地质科技通报 (Nov 2023)
On the reliability of drilling core reorientations using palaeomagnetic methods: A case study from the boreholes in the Tarim Basin
Abstract
Objective This study explores the accuracy of drilling core reorientations by using remanent magnetization. Methods 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 (Kmax)axis of the anisotropy of magnetic susceptibility (AMS) using remanent magnetization was analyzed. Results 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 Kmax 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 Kmax and geological evidence. Conclusion 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 4th, 5th and 6th 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.
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