The Genesis Analysis And Geological Significance Of High Natural Gamma Anomaly From Benxi To Shanxi Formation,Northeastern Ordos Basin

The characteristics, genesis and geological significance of high natural gamma anomaly stratum from Benxi to Shanxi Formation is researched based on the comprehensive analysis of cores, logging data and text result of more than 50 wells. Eight stratums of high natural gamma anomaly are recognized(numbered Ga~Gh from bottom to top) from Benxi to Shanxi Formation, among which the bottom of Benxi Formation(Ga), the 8, 9#coal seam floor of Jinci Formation(Gc) and the 5# coal seam floor of Shanxi Formation(Ge) are traceable and correlated in the study area. Based on the comprehensive analysis of clay minerals composition, well logging response and radioactive elements association, the rock type of these three main anomaly stratums is volcano alteration clay rock. The spatial distribution rule of Ga, Gc and Ge are determined which show the tendency of thinning and decreasing from NE to SE, according to the parameters of thickness and amplitude of anomaly stratums. In addition, O ther types of rock such as carbonaceous mudstone, volcano alteration tuff and intrusive rock causing high natural gamma anomaly are also described in this paper.Controlling factors of the development of high natural gamma anomaly stratums are discussed, according to the research findings of synsedimentary magmatism of Inner Mongolia Orogenic Belt and the distribution of sedimentary facies in research area. The study shows that there generated multi-period intermediate-acid volcanic eruptive during Benxi stage and Jinci stage of Late Carboniferous, Shan23 stage of Early Permian in Inner Mongolia Orogenic Belt, under the effect of Paleo-Asian Ocean Plate’s subduction beneath North China Plate. These volcanic eruptive formed some volcanic materials containing abundant U and Th elements, which provides material base for high natural gamma anomaly of Ga, Gc and Ge stratums. The migration and enrichment of radioactive elements like U and Th can be restricted by sedimentary environment. Sedimentary environment(underwater distributary channel, barrier bars, etc.) with stronger hydrodynamic condition and oxidation state is beneficial to the migration of U and Th radioactive elements; However, other(lagoon, mudflat, peat swamp, interdistributary bay, etc.) with weaker hydrodynamic condition and reduction state is favorable to the formation of dark muddy sediments which adsorb radioactive elements strongly, promoting the precipitation, enrichment of U and Th and then the development of high natural gamma anomaly stratums.Geological significance of high gamma anomaly is analyzed considering the evolution of source rock and fine classification and correlation of stratum. The research shows that the average heat production of Ga, Gc and Ge stratums respectively account for 74%, 36% and 32% of the rock stratum’ mean heat flux in the basin. The heat released from U, Th and other radioactive elements contained within abnormal stratum makes significant contribution to the thermal flux and accelerates the thermal evolution of source rock; The natural gamma curve is ―finger-type or sword-shaped‖, displaying high content of U and Th, high acoustic time, low resistivity, media- low density, stable horizon, wide distribution and other significant identification marks. It can be applied to refined sub- layer classification and correlation assisted by other logging curves in drillings without core and continental well area lacking marine marker bed.