青海木里冻土区AMT探测天然气水合物正演模拟研究

AMT forward modeling research on detecting natural gas hydrate in Muli permafrost area,Qinghai Province

自2008年首次在中低纬度带青海木里地区发现天然气水合物以来,开展了一系列的地球物理与地球化学勘查工作,但该区的水合物储层呈现低孔低渗的特征,给勘探工作带来了诸多挑战。为了研究冻土区的天然气水合物模型的电性响应特征,开展了冻土区天然气水合物AMT正演模拟研究。针对木里地区天然气水合物薄层分布的特点,结合该区天然气水合物层测井电阻率值分布特征构建了砂岩与泥岩两类模型,通过对不同参数的两类模型开展AMT正演模拟与反演研究,得到了两类模型的电性响应特征,结果表明:当水合物层的厚度/埋藏深度和水合物层/围岩电阻率比值满足一定条件时,无论是砂岩模型还是泥岩模型,可通过一维反演曲线分辨水合物层;在木里地区水合物稳定带内,无论是砂岩模型还是泥岩模型,对于水合物层的电阻率为含水合物砂岩层、泥岩层的均值时,当其厚度/埋藏深度≥10%时,可通过一维反演曲线分辨水合物层;而当水合物层厚度/埋藏深度≤5%,则无法分辨水合物层。

Since the discovery of natural gas hydrate in Muli area of Qinghai Province for the first time in 2008, a series of geophysical and geochemical exploration work has been done. However, the fact that the gas hydrate reservoir in this area has features of low porosity and low permeability has brought many challenges to exploration work. To obtain the electrical response characteristics of the natural gas hydrate models in the frozen soil area, the authors conducted the AMT forward modeling of the gas hydrate in this paper. Based on the thin layer distribution characteristics of gas hydrate and the distribution range of the logging resistivity values of gas hydrate, the au- thors constructed two types of models, i.e., sandstone and mudstone models. Through the AMT forward modeling and inversion studies of the two models, the electrical response characteristics of the two models were obtained. The results are as follows ： （ 1 ） When the ra- tio of the thickness of the hydrate layer to the depth of the hydrate layer and the ratio of the resistivity of the hydrate layer to the resistivity of surrounding rock meet certain conditions, both the sandstone models and the mudstone models can be used to distinguish the hydrate layer by 1D inversion curve. （2） In the hydrate stability zone of Muli area, the resistivity value of the hydrate layer is equal to 400Ω·m or 200Ω· m; when the ratio of the thickness of the hydrate layer to the depth of the thin hydrate layer is equal to 10%, the onedimensional inversion can be used to distinguish the hydrate layer, and when the ratio of the thickness of the hydrate layer to the depth of the thin hydrate layer ≤5%, the one-dimensional inversion can not distinguish the hydrate layer.