Reservoir petrology and diagenesis of Lower Shihezi Formation and Shanxi Formation in northern Ordos Basin

The Lower Shihezi Formation and Shanxi Formation are the key strata for natural gas exploration in Ordos Basin, but petrology and reservoir sandstones are poorly studied because of few drillings and core data. The new drillings now have shown the good prospect for natural gas exploration. This study based on the new core data indicates the petrology and the main diagenetic events. The petrology mainly consists of mainly coarse to middle grained sandstone, minor siltstone and mudstone. The diagenesis includes alteration of feldspars, compaction, calcite cementation, quartz overgrowth and replacement of feldspar and lithic fragments. Isotopic data shows that the quartz overgrowth is at a deep burial depth and high temperature. Calcite cement may occur relatively early in the diagenetic process, but the isotopic data show that the calcite cement should be late, because the temperature of calcite cement calculated from the isotopic value is 93 ∼ 140°C. This is in contradiction to the observation of thin section and SEM. This may be the cause of isotopic fractionation. The process of isotopic fractionation during diagenesis is still unknown. The diagenetic model has been supposed for further studies.