羌塘盆地冻土结构特征及其对天然气水合物成藏的影响

Permafrost structures and their effects on the accumulation of the natural gas hydrates in the Qiangtang Basin, northern Xizang

关键词:  羌塘盆地  冻土结构  含冰饱和度  天然气水合物

Key words:  Qiangtang Basin  permafrost structure  ice saturation  gas hydrates

摘要:

羌塘盆地是青藏高原最大的含油气盆地,多年冻土广泛分布,具备良好的天然气水合物形成条件和找矿前景。基于羌塘盆地天然气水合物钻探试验井资料,从影响天然气水合物成藏角度提出了羌塘盆地3种主要的冻土结构类型,其中由冻融层、含冰沉积物冻土层、含冰基岩冻土层、非含冰基岩冻土层所组成的冻土结构最为常见。研究表明,冻土层结构对天然气水合物温压条件具有一定影响,当非含冰基岩冻土层存在时,其下伏的非冻土层的孔隙流体压力与上部冻土层的微孔和微裂隙特征紧密相关,有利于浅层烃类气体的封存和水合物的成藏。含冰冻土层冰地球化学特征指示冻土层形成的过程是大气降雪融化成水后未经蒸发作用直接渗入地下,受气候变冷影响,地层由浅往深逐渐冻结形成。同时,矿化度和阴、阳离子浓度的高低在一定程度上反映了不同深度沉积物的物化性质。含冰冻土层对于浅层烃类气体封盖作用的定量评价显示,随着含冰饱和度的增加,甲烷气体渗透率降低,当含冰饱和度达到80%时,冻土层能完全有效地限制甲烷气体运移。由于在气候变暖因素的驱动下,冻土层不仅能通过温压条件来控制天然气水合物矿藏存在的空间范围,而且还限制着来自部分水合物分解所产生的烃类气体向浅部运移。因而推测,在青藏高原冻土区可能存在一个由断裂体系相关联的深部烃类储层、中部天然气水合物储层和浅部天然气藏组成的油气系统。

Abstract:

The permafrost regions constitute much of the Qiangtang Basin on the Qinghai-Xizang Plateau, and are considered to be favourable areas for the accumulation of the natural gas hydrates with a great potential. Based on the data from the test drillings, three structural types are recognized for the permafrost regions, of which the most common one is the structural type composed of the active layers, ice-bearing sediments permafrost layer, icebearing basement permafrost layer, and ice-free basement permafrost layer. While in the case of the ice-free basement permafrost layer, the pore fluid pressures in the underlying ice-free permafrost layers tend to be closely related to the structures of the micropores or microfractures in the overlying the ice-bearing permafrost layers, which may facilitate the sealing of the near-surface hydrocarbon gas and accumulation of gas hydrates. The geochemical signatures of ice in the ice-bearing permafrost layers have reflected that the water from the melted atmospheric snowfall directly percolates into the soil and rock layers, and results in the gradually freezing of the strata from shallower to deeper depths due to the decrease of temperatures. Meanwhile, the mineralization degrees and antion and cation concentrations in water may give a reference for the assessment of physical and chemical properties of the sediments. As indicated by the experimental simulation, the methane permeability tends to decrease with the increase of ice saturation in the ice-bearing permafrost layers, and may go into ice-free permafrost layers as the ice saturation exceeds 80%. Influenced by the climatic changes, the permafrost layers may exercise a major control not only on the accommodation spaces of the gas hydrates but also on the migration of the near-surface hydrocarbon gases. It follows that within the permafrost regions in the Qiangtang Basin on the Qinghai-Xizang Plateau, there may be the petroleum systems composed of the fault-controlled deep-seated hydrocarbon reservoirs, medium-deep gas hydrate reservoirs and shallow-seated gas reservoirs.