Kaolinite-to-Chlorite Conversion from Si,Al-Rich Fluid-Origin Veins/Fe-Rich Carboniferous Shale Interaction

The kaolinite-to-chlorite conversion is one of the chloritization processes that occurs in low temperature diagenetic and hydrothermal systems. The mechanism of this mineralogical transformation is still under discussion, since direct transformation, conversion via berthierine as intermediate phase or direct formation of berthierine/chlorite mix, either by dissolution-crystallization or by solid state transformation (or a combination of both), are all hypotheses put forward. In this context, each description of a kaolinite-to-chlorite conversion occurrence becomes an opportunity to shed new light and to renew this debate. Studying Carboniferous shale–crosscut by large quartz-kaolinite veins–from the mining basin of the North of France, we report therefore an uncommon kaolinite-Fe-rich chlorite assemblage. This assemblage appears as a chlorite fringe 20 µm wide along the interfaces between the shale and the quartz-kaolinite veins. All petrographical, mineralogical and chemical data suggest that the Fe-chlorite results from the interaction between the shale, providing the Fe,Mg supply, and the Si,Al-rich veins, leading to the chloritization of the kaolinite at a small scale via at least one dissolution-recrystallisation step. High-resolution observations highlight that neoformed Fe-rich chlorite contains some 7Å isochemical layers, as relict of berthierine. Therefore, we advance that the conversion takes place either through the precipitation of berthierine following by a second step involving solid state berthierine-chlorite conversion, or through the direct precipitation of a chlorite-rich/berthierine-poor mix driven by the Fe/(Fe + Mg) ratio, at low temperature and in reducing conditions. The comparison of our data with the recent literature allows to prefer the second hypothesis.

Upper Carboniferous Shale Gas Potential Analysis in Eastern Qaidam Basin, Northwestern China

Shale of rich organic matter presents in Upper Carboniferous in Qaidam Basin, Northwest of China. Carboniferous shale thickness is between 100 ~ 300m in the Qaidam Basin, the shale includes silty mudstone shale, calcareous mudstone, shale and carbonaceous shale, and it is very favorable lithology type for shale gas. According to the shale organic geochemical analysis, the abundance of organic matter reaching the middle - good degree of hydrocarbon source rocks; the type of organic matter is mainly II2 and III type. The maturity of organic matter is mainly between 1 % -1.3 %. The Upper Carboniferous shale thermal evolution is in mature oil and gas stage. The Upper Carboniferous hydrocarbon-rich shale distribute stability, with great thickness. Shale gas potential in Upper Carboniferous is quite large.