Institute of Sedimentary Geology,Chengdu University of Technology
Research Institute of Exploration and Development ofChangqing Oilfield Branch Company,PetroChina
National Engineering Laboratory for Exploration and Development of Low⁃permea⁃bility Oil / Gas Fields

In order to explore the marker anomaly and genesis of the late diagenesis stage division of the Upper Paleo⁃zoic coal bearing strata in the Longdong area of Ordos Basin, the samples were analyzed by means of thin section iden⁃tification, cathodoluminescence, X⁃diffraction, scanning electron microscope, vitrinite reflectance (Ro ) and wrap⁃ping temperature. The results show that the average Ro of source rocks in the Upper Paleozoic coal bearing strata is2.3%, and the average value of the maximum pyrolysis temperature Tmax is 555.1 ℃. The clay minerals are mainlyI/ S, illite, kaolinite and chlorite. The sandstone has the characteristics of late diagenesis stages such as light meta⁃morphic sericitization, grade III-IV enlarged edge of quartz, mineral orientation and dry gas reservoir. However, it isfound 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 diageneticstage. In the Upper Paleozoic coal bearing strata in the Longdong area, the degree of thermal evolution of source rocksis 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, resultingin 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 providesmectite, and kaolinite are all transformed into illite, while smectite and kaolinite will be retained, resulting in alarge number of I/ S. Moreover, the tuffaceous rock debris in the stratum is argillized, which increases the content ofsmectite and also leads to the high content of I/ S in the stratum. The salinity of inclusions in the study area is nega⁃tively 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 in⁃teraction results in only a small amount of temperature inclusions recorded in the late diagenetic stage, so mostof the measured temperatures stay in the middle diagenetic stage, and only a few temperatures are in the late diagenet⁃ic stage.