Mosuowan area has developed a rather complicated formation water including Na

2

SO

4

NaHCO

3

MgCl

2

and CaCl

2

four types.There are two pressure systems in the vertical direction

of which the lower high-pressure system shows reverse hydrochemical depositional sequence

while the upper part with normal pressure system has a little different hydrochemical depositional sequences in different areas.Study on regional structure features of the area and analysis of formation water types and salinities show that formation water in Mosuowan area is within hydrogeologic system of NaHCO

3

type.The CaCl

2

-type formation water is sourced from the deep strata and migrates into the reservoir through faults

while the Na

2

SO

4

-and MgCl

2

-type formation waters are mixtures of CaCl

2

-type water of high salinity and NaHCO

3

-type water of low salinity.The difference between the geological conditions in Peng-5 well-block and Pengcan-2 wellblock is the main cause of the different hydrochemical depositional sequences of the strata with normal pressure in these two wellblocks.Diagenesis and hydrocarbon generation are the reasons behind the reverse hydrochemical depositional sequences of the strata with high pressure.