The Application Of Paleomagnetic Method On Fracture Orientation Of Volcanic Rocks, Xujiaweizi In Daqing

In the exploration of gas and oil fields, the orientation of the drilling core is of great importance. Paleomagnetism is a very effective and economic way for core orientation developed recent years and has good prospects in its application.In this article, we collected about 500 core samples from Lower Cretaceous Yingchen Formation over 9 drilling wells in Xujiaweizi, Daqing. Cracks on the cores were measured systematically. Representive samples were measured for rock magnetic properties and systematic thermal cleaning were carried out on all the samples.Rock magnetic analysis such as isothermal remnant magnetization and block temperature spectrum shows that the dominantly magnetic carrier is magnetite combined with some hematite, occasionally with pyrrhotite or titanomagnetite in the Lower Cretaceous Yingchen Formation. Among these lava, rhyolite and dacite are mainly magnetite, while tuffaceous rocks are mainly hematite. Systematic thermal demagnetization analysis shows two remnant magnetism components which are characteristic remnant magnetization and viscous remnant magnetization. Remnant magnetic feature of lava shows that the viscous remnant magnetism component cleaning at below 350°C from the lava is consistency with the present direction of the Earth's magnetic field. We can recover the original underground position of the core by correct the viscous remnant magnetization component of the core to the current earth's magnetic field direction. And then we can obtain the orientation of the core cracks.In order to test the effectiveness of the viscous remnant magnetization, we isolated characteristic remnant magnetization (ChRM) from some selected samples. The average direction of the characteristic remnant magnetization is D=35.2°,I=71.7°,α95=12.1°, k = 22; and from this we can calculate that the position of the paleomagnetic pole isψ= 175.9°,λ=65.9°,dp=18.8,dm=21.4, palaeolatitude is 56.5°. This result is parallel to the already exist paleomagnetic pole of the Early Cretaceous of the Songliao region and thus pass the ChRM test which shows the re-orientation effectiveness by viscous remnant magnetization. We then use the viscous remnant magnetization to re-oriente cracks direction of the lava core in the study area, and make the rose-diagram of crack strikes of the corresponding well which can directly reflects the preferential direction of the crack in the statistical point of view. There are differences among the preferential developed directions of different wells, however, there are mainly four preferential developed directions of cracks in the lava. They are northeast (35°-50°), northwest(310°-320°) , nearly N-S and nearly E-W, among which the northeast and the northwest are more dominant. The direction of Songxi Fault in Xingcheng-Shengping district is NNW (330°) , which is a sinistral slip strike fault,and others are driviative faults drove by Songxi fault.