  We present the sensitivity of the Theia experiment to low-energy geo- and
reactor antineutrinos. For this study, we consider one of the possible proposed
designs, a 17.8-ktonne fiducial volume Theia-25 detector filled with
water-based liquid scintillator placed at Sanford Underground Research Facility
(SURF). We demonstrate Theia's sensitivity to measure the geo- and reactor
antineutrinos via Inverse-Beta Decay interactions after one year of data taking
with $11.9\times10^{32}$ free target protons. The expected number of detected
geo- and reactor antineutrinos is $218\,^{+28}_{-20}$ and $170\,^{+24}_{-20}$,
respectively. The precision of the fitting procedure has been evaluated to be
6.72% and 8.55% for geo- and reactor antineutrinos, respectively. We also
demonstrate the sensitivity towards fitting individual Th and U contributions,
with best fit values of $N_\text{Th}=39\,^{+18}_{-15}$ and
$N_\text{U}=180\,^{+26}_{-22}$. We obtain $(\text{Th}/\text{U})=4.3\pm2.6$
after one year of data taking, and within ten years, the relative precision of
the (Th/U) mass ratio will be reduced to 15%. Finally, from the fit results of
individual Th and U contributions, we evaluate the mantle signal to be
$S_\text{mantle} = 9.0\,\pm [4.2,4.5]$NIU. This was obtained assuming a
full-range positive correlation ($\rho_c\in[0, 1]$) between Th and U, and the
projected uncertainties on the crust contributions of 8.3% (Th) and 7.0% (U).
When considering systematic uncertainties on the signal and background shape
and fluxes, the mantle signal becomes $S_\text{mantle} = 9.3\,\pm
[5.2,5.4]$NIU.
