广域电磁法在低阻覆盖区的应用与评价——以河南中牟为例
The application and evaluation of the wide field electromagnetic method in the low resistance coverage area-Exemplified by Zhongmu area of Henan Province
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收稿日期: 2020-03-20 修回日期: 2020-08-28 网络出版日期: 2020-10-20
Received: 2020-03-20 Revised: 2020-08-28 Online: 2020-10-20
作者简介 About authors
田红军(1988-),男,2015年毕业于中南大学,主要研究方向为电磁法理论与数据处理。Email:
在分析和总结河南中牟含气页岩层电阻率等物理性征基础上,将广域电磁法勘探技术用于南华盆地低阻覆盖区页岩气探测,获得工作区地下5 km内地层电阻率分布规律和特征,查明了工作区的构造展布,同时揭示了以二叠系太原组、山西组以及上、下石盒子组为主要目的层的含气页岩发育特征。此次探测工作表明,在低阻覆盖区广域电磁法是获取南华盆地深部含气页岩埋深和分布范围的有效探测手段,可为后期有利区带评价、区块优选页岩气层提供地球物理参数。
关键词:
Based on an analysis and summary of the physical properties of the gas-bearing shale layer in Zhongmu area of Henan Province, the authors used the wide-area electromagnetic exploration technology to detect shale gas in the low-resistance coverage area of the Nanhua Basin, obtained the distribution regularity and characteristics of the stratigraphic resistivity within 5km under the work area, and ascertained the structural distribution of the work area. In addition, the development characteristics of gas-bearing shales were revealed, with the main target layers being the Permian Taiyuan Formation, Shanxi Formation, Upper Shihezi Formation and Lower Shihezi Formation. The results show that, in the low-resistance coverage area, the wide-field electromagnetic method is an effective detection method to obtain the buried depth and distribution range of deep gas-bearing shale in the Nanhua Basin, providing geophysical parameters for the evaluation of favorable zones in the later period and the optimization of shale gas layers in the block.
Keywords:
本文引用格式
田红军, 尹文斌, 刘光迪, 蒋永芳, 游文兵.
TIAN Hong-Jun, YIN Wen-Bin, LIU Guang-Di, JIANG Yong-Fang, YOU Wen-Bing.
0 引言
为了深化对低阻覆盖区构造复杂区深部地质的认识,同时探讨适合于低阻覆盖区电磁勘探方法,在河南中牟工作区开展了广域电磁法勘探。通过深部页岩气勘探,落实工作区断裂展布和构造单元划分,为确定地层展布、深大断裂的分布、有利区带评价和区块优选提供技术支撑,为后期深部勘探提供地球物理参数。
1 工作区地质地球物理特征
根据目前所掌握的地、物、科研等资料,结合区域地质特征综合分析,暂推定工作区内发育地层主要为奥陶系、石炭系、二叠系、三叠系、侏罗系、白垩系、古近系、新近系和第四系。其中,二叠系由太原组、山西组、下石盒子组、上石盒子组、平顶山组及孙家沟组构成,太原组、山西组、下石盒子组为页岩气主要目的层系。
由于中牟区块还没有开展相关电磁法勘探工作,未获得相关的地表电性参数资料,主要参考牟页1井的电测井资料(表1):第四系—馆陶组为低阻覆盖区,平均电阻率<10 Ω·m,厚度为1 400 m左右;和尚沟组—平顶山组为中低阻,平均电阻率约为70 Ω·m,厚度约600 m;平顶山组为中阻,平均电阻率约为200 Ω·m,厚度约100 m;目的层上石盒子组、下石盒子组为低阻层,平均电阻率约为20 Ω·m,厚度为630 m左右。目的层之下的山西组—马家沟组为电性层由中低阻向高阻过渡层,其中马家沟组地层可视为本次勘探的基底。总体而言,工作区地表1 400 m为厚层状的低阻覆盖区。
表1 牟1井电测井资料
Table 1
层号 | 顶深/m | 底深/m | 层厚/m | 有效厚度/m | RLLD/(Ω·m) | RLLS/(Ω·m) | MFSL/(Ω·m) |
---|---|---|---|---|---|---|---|
第四系 | 0 | 197.8 | 197.8 | 197.8 | |||
明化镇组 | 197.8 | 873 | 675.2 | 675.2 | |||
馆陶组 | 873 | 1482.8 | 609.8 | 609.8 | 5.556 | 3.797 | 2.812 |
和尚沟组(刘家沟) | 1482.8 | 1893.3 | 410.5 | 410.5 | 91.44 | 84.863 | 62.046 |
孙家沟组 | 1893.3 | 2071.1 | 177.8 | 177.8 | 52.055 | 49.394 | 34.502 |
平顶山组 | 2071.1 | 2174.1 | 103 | 103 | 214.114 | 202.666 | 50.647 |
上石盒子组 | 2174.1 | 2495.2 | 321.1 | 321.1 | 24.398 | 23.361 | 10.704 |
下石盒子组 | 2495.2 | 2798.4 | 303.2 | 303.2 | 17.75 | 16.74 | 8.407 |
山西组 | 2798.4 | 2889.8 | 91.4 | 91.4 | 49.443 | 45.289 | 21.062 |
太原组 | 2889.8 | 2971.8 | 82 | 82 | 646.303 | 301.79 | 21.097 |
本溪组 | 2971.8 | 2987.8 | 16 | 16 | 2381.969 | 2137.384 | 193.5 |
马家沟组 | 2987.8 | 3020.8 | 33 | 33 | 183.625 | 170.723 |
2 广域电磁法的有效性分析
本次野外采用广域电磁法的工作模式是测量电场水平分量Ex:
根据式(1)定义广域意义上的视电阻率:
式中:Ex为x方向的电场水平分量;ρa为广域视电阻率;I为供电电流;dL为供电点AB的直线长度;φ为测点与供电点AB中点的连线的夹角;r为测点到供电点AB中点的距离;i为虚数单位;k为波数。
根据牟页1井物性参数统计资料及其他地质资料,建立了工作区的地球—地质物理模型:第一层为第四系—新近系馆陶组,厚度为1 400 m,电阻率为5 Ω·m;第二层为三叠系和尚沟组—二叠系平顶山组,厚度为600 m,电阻率为70 Ω·m;第三层为二叠系以中粗粒长石石英砂岩为主的平顶山组,厚度为100 m,电阻率为200 Ω·m;第四层为本次勘探的目的层——以页岩为主的二叠系上石盒子组和下石盒子组,厚度为630 m,电阻率为20 Ω·m;第五层为山西组—马家沟组,电阻率为1 000 Ω·m。
图1
图1
广域电磁法正演曲线及异常百分比曲线
a—视电阻率曲线对比;b—异常幅度百分比
Fig.1
Forward curve and abnormal percentage curve of wide field electromagnetic method
a— comparison of apparent resistivity curves; b— percentage of exceptions
3 工作部署
测线布置如图2所示,布置了2条平行测线L54和L55.4,每条测线长10 km,线距为1 km,点距为100 m,广域电磁法物理点共计100个。采用的测量频率为0.011 7~8 192 Hz,适合探测从地表到地下7 000 m范围的目标体。发射电流为146 A,发射电压为950 V,收发距离分别为17 km(L54线)和18 km(L55.4线)。
图2
图2
广域电磁法测线施工布设示意
a—广域电磁法野外施工示意;b—测线部署
Fig.2
Layout of widefield electromagnetic survey line
a—field construction schematic diagram of wide-area electromagnetic method; b—line deployment chart
4 资料处理与解释
图3
图3
L55.4线综合成果断面
a—拟地震断面;b—反演成果及地质解释断面
Fig.3
Line55.4 comprehensive achievement profile
a—quasi seismic section; b—inversion results and geological interpretation profile
图4
图4
L54线综合成果断面
a—拟地震断面;b—反演成果及地质解释断面
Fig.4
Line54 comprehensive achievement profile
a—quasi seismic section; b—inversion results and geological interpretation profile
整体上,广域电磁法反演成像揭示了3套低阻层、3套中阻层和1套高阻层共计7套近似水平分布的电性层。其中,L54线随着反演深度加深,电性层呈缓倾斜状变化,低阻层电阻率变化范围在3~9 Ω·m,中阻层电阻率变化范围10~100 Ω·m,高阻层电阻率>100 Ω·m。拟地震图揭示了本区地层界面近似水平,构造单一,L54线大号点向低频倾斜。
结合牟页1井和地质解释资料,广域电磁法反演成像结果显示埋藏深度2 000~3 500 m为含页岩层的有利地段。牟页1井电测井资料以及地质资料揭示了二叠系下统太原组(P1t)、山西组(P1s)和二叠系上、下石盒子组(P2sh、P2x)的电阻率值在3~9 Ω·m,为主要的页岩层。
根据牟页1井揭示的地层特征,推断埋深2 000~3 500 m的石炭系本溪组(C2b)至二叠系下统太原组、山西组和二叠系上、下石盒子组表现为低阻,电阻率值在3~9 Ω·m;视厚800 m及800 m以上的稳定低阻层是本区最有前景的目的层,以4 Ω·m为限圈定的下石盒子组高有机含量低阻带是本区最有前景的页岩气层。埋深在750~1 500 m的中新统馆陶组(N1g)其视厚度达400 m,因电性稳定,厚度稳定,可视为本区又一重要的页岩气目的层。
L55.4和L54线反演成像揭示测区发育有多条断裂构造,共计推测出10条断裂构造。其中,F2、F3断裂是区内重要断裂构造,决定了中二叠世—三叠纪的隆起、凹陷的分布。其他断裂构造未对低阻目的层造成明显破坏。
查明了工作区内基底起伏、埋藏深度和地层在空间上的主要分布。L55.4线基底呈背形构造特征变化,埋藏深度在3 200 m左右,L54线基底呈WS高EN低的倾斜构造特征变化,WS方向埋藏深度在3 200 m,EN方向200号点未见基底,说明基底埋藏深度在5 000 m以下;电性特征表现为高阻。
5 结论
在低阻覆盖区开展高精度大深度广域电磁法探测,采用自主研发的“重磁电三维反演成像解释一体化系统”进行广域电磁法反演成像,有效揭示了工作区地下5 km电阻率分布规律和特征。探测结果表明,埋藏深度2 000~3 500 m为含页岩层的有利地段。
通过本次野外工作,形成了适用于河南中牟低阻覆盖区页岩气勘探的广域电磁法数据采集及处理流程,表明广域电磁法能够克服低阻覆盖层的影响,是获取页岩层分布范围和埋深的一种有效探测手段,为后期有利区带评价和区块优选提供了地球物理参数和技术支撑。
致谢
感谢河南省地质调查院提供试验区块,感谢中南大学李帝铨教授对修改论文提出宝贵的意见,感谢张贤对论文的修改提供帮助。
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