OS5B-6:GAS HYDRATE GEOLOGICAL SYSTEM IN THE QILIAN MOUNTAIN PERMAFROST

Zhengquan LU1,2, Zewen LIAO3, Youhai ZHU1, Hui LIU1, Ting WANG1, Shiqi TANG1, Panpan TAN2
1. Oil & Gas Survey, China Geological Survey, China; 2. Institute of Mineral Resources, Chinese Academy of Geological Sciences, China; 3. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China

    Foreign known gas hydrates are characteristic of a geological system composed of a hydrocarbon-generation subsystem, a fluid-migration subsystem and a gas-hydrate-accumulation subsystem (Lu et al., 2008). A hydrocarbon-generation subsystem provides gas sources for gas hydrate to form; a fluid-migration subsystem provides paths for fluids or hydrocarbon gases to migrate upwards or laterally; a gas-hydrate-accumulation subsystem couples hydrocarbon gases under low temperature and high pressure environments and the enrichment space for gas hydrate to accumulate.

    Based on the detailed studies of the known gas hydrate drilling holes, the gas hydrate occurrences are the coupling product of a hydrocarbon-generation subsystem, a fluid-migration subsystem and a gas-hydrate-accumulation subsystem in the Qilian Mountain permafrost. The gas hydrate borehole data show that the gas hydrate occurrences are closely associated with faults not only in the horizon but also in the borehole profile in the Qilian Mountain permafrost. Correlation of gas components and methane carbon isotopic compositions in headspace gases from cores with occurrences of gas hydrate and its anomalies together with faults or fractures with depth in DK-8 gas hydrate drilling hole indicates that faults or fractures have an obvious effect on the migration of hydrocarbon gases for gas hydrate to form in the Qilian Mountain permafrost. Gas concentrations in cores behave their heights respectively at intervals of 149~167m, 228~299m, 321~338m and 360~380m, which well corresponds to intervals of gas hydrate and its anomalies. Hydrocarbon gas volumetric percentage, methane carbon isotopic composition, and ratios of C1/ƩC1-5, C1/ΣC2-5, C1/ΣC2-3, C1/C2, C2/C3, C2/ΣC3-4, iC4/nC4 and iC5/nC5 in headspace gases from cores collected at different distance from faults or fractures, indicate that deeper faults or fractures may serve as migration paths for hydrocarbon gases and middle to shallower faults or fractures may provide the space for gas hydrate to accumulate.

    Hydrocarbon gases are considered to come from the deep. Gas composition and methane carbon isotope composition of gases from gas hydrate show that hydrocarbon gases are oil-typed rather than coal-typed. Organic geochemical results indicate that mudstone, oil shale, coal within gas hydrate occurrence zone are not mature or below an over-mature level, insufficient to produce enough gases for gas hydrate to form. A comparison of gas composition and carbon isotopes between gases from gas hydrate and those thermally stimulated from mudstone, oil shale and coal within gas hydrate occurrence zone, suggests that gas sources for gas hydrate are produced by the deep mudstone and oil shale at the depth of about 1500~200m, where the Triassic mudstone / oil shale and the Permian mudstone are identified in the seismic profile.

    When the deep hydrocarbon gases migrate upward along faults or fractures, the accumulation of gas hydrate is essentially dependent on the temperature and pressure conditions, namely the permafrost environments. Gas supply conditions are dependent on the hydrocarbon-generation subsystem and the fluid-migration subsystem. The permafrost conditions are parts of the gas-hydrate-accumulation subsystem. The effect of coupling and matching among the hydrocarbon-generation subsystemm, the fluid-migration subsystem and the gas-hydrate-accumulation subsystem is finally decisive to gas hydrate formation and accumulation. Since the gas hydrate geological system coupled by the hydrocarbon-generation subsystemm, the fluid-migration subsystem and the gas-hydrate-accumulation subsystem has a great variety in different locations in the Qilian Mountain permafrost, gas hydrate occurrences are heterogeneous both horizontally vertically in this area.