利用古地磁学方法恢复钻孔岩心原始方位可靠性的探讨:以塔里木盆地钻井为例

On the reliability of drilling core reorientations using palaeomagnetic methods:A case study from the boreholes in the Tarim Basin

针对非定向钻孔岩心原始方位难以确定的科学问题,利用塔里木盆地5口钻井(TKQ101井、SHUN9井、TAT19井、TZ18井和TS108井)志留纪无定向砂岩样品,对典型样品进行了岩石磁学、扫描电镜(SEM)和能谱分析(EDS)等实验,明确了样品的主要载磁矿物,之后对43块样品进行了退磁处理,并分析了校正后的磁化率各向异性(AMS)最大轴(K_(max)轴)指示的古水流方向,探讨了利用剩磁恢复岩心原始方位的可靠性。AMS结果揭示的沉积组构支持整个钻孔回次地层近似水平;岩石磁学、SEM和EDS实验结果表明TKQ101井样品主要载磁矿物为磁铁矿、含有少量针铁矿和赤铁矿,其他钻井样品主要载磁矿物为磁黄铁矿和磁铁矿。43块样品的系统退磁实验结果显示,TKQ101井岩心样品可分离出可靠的现代场黏滞剩磁方向(VRM)和志留纪地层原生剩磁方向(ChRM),经VRM和ChRM各自计算获得岩心原始方位旋转量(R,R′,其中R,R′分别为利用VRM和ChRM磁偏角获得的岩心原始方位旋转量)一致,校正后的K_(max)指示的古水流方向也与地质证据相吻合,支持TKQ101井岩心原始标志线方位需逆时针旋转258.0°~262.0°;其他4口钻井岩心退磁结果呈现单分量特征,综合分析明确其经历了由喜山期油气运移、聚集等流体活动导致的化学重磁化,携带的剩磁为现代地磁场的黏滞剩磁和喜山期重磁化成分叠加结果。通过校正后K_(max)轴指示的古水流方向和地质证据验证后,揭示利用VRM获得的恢复岩心原始方位旋转量R较为可靠。综上,钻井岩心原始方位恢复需要旋转的角度如下:TKQ101井岩心在逆时针旋转258.0°~262.0°后即可获得可靠的原始标志线方位;SHUN9井第4,5,6回次岩心标志线的方位需分别逆时针旋转148.1°,221.2°和318.2°;TAT19钻井第3,5回次岩心标志线的方位需分别逆时针旋转269.8°,155.9°;TS108井和TZ18井岩心标志线的方位需分别逆时针旋转239.3°,256.6°。

This study explores the accuracy of drilling core reorientations by using remanent magnetization.To this end,paleomagnetic analyses were carried out on 43 Silurian sandstone samples collected from five boreholes(TKQ101,SHUN9,TAT19,TZ18,and TS108)in the Tarim Basin.Meanwhile,rock magnetic measurements,scanning electron microscope(SEM)and energy dispersive spectral(EDS)observations were conducted on representative samples to identify the predominant magnetic carriers.Furthermore,the paleocurrent direction inferred from the corrected maximum magnetic susceptibility(K_(max))axis of the anisotropy of magnetic susceptibility(AMS)using remanent magnetization was analyzed.AMS results indicate a sedimentary fabric preserved in the studied drilling cores,suggesting their stratigraphy are overall horizontal.Rock magnetic results,SEM and EDS observations reveal that magnetite is the dominant magnetic carrier for the TKQ101 samples,with small amounts of goethite and hematite,while pyrrhotite and magnetite are the dominant magnetic carriers for the other samples.The demagnetization results indicate that the viscous remanent magnetization(VRM)acquired in the present geomagnetic field and the characteristic remanent magnetization(ChRM)of the Silurian formation can be isolated for the TKQ101 samples,where the original azimuth rotations(R,R′)estimated by VRM and ChRM are consistent.Furthermore,the paleocurrent direction inferred from the corrected K_(max)is supported by the geological evidence,suggesting a counterclockwise rotation of 258.0°-262.0°of the TKQ101 drilling cores.Only one remanence component was isolated for the majority(~90%)of samples from the other four boreholes,which is a superposition component of the VRM acquired in present geomagnetic field and the chemical remagnetization caused by fluid activities,such as oil-gas migration and accumulation,during the Himalayan period.Therefore,it is more reliable to reorient these drill cores by using the VRM component,with confirmation of the paleocurrent direction inferred by the corrected K_(max)and geological evidence.In summary,to restore the original orientations of these drilling cores,the following rotation angles are required:258.0°-262.0°counterclockwise rotation for the TKQ101 drill cores;148.1°,221.2°,and 318.2°counterclockwise rotation for the 4 th,5 th and 6 th sections from the borehole SHUN9,respectively;269.8°and 155.9°counterclockwise rotation for the sections 3 and 5 from the borehole TAT19,respectively;239.3°and 256.6°counterclockwise rotation for drill cores from the boreholes TS108 and TZ18,respectively.