地球深部页岩气储层的大地电磁响应特征

Magnetotelluric Response Characteristics of Shale Oil and Gas in Deep Earth

为了探究大地电磁测深法(magnetotelluric,MT)对地球深部页岩气储层的探测能力,进行数值模拟研究。首先,将3000、3500、4000、4500、5000 m设为目标页岩气储层埋深,每种深度条件下设定11种围岩电阻率3、5、25、35、70、100、200、300、500、800、1200Ω·m,6种目标页岩气储层电阻率10、15、20、30、40、50Ω·m,4种目标页岩气储层厚度(50、100、180、300 m),共建立1320种地电模型。然后,利用正演模拟计算各模型的大地电磁响应特征,获得理论计算的视电阻率资料,分析各种条件下存在目标页岩气储层时引起的视电阻率曲线差异大小。数值结果表明:利用MT法对深部页岩气储层进行勘探时,围岩电阻率的大小、页岩气储层的厚度对MT法分辨力影响很大。一般来说,随着埋深变大,分辨能力会明显变小,但当围岩电阻率较大或页岩气储层较厚时,会弥补埋深变大带来的这一不足。从数值模拟的角度证实了MT法勘探深部页岩气储层的有效性,为MT法精细勘探深层页岩气资源提供了理论参考。

In order to explore the ability of mgnetotelluric(MT)to detect shale gas reservoir in deep earth,numerical simulation was carried out.Firstly,3000,3500,4000,4500,5000 m was set as the target shale burial depth,and 11 kinds of surrounding rock resistivity were set at each depth(3,5,25,35,70,100,200,300,500,800,1200Ω·m),6 target shale gas reservoir resistivity(10,15,20,30,40,50Ω·m),4 target shale gas reservoir thickness(50,100,180,300 m),a total of 1320 geoelectric models were established.Then,the magnetotelluric response characteristics of each model were calculated by forward modeling to obtain the theoretical calculated apparent resistivity data and analyze the difference of apparent resistivity curves caused by the existence of target shale gas reservoir under various conditions.The numerical results show that the resistivity of surrounding rock and the thickness of shale gas reservoir have great influence on the resolution of MT method when it is used to explore deep shale gas reservoir.Generally speaking,as the burial depth increases,the resolution decreases significantly,but when the surrounding rock resistivity is higher or the shale gas reservoir is thicker,this deficiency will be compensated.The effectiveness of MT method in deep shale gas exploration is verified by numerical simulation,which provides a theoretical reference for MT method in the field of fine exploration of deep shale gas.