  Leakage of polarized Galactic diffuse emission into total intensity can
potentially mimic the 21-cm signal coming from the epoch of reionization (EoR),
as both of them might have fluctuating spectral structure. Although we are
sensitive to the EoR signal only in small fields of view, chromatic sidelobes
from further away can contaminate the inner region. Here, we explore the
effects of leakage into the 'EoR window' of the cylindrically averaged power
spectra (PS) within wide fields of view using both observation and simulation
of the 3C196 and NCP fields, two observing fields of the LOFAR-EoR project. We
present the polarization PS of two one-night observations of the two fields and
find that the NCP field has higher fluctuations along frequency, and
consequently exhibits more power at high-$k_\parallel$ that could potentially
leak to Stokes $I$. Subsequently, we simulate LOFAR observations of Galactic
diffuse polarized emission based on a model to assess what fraction of
polarized power leaks into Stokes $I$ because of the primary beam. We find that
the rms fractional leakage over the instrumental $k$-space is $0.35\%$ in the
3C196 field and $0.27\%$ in the NCP field, and it does not change significantly
within the diameters of $15^\circ$, $9^\circ$ and $4^\circ$. Based on the
observed PS and simulated fractional leakage, we show that a similar level of
leakage into Stokes $I$ is expected in the 3C196 and NCP fields, and the
leakage can be considered to be a bias in the PS.
