Chemical analysis of formation water in the 6

th

member of Yanchang Formation(Chang 6 member)in the sou-thern Ordos Basin reveals that the formation water is high in total salinity;Cl

-

(Na

+

+K

+

)and Ca

2+

are dominant

and thus the water is of CaCl

2

type.Vertically

the total salinity

mainly controlled by ions of Cl

-

and(Na

+

+K

+

)

tends to increase with increasing depth

demonstrating the ever-condensing feature of formation water with depth and the ever-growing sealing conditions subsurface.Laterally

the distribution of the total salinity is relatively consistent with the sand body thickness

and is controlled by sedimentary facies.Given that the formation water in Chang 6 member is characte- rized by low sodium-chloride ratio

magnesium-calcium ratio

coefficient of desulfurization and exchange coefficient of cations

and high metamorphism coefficient in chemistry

we are sure that the formation water therein is located in alternating stagnant zones

a closed anoxic and reducing environment

which is conducive for hydrocarbon accumulation and preservation.Besides

the formation water therein belongs to the enclosed deep water isolated from the surface water

and both its total salinity and Ca

2+

concentration have a certain positive correlation with porosity of Chang 6 member.Accor-dingly we may conclude that the origin of formation water in the area is closely related with the diagenesis and thermal evolution of organic matters

based on the regional thermal evolution history.What’s more

it is original sedimentary water

having undergone strong condensation and metamorphism and been affected by organic fluids such as oil and gas.All in all

the formation water associated with oil and gas in the 6

th

member of Yanchang Formation in the southern Ordos Basin manifests that the hydrocarbon reservoirs in the southern part of the basin is in a stable and closed environment

and the prospects for its oil-gas exploration are brighter.