四川盆地川中古隆起及周缘下寒武统筇竹寺组页岩有机质石墨化区预测

Distribution prediction of graphitized organic matter areas in the Lower Cambrian Qiongzhusi shale in the Central Sichuan paleo-uplift and its surrounding areas in the Sichuan Basin

有机质石墨化是造成我国部分地区页岩气勘探效果不佳的主要原因之一,目前关于有机质石墨化的程度、深度下限及导致有机质发生石墨化的主要地质原因尚不清楚。为此,以四川盆地川中古隆起及周缘下寒武统筇竹寺组为主要研究对象,利用电阻率测井响应和激光拉曼光谱分析法,对筇竹寺组页岩开展了有机质石墨化表征的电性特征研究;在此基础上,预测了页岩有机质石墨化的深度下限和分布范围,以期为页岩气勘探选区提供依据。研究结果表明:(1)川中古隆起高部位页岩电阻率测井曲线为正常扁平状特征,有机质拉曼光谱未出现G'峰,未发生有机质石墨化作用;(2)早寒武世内裂陷区5 200 m以深页岩电阻率测井曲线为低阻特征,拉曼光谱出现G'峰,Ro> 3.5%,表明5 200 m以深的有机质已石墨化,但石墨化程度较长宁地区弱;(3)威远—资阳地区有机质石墨化深度下限值变化大(介于4 000～4 600 m),磨溪—高石梯地区有机质石墨化深度下限值较稳定(5 200 m左右)。结论认为:(1)四川盆地川中古隆起大部分地区筇竹寺组页岩的有机质已石墨化,不利于页岩气勘探;(2)威远—资阳、磨溪—高石梯等构造主体部位为非石墨化区,具有较好的页岩气勘探远景。

Organic matter graphitization is one of the main reasons for the poor results of shale gas exploration in some areas in China.At present,however,the graphitization degree of organic matter,the lower limit of depth and the main geological reasons for organic matter graphitization are still unclear.In this paper,the Lower Cambrian Qiongzhusi Fm in the central Sichuan paleo-uplift and its surrounding areas were taken as the main research objects.The electric characteristics study of organic matter graphitization were carried out on the Qiongzhusi shale by using resistivity log response and laser Raman spectroscopy.On this basis,the lower depth limit and distribution range of graphitized organic matter were predicted so as to provide a basis for the selection of shale gas exploration areas.And the following research results were obtained.First,The resistivity log of the shale in the high part of Central Sichuan paleo-uplift is normally flat,and there is no G'peak in Raman spectra of organic matter,indicating no organic matter is graphitized.Second,the resistivity of shale below 5 200 m in the Lower Cambrian aulacogen area is low,G'peak occurs in Raman spectra and Ro is over 3.5%.It is indicated that the organic matter below 5 200 m in this area has been graphitized,but its degree of graphitization is lower than that in the Changning area.Third,the lower depth limit of the graphitization of organic matter varies greatly(4 000–4 600 m)in Weiyuan–Ziyang area,but stays stable(about 5 200 m)in the Moxi–Gaoshiti area.In conclusion,the organic matter in Qiongzhusi Fm shale in the most parts of the Central Sichuan paleo-uplift in the Sichuan Basin has been graphitized,so it is unfavorable for shale gas exploration.In addition,the Weiyuan–Ziyang and Moxi–Gaoshiti areas are non-graphitized areas and they present good prospects of shale gas exploration.