The mid-upper part of the fault or fracture zone can possibly serve as gas hydrate reservoir. The faults with different properties, the various genetic types of gas source,and the effective supply of gas source are the main factors of the formation and distribution of the gas hydrate.

The source rock of the gas hydrate experienced one stage of the gas generation during the Middle Jurassic (Zuo YH et al., 2016). The gas hydrate formation time should be no later than the Early to Middle Pleistocene (Qi BS et al., 2014). The gas hydrate accumulation pattern is summarized in Fig. 1.

Gas hydrate layers were newly obtained at the intervals of 188.20–209.45 m, 259.77–271.89 m, 300.09–302.19 m and 357.50–367.59 m below the ground surface in one hole (Fig.2a). An oil-bearing formation was encountered in one new hole from 362.79 m to 370.58 m below the ground surface in the sandstones (Fig. 2b). A strong gas blowout took place in another new hole at the depth of 52.9 m below the ground surface in sandstone. The flame was as high as 3–4 m (Fig.2c), lasting for more than 48 hours. In the field the gas flow was estimated greater than 4800 m3/d.

4. Conclusion

The target gas hydrate source rock is defined in this project. The migration of gas hydrate is revealed through fracture analysis. Three main geological controlling factors of the gas hydrate accumulation are found out. A gas hydrate accumulation model is established. Several gas hydrate layers were successfully obtained. A new discovery in oil prospecting was first made in Qilian Mountain permafrost zone.

Acknowledgement

This work was financially supported by National State Theme Project on Exploration and Test-production of Gas Hydrate (GZHL 20110311, DD20160223).