The location of a downhole gauge may not match the formation top.

In order to translate it to the Datum one needs:

1. Estimate gauge readings at formation top using the wellbore fluid density gradient:  $//<![CDATA[ \begin{array}{l}\displaystyle p_{\rm top} = p(z) + g \cdot \int_z^{z_{\rm top}} \rho_f(z) dz\end{array} //]]>$
   
   
2. Recalculate the pressure at formation top to the Datum using regional hydrostatic pressure gradient:  $//<![CDATA[ \begin{array}{l}\displaystyle p_{\rm dat} = p_{\rm top} + GP \cdot (z_{\rm dat} - z _{\rm top})\end{array} //]]>$

See Also

Petroleum Industry / Upstream / Subsurface E&P Disciplines / Pressure Testing & Production Analysis