but I don't know if voltage divider will work with high resistor values...
The conventional method would be to make the high impedance resistor ladder and then run the output through an opamp to buffer the output (lower the impedance). The opamp takes the weak signal and makes it strong enough to feed into the ADC of your micro. Plus you can tweak the gain to get the voltages in the right range for your ADC to maximize the resolution.
The datasheet will show the maximum recommended signal impedance for your ADC. At 15Mohm resistance of your sensor leg of the ladder, yeah, you will mostly likely need to buffer the output.
Bitslice's suggestion has some pros and cons. The tolerance range of capacitors is a major con, if you plan to make more than one of these.