Oil & Gas Geology ›› 2017, Vol. 38 ›› Issue (5): 952-962.doi: 10.11743/ogg20170514

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Petrologic and geochemical records of interaction between reducing fluids and mudstone caprocks:A case from the mudstone in the Qingshankou Formation of Wangfu Depression in southern Songliao Basin

Ming Xiaoran1, Liu Li1, Song Tushun2, Liu Na1, Yang Huidong3, Yu Lei1, Bai Hanggai1   

  1. 1. College of Earth Sciences, Jilin University, Changchun, Jilin 130061, China;
    2. North China University of Science and Technology, Tangshan, Hebei 063009, China;
    3. Research Institute of Exploration and Development, Jilin Oilfield Company, PetroChina, Songyuan, Jilin 138000, China
  • Received:2017-03-31 Revised:2017-08-12 Online:2017-10-28 Published:2017-11-10

Abstract: Reducing fluids percolated from natural gas reservoirs or fields may cause composition variation (especially the macro and trace elements) of mudstone caprocks.The variation records the interaction between the reducing fluids and mudstone caprocks and may be used as an indicator of the locations of oil and gas reservoirs.By resorting to natural analogy,we studied the mudstone samples from the the Qingshankou Formation of Wangfu Depression in southern Songliao Basin to analyze the variation of mudstone composition caused by reducing fluids and the migration of the fluids in cap rocks of gas fields (reservoirs).The results show that the reducing components in the fluids are mainly methane,followed by hydrogen sulfide.During the fluid-rock interaction process,the red Fe3+ in the mudstone was reduced to colorless Fe2+,causing a phenomenon called bleach.The Fe2+ then migrated in the form of colloids with the fluids and reprecipitaed as iron concretions elsewhere.During the bleach process,the increasing U content,and decreasing Ga and Sc contents,as well as the veritical changes of the U,Mo,V and Ni contents,are suggested to be connected with the reductivity of the fluids.In mudstone caprocks,the reducing fluids were driven by buoyancy to migrate vertically and the uplifting of strata might cause dropping of fluid surface.The regular variation of trace elements in the mudstone caprocks may be used as indicators to the flow pathways and directions of the reductive fluids.

Key words: mudstone caprock, reducing fluid, bleached mudstone, iron concretion, natural analogy, natural gas field (reservoir), Songliao Basin

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