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

where 

When wellbore fluid density is fairly condant between the gauyge location and formation top then ine can simplify the datum pressure calculation to:

See Also

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