High pass filter to opamp buffer causing an offset (op-amp specs)
Dwayne is correct.  Because you want a very low frequency filter, the resistors and capacitors have to be very large.  All real op amps have currents that flow into or out of  their input terminals. The large resistors cause this current to create a DC offset on the op amps.  You want to use an op amp with a CMOS, or perhaps JFET input stage, so the input current is small.  Lowering the resistor value is good, too, although then the capacitors get even bigger.  It is possible that adding a resistor of 3.6 meg in series with the (-) terminal of the op amp would help, but that is not always the case, and the big resistors add noise.
   In summary, you have chosen a very inappropriate op amp for this application: it has large input currents because it uses bipolar transistors (it is designed for high bandwidth.)  Choose an op amp with the adjective "precision" in its marketing description, this will be more appropriate for low frequency work.