The degree of thermal evolution of the Middle-Upper Proterozoic to Lower Paleozoic in the Ordos Basin:Constraints from illite crystallinity and I/S mixed-layer
The understanding of the thermal evolution degree of the Middle-Upper Proterozoic-Lower Paleozoic in the Ordos Basin has always been limited due to the lack of effective paleotemperature scales.This article conducts systematic tests on the crystallinity of authigenic illite and the I/S mixed-layer in the Middle-Upper Proterozoic-Lower Paleozoic in over 20 drilling wells in the basin,attempting to analyze the degree of thermal evolution from a new perspective.The data suggest that there are notable variations in the correlation between crystallinity and depth of illite across different regions of the basin,with a general tendency for the correlation to decrease with depth,primarily influenced by burial depth and paleogeothermal gradients.The illite crystallinity in the sedimentary rocks of the basin is generally greater than 0.25,an indication that it is not yet in the greenschist facies shallow metamorphic stage.The crystallinity of illite in the Lower Paleozoic Ordovician of the basin is greater than 0.42,indicating that it is in the stage of diagenetic evolution.The Cambrian samples have slightly lower crystallinity than Ordovician,placing them in the middle to late stage of metamorphism in the diagenetic belt.Similarly,a higher degree of thermal evolution is indicated by the relatively low illite crystallinity in the Middle Proterozoic.Furthermore,we compared the maximum paleogeothermal results obtained from illite crystallinity and equivalent vitrinite reflectance,and found that they were consistent within the error range.These results indicate that this method is highly feasible for studying the thermal evolution of ancient strata in China.
万方数据
维普
