The Special lithologic Sections of Permian Xiazijie Formation in Junggar Basin: Geophysical Characteristics and Origin

According to the GR log response,the special lithologic sections could be easily identified from the sandy conglomerates in Permian Xiazijie formation of Junggar basin,in which the GR log shows alternate abnormal high GR and low GR responses.The 3D seismic and dip log show that the distribution of the special lithologic sections is spatially related to the regional fault.The borehole resistivity image indicates that the abnormal high GR within the special lithologic sections usually located in naturally fractured zone or in the erosion surface in bottom of sandy conglomerates.The statistical analysis of GR log values manifests that the average value of GR of the special lithologic sections is close to that of Xiazijie formation with normal GR response.It is consequently supposed that the abnormal high GR in the specail lithologic sections mainly results from the redistribution of radioisotopes of the fragmental deposits within the formation.Several typical minerals such as hydromica,ankerite and pyrite have been identified by thin section analysis from rock samples of the special lithologic sections.Therefore,it is concluded that the identified special lithologic sections in Permiean Xiazijie formation are hydrothermal altered bodies caused by hydrothermal alteration conducted by hydrothermal fluids coming from the deep regional fault.This paper puts forward the hypothesis that the abnormal high GR reading in the special lithologic sections mainly created by the precipitations of uranium isotopes dissolved and migrated by the hydrothermal fluids related to various permeable bodies,that is,faults,fractured formation and erosion surfaces.