I agree with Andre, but I would further qualify his response with a few more potential gotchas. If your reservoir interval has conductive minerals such as glauconite, you'll need to correct your resistivity log. Thin bed effects (bedding thickness below the tool resolution) can also throw off your resistivity log.

EC = 10000/Rw

F (formation factor) = Rf/Rw

maybe these two works can help you:

Hayashi M.; 2004: Temperature-electrical conductivity relation of water for environmental monitoring and geophysical data inversion. Environmental Monitoring and Assessment, 96, 119-128.

Wagner, R.J., Boulger, R.W., Jr., Oblinger, C.J., and Smith, B.A., 2006, Guidelines and standard procedures for continu¬ous water-quality monitors—Station operation, record computation, and data reporting: U.S. Geological Survey Tech¬niques and Methods 1–D3, 51 p. + 8 attachments; accessed April 10, 2006, at http://pubs.water.usgs.gov/tm1d3

Even for Standard Seawater, this problem is still on the table of the recently established IAPWS/SCOR Joint Committee on Seawater. There will be a related salinity workshop at the 16th ICPWS (www.icpws16.org). Take a look at the brief explanations given in (free access):

http://www.ocean-sci.net/8/161/2012/os-8-161-2012.pdf

and

http://www.teos-10.org/index.htm

Rainer

Besides the resistivity log the SP log also can be used for formation water salinity estimation based on the Nerst-equation.