Genesis of formation water of 8

th

Member of Xiashihezi Formation (H8) and their relationships with gas accumulation in western Sulige gas field of Ordos Basin in central China was studied by means of analysis of the geochemical behaviors of formation water samples from 39 gas wells.The distribution regularity of oil

gas and water was investigated.There is an obvious difference in the concentration(r)of anion and cation of formation water.Content of Cl

-

is very high and dominant among the anions of the water

while that of HCO

3

-

and SO

4

2-

are very low.Content of Ca

2+

is the highest among the cations of formation water

followed by K

+

+Na

+

while the content of Mg

2+

is low.The formation water show property of weak acidity with medium-high salinity.The calcium chloride Ⅴ-type water is predominant

with the calcium chloride Ⅲ-type and Ⅳ-type water occurring locally.The chemical characteristic parameters of the formation water are characterized by low sodium-chloride ratio (r

Na

+

/r

Cl

-

)

high coefficient of desulfurization (2×100×r

SO

4

2-

/r

Cl

-

)

high metamorphic coefficient[1/2(r

Cl

-

-r

Na

+

)/r

Mg

2+

]and low magnesium-chloride ratio (r

Mg

2+

/r

Ca

2+

).The formation water of H8 in western Sulige gas field has the characteristics of gas-associated water.It could be residual formation water formed when gas was charged under a closed and reducing environment.The distribution of formation water is controlled by sand body and reservoir conditions.Three genetic types of formation water are identified

namely the closed formation water within isolated sand body

the residual water with weak dynamic and incomplete gas-water displacement

and the stagnant water at lower position of large sand body.