Geological constraint on gas hydrate accumulation in Qilian Mountain permafrost----a case in DK-9 hole

摘要

Gas hydrate was successfully sampled again in DK-9 hole in QilianMountain permafrost. However gas hydrate occurrences areheterogeneous both in horizon and in section within a limited area.What geological constraint affects gas hydrate occurrences is notknown yet. Since features of gas hydrate and other related geologicalinformation were well recorded in DK-9, a possible answer is putforward based on analyses of geochemical and geological data in thishole. In this paper, features of hydrocarbon in headspace gases fromcores at various depths are compared with occurrences of gas hydrate,faults or fractures in DK-9 in the Qilian Mountain permafrost. Theresults show that gas contents are relatively clearly higher at theintervals of 180.26m~308.50m, 356.45m~399.32m, 458.55m~508.65mthan other intervals. Interestingly gas hydrate and its related anomaliesfall into the intervals with higher gas contents. In the meanwhile,hydrocarbon concentrations in headspace gases within the faults orfracture zones are higher than those above the faults or fracture zones.This may suggest that faults or fracture zones have great impact on gashydrate occurrences. Further comparative analyses on spatial relation ofgas hydrate occurrences to faults or fracture zones, reveal that faults orfracture zones serve as migration paths for gases in the deep andprovide accumulation space for gas hydrate in the shallow in the QilianMountain permafrost. Actually geological data show that gas hydrateoccurrences are limited within the belt controlled by faults of F1 and F2in a plane and are mainly within the lower block of these faults in aprofile. Hence it is preliminarily drawn out that faults of F1 and F2controlled gas hydrate accumulation in Qilian Mountain permafrost.