Geological Constraint on Gas Hydrate Accumulation in Qilian Mountain Permafrost - A Case in DK-9 Hole

Gas hydrate was successfully sampled again in DK-9 hole in Qilian Mountain permafrost. However gas hydrate occurrences are heterogeneous both in horizon and in section within a limited area. What geological constraint affects gas hydrate occurrences is not known yet. Since features of gas hydrate and other related geological information were well recorded in DK-9, a possible answer is put forward based on analyses of geochemical and geological data in this hole. In this paper, features of hydrocarbon in headspace gases from cores at various depths are compared with occurrences of gas hydrate, faults or fractures in DK-9 in the Qilian Mountain permafrost. The results show that gas contents are relatively clearly higher at the intervals of 180.26m~308.50m, 356.45m~399.32m, 458.55m~508.65m than other intervals. Interestingly gas hydrate and its related anomalies fall into the intervals with higher gas contents. In the meanwhile, hydrocarbon concentrations in headspace gases within the faults or fracture zones are higher than those above the faults or fracture zones. This may suggest that faults or fracture zones have great impact on gas hydrate occurrences. Further comparative analyses on spatial relation of gas hydrate occurrences to faults or fracture zones, reveal that faults or fracture zones serve as migration paths for gases in the deep and provide accumulation space for gas hydrate in the shallow in the Qilian Mountain permafrost. Actually geological data show that gas hydrate occurrences are limited within the belt controlled by faults of F₁ and F₂ in a plane and are mainly within the lower block of these faults in a profile. Hence it is preliminarily drawn out that faults of F₁ and F₂ controlled gas hydrate accumulation in Qilian Mountain permafrost.