基于富有机质页岩复电阻率的几种导电模型的探讨

Discussion of Several Conductive Models Based on the Complex Resistivity of Organic-Rich Shale

利用SI-1260阻抗分析仪在室温常压下对南方页岩气探区中某井多块富总有机碳(TOC)含量的饱和页岩岩芯进行多次复电阻率扫频测量,最终选择一组数据分别使用单Cole-Cole、双ColeCole、Cole-Cole乘Brown以及Dias模型对其复电阻率幅值以及相位进行频谱参数联合反演。反演结果表明双Cole-Cole和Cole-Cole乘Brown模型相对于其他模型具有较小拟合误差,能够很好地描述页岩岩芯的复电阻率以及相位曲线,且能从反演结果中较准确地得到各频谱参数。测试与分析结果表明高TOC的页岩岩样的宽频复电阻率参数表现为低频时的高极化率异常,总结出高TOC页岩储层具有低电阻率、高极化率的异常特征。其对使用时频电磁勘探技术和频谱激电法寻找富总有机碳含量的优质页岩储层以及页岩储层复电阻率测井方法的实现都具有重要的指导与借鉴意义。

Recently,shale gas entered our exploration area as an unconventional energy source.Geological study has shown that the shale in southern China has good qualities for exploration.Because of the unique storage conditions and continuous complicated accumulation patterns of shale gas,it is difficult to use conventional geophysical methods to predict sweet spots.According to geological deposition theory,the total organic carbon（TOC）is the basic element that controls the formation of pyrite in marine sediment under a deep water deposition environment.There is a close relationship between the TOC and pyrite in such deep water deposition environments.In this paper we have found that pyrite plays an important role in the prediction of TOC.Through laboratory measurement analysis and joint inversion of the complex resistivity and phase,the marine organic-rich shale samples showed low resistivity and high polarizability.We think that the induced polarization（IP）electromagnetic exploration method can evaluate shale gas formation effectively,and application has shown that IP results are in good agreement with the seismic prediction for sweet spots.With the growth in global demand for oil and gas resources,conventional oil and gas exploration and development potential is very narrow,but unconventional reservoirs are open.Recently,shale gas has become the new target of global oil and gas exploration and development.Exploitation of shale gas and tight oil has brought a major change to the world which is gradually affecting the pattern of world energy supply and demand.It is estimated that there are rich marine shale gas resources in the south of China.Geologically,there are six sets of marine shale formations in Proterozoic-Paleozoic strata in southern China.Specifically,in the Silurian Longmaxi group（S1l）and the Cambrian Qiongzhusi group（∈ln）,the shale has a good quality of TOC and thickness,which indicate the huge potential reserves of unconventional gas.The seismic method is certainly the first choice to predict sweet spots,but unfortunately,it has run into difficulties in southern China because of rugged tomography and complicated geology.Because of the low resistivity and strong polarization of marine organic-rich shale compared with the surrounding rock,the IP electromagnetic exploration method,which has been considered as an effective supplementary method,can be used to do that.At room temperature and atmospheric pressure,the complex resistivity sweep measurement is obtained for a saturated shale core by the SI1260 impedance analyzer.Finally,the joint inversion of complex resistivity amplitude and phase is conducted using single Cole-Cole,double Cole-Cole,Cole-Cole multiply Brown,and Dias models.Inversion results indicate that the double Cole-Cole and Cole-Cole multiply Brown models can describe shale cores＇ complex resistivity and phase curves with smaller fitting errors than the other models.We can get accurate spectral parameters from each inversion.It is useful to find shale reserves by the IP method and to realize shale gas complex resistivity logging.