西藏羌塘盆地鸭湖地区天然气水合物成藏条件

An analysis of gas hydrate accumulation condition in the Duck Lake area,Qiangtang Basin,northern Tibet

近年来中国陆域冻土区天然气水合物调查研究结果表明,气源条件是制约羌塘盆地天然气水合物找矿突破的关键因素。为明确鸭湖地区天然气水合物成藏潜力,基于近年来的钻探调查成果,从陆域冻土区天然气水合物成藏系统理论出发,系统分析了影响天然气水合物成藏的冻土、气源、储集、构造等地质因素。分析结果显示,鸭湖地区局部具有较好的冻土、地温、气源、储集、构造及水源条件,具备一定的天然气水合物成藏潜力,继续寻找充足的烃类气源是下一步天然气水合物调查的主要方向。同时,选取钻探调查获取的地温梯度、气体组分等参数,结合音频大地电磁测深(AMT)冻土厚度调查成果,对鸭湖地区天然气水合物稳定带的厚度和底界深度进行了预测。结果显示,当甲烷为85%、乙烷为9%、丙烷为6%时,天然气水合物稳定带厚度与冻土厚度分布变化基本一致,稳定带厚度400~630m,底界深度400~680m。当甲烷为98%、乙烷为2%时,天然气水合物稳定带厚度急剧减薄,大部分地区仅有0~30m,最厚仅有150m,局部地区稳定带底界最深仅为240m。结合气测录井结果,认为渐新世唢呐湖组比上三叠统土门格拉组更具备天然气水合物成藏潜力,土门格拉组自身具备较强的生排烃能力,可作为寻找常规油气或页岩气的一个重要层位。

In recent years,the gas hydrate investigation in the permafrost region of China shows that the gas source condition is the key factor for controlling the breakthrough of gas hydrate exploration in the Qiangtang Basin.In order to further clarify the potential of gas hydrate accumulation in the Duck Lake area,the authors systematically analyzed such geological factors as permafrost,gas source,reservoir and structure based on the drilling results in recent years.The results show that there are good conditions of permafrost,geothermal gradient,gas source,reservoir,structure and water source in some areas,indicating that Duck Lake area has a certain gas hydrate accumulation potential.Finding sufficient hydrocarbon gas sources will be the main direction of the next gas hydrate investigation.In addition,the thickness of the gas hydrate stability zone(GHSZ)and the depth of bottom of GHSZ in the Duck Lake area were predicted by the data of the geothermal gradient and the gas composition obtained from the drilling and AMT results.Predictive results show that,when the methane is85%,ethane is9%and propane is6%,the thickness distribution of GHSZ is basically the same as that of the permafrost.The thickness of GHSZ is between400m and630m,and the depth of the bottom of GHSZ is between400m and680m.Where methane is98%and ethane is2%,the thickness of GHSZ is sharply thinned,only0~30m in most areas and only150m thick in certain areas,and the depth of the deepest bottom of GHSZ is only240m.Based on the results of gas logging,it is concluded that the Oligocene Suonahu Formation has more potential gas hydrate accumulation potential than the Upper Triassic Tumengela Formation in Duck Lake area,while Tumengela Formation has strong hydrocarbon generation and expulsion capability,which thus can be regarded as an important horizon for exploration of conventional oil and gas or shale gas.