Late diagenesis stage division and genesis of Upper Paleozoic coal bearing strata in Longdong area, Ordos Basin

In order to explore the marker anomaly and genesis of the late diagenesis stage division of the Upper Paleozoic coal bearing strata in the Longdong area of Ordos Basin, the samples were analyzed by means of thin section identification, cathodoluminescence, X-diffraction, scanning electron microscope, vitrinite reflectance(R_o) and wrapping temperature. The results show that the average Ro of source rocks in the Upper Paleozoic coal bearing strata is 2.3%, and the average value of the maximum pyrolysis temperature T_max is 555.1 ℃. The clay minerals are mainly I/S, illite, kaolinite and chlorite. The sandstone has the characteristics of late diagenesis stages such as light metamorphic sericitization, grade III-IV enlarged edge of quartz, mineral orientation and dry gas reservoir. However, it is found that kaolinite is abundant in the relative content of clay minerals in sandstone, the content of I/S is high, and the homogenization temperature of inclusions is lower than the diagenetic temperature of normal late diagenetic stage. In the Upper Paleozoic coal bearing strata in the Longdong area, the degree of thermal evolution of source rocks is high. Oil and gas carry acid fluid into sandstone, causing the dissolution of feldspar and other aluminosilicates. The dissolution of feldspar aluminosilicate minerals provides a large amount of Al³⁺ for authigenic kaolinite, resulting in a large amount of kaolinite in the formation. The low content of K⁺ also hinders the illitization of kaolinite. Due to the limited content of potassium rich minerals such as K-feldspar in the stratum, it is not enough to provide smectite, and kaolinite are all transformed into illite, while smectite and kaolinite will be retained, resulting in a large number of I/S. Moreover, the tuffaceous rock debris in the stratum is argillized, which increases the content of smectite and also leads to the high content of I/S in the stratum. The salinity of inclusions in the study area is negatively correlated with homogenization temperature, and the salinity level of inclusions reaches medium salinity, which makes most of the diagenetic temperature lower than that in the late diagenetic stage. The inactive water rock interaction results in only a small amount of temperature inclusions recorded in the late diagenetic stage, so most of the measured temperatures stay in the middle diagenetic stage, and only a few temperatures are in the late diagenetic stage.