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 Fe

3+

in the mudstone was reduced to colorless Fe

2+

causing a phenomenon called bleach.The Fe

2+

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.