祁连山冻土区天然气水合物DK-8孔岩芯顶空气地球化学特征及其运移指示意义

Gas Geochemical Features of the DK-8 Gas Hydrate Drilling Hole in the Permafrost of Qilian Mountain,and their Indicative Significance

通过对祁连山木里冻土区天然气水合物DK-8孔不同深度岩芯中气体组分和甲烷碳同位素分析测试,对比分析其变化特征与天然气水合物及异常现象产出层段、断层或破碎带分布之间的空间关系,指出它们对天然气水合物及烃类运移作用的地球化学指示意义。岩芯中气体含量(μL/kg)在149～167m、228～299m、321～338m、360～380m等深度段具异常值特征,它们与天然气水合物及主要异常现象产出层段基本一致,表明岩芯中烃类含量的异常值特征主要是天然气水合物及异常现象的反映。根据离断层或破碎带不同距离岩芯烃类总体积百分比(vol%)、甲烷碳同位素δ13C1值(‰PDB)及C1/C1-5、C1/ΣC2-5、C1/ΣC2-3、C1/C2、C2/C3、C2/ΣC3-4、iC4/nC4、iC5/nC5等特征,代表天然气水合物及其异常的具异常值特征的岩芯中烃类主要由运移而来,它们与断层或破碎带关系密切,下部断层或破碎带是主要烃类运移通道,中上部断层或破碎带可成为天然气水合物赋存空间。

Gas component and methane carbon isotope were analyzed for the cores at different depths in DK-8 gas hydrate drilling hole in the Muli permafrost of the Qilian Mountain.Their changing features are correlated with the intervals of gas hydrate anomalies and with spatial distribution in faults or fractures.Out study shows that these characteristics have indicative significance in gas hydrate occurrence and hydrocarbon migration.Gas contents （μl/kg） in the cores shows anomalies at depths of 149～167m,228～299m,321～338m and 360～380m,which are basically consistent with intervals of gas hydrate and its anomalies,suggesting that high values of gas content were caused by gas hydrate and its anomalies.Based on total volumetric percentage of hydrocarbon gases in core （vol%）,carbon isotope of methane （δ13C1,‰PDB）,and ratios of C1/ΣC1-5,C1/ΣC2-5,C1/ΣC2-3,C1/C2,C2/C3,C2/ΣC3-4,iC4/nC4 and iC5/nC5 from samples with different distance away from faults or fractures,it is suggested that hydrocarbon gases in cores with high values migrated mainly from deep sources and were closely related with faults or fractures.Deeper faults or fractures may serve as migration paths for hydrocarbon gases,and middle to shallower faults or fractures may provide accumulation space for gas hydrate.