Geochemical dynamics of the gas hydrate system in the Qilian Mountain Permafrost, Qinghai, Northwest China

Abstract

The gas hydrate system in the Qilian Mountain permafrost region exists as an epigenetic hydrocarbon reservoir situated over a deep-seated potential hydrocarbon reservoir. This system is characterized by a particular series of geochemical processes and elemental behaviors, which control the origin of the gases, associated mineralization processes, and the geochemical migration and enrichment of different elements. In this study, we discuss these processes based on compositional analyses of the hydrate-bound gases, associated minerals, fracture fluid, and major, trace, and rare earth element behaviors. Most of the hydrate-bound gases are thermogenic in origin, with minor additional mixed microbial and thermogenic methane that is sourced from sapropelic kerogens in the underlying hydrocarbon reservoir. Episodes of mineralization that formed pyrite, alstonite, and ankerite along rock fractures were likely caused by decomposition of nearby gas hydrate, due to shrinkage of the gas hydrate stability zone. Major element concentration anomalies partly result from ion-precipitation that occurred in the pore and fracture fluid during hydrate formation and decomposition. Most of the anomalies in the measured trace element concentrations are interpreted to demarcate the base of the gas hydrate layer, as low-temperature hydrate has a strong sealing ability. Some of these enriched elements are shown to have migrated into the pyrite crystals, causing their anomalously high concentrations. In addition, the pyrite REE pattern varies with depth and, for pyrites with the same origin, the presence of a hydrate reservoir may be the major cause of fractionation of REE into pyrite. This study provides a new description of the geochemical dynamics of a gas hydrate system operating in a region of permafrost.