Mechanism and significance of hydrothermal activity in the Majiagou Formation,Fuxian area, Ordos Basin
QI Rong1, LI Guorong2, ZHANG Juntao3, WANG Fubin1, LI Yuxiang1, YANG Fei1, LIU Lu1, HE Sai2,4, TIAN Jiaqi2, LI Xiaoxiao2
1 North China Company,Sinopec,Zhengzhou 450006,China; 2 College of Energy,Chengdu University of Technology,Chengdu 610059,China; 3 Research Institute of Petroleum Exploration and Development,Sinopec,Beijing 102206,China; 4 Sichuan Zhongcheng Institute of Coalfield Geophysical Engineering,Chengdu 610072,China
Abstract：It is of great significance to study the mechanism of hydrothermal activity,related to the Yanshanian thermal events in Majiagou Formation,Fuxian area of the Ordos Basin,to reveal the origin of reservoirs. The two stages of hydrothermal activity related to the Yanshanian thermal events in Majiagou Formation,Fuxian area are identified,based on a comprehensive analysis of petrology and geochemistry,including core and thin section observation and cathode luminescence,homogenization temperature of fluid inclusions,carbon and oxygen stable isotopes,strontium isotope,electron probe microanalysis and rare earth element analyses. The results show that the stage-Ⅰ hydrothermal activity is characterized by the development of saddle dolomite in the fissure-cavern systems,with low and negative δ13C and δ18O values,higher strontium isotope value than normal marine limestone,negative abnormal Eu and La in rare earth elements,rich content of MnO,FeO,SrO and poor content of K2O and Na2O in trace elements,and high homogenization temperature of fluid inclusions. The geochemical characteristics reveal that the saddle dolomite was formed by the combined action of internal fluids in the Ordovician strata,shallow fluids in the Carboniferous-Permian strata,and deep hydrothermal fluid under Early Cretaceous extensional fault activity,in which the deep hydrothermal fluid is post-magmatic hydrothermal fluid related to basalt eruption. The stage-Ⅱ hydrothermal activity is characterized by quartz precipitation in fracture-vug systems,and the development of fractures calcite that cut saddle dolomite,with δ13C values close to the background value of Ordovician carbonate rocks,low δ18O value,higher strontium isotope value than saddle dolomite,negative abnormal La and positive abnormal Eu in rare earth elements,rich content of MnO,FeO,K2O,Na2O in trace elements,and high homogenization temperature of fluid inclusions in quartz. The geochemical characteristics reveal that the calcite and quartz were formed by the combined action of internal fluids in the Ordovician strata and deep hydrothermal fluid under Early Cretaceous extensional fault activity,in which the deep hydrothermal fluid is the post-magmatic hydrothermal fluid related to the activities of the Early Cretaceous monzonite and monzonitic diorite magmatic intrusion. Hydrothermal activity related to Yanshanian thermal events has both constructive and destructive effects on the formation and preservation of reservoir space in the Majiagou Formation,but the overall sphere of influence is limited.
QI Rong,LI Guorong,ZHANG Juntao et al. Mechanism and significance of hydrothermal activity in the Majiagou Formation,Fuxian area, Ordos Basin[J]. JOPC, 2024, 26(3): 632-643.
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