Correction of Single Log-Derived Temperatures in the Danish Central Graben, North Sea

Abstract

An equation for correcting log-derived temperatures measured at well depths between 1200 and 5400 m has been derived by comparing log-derived temperatures from wells in the Danish Central Graben (North Sea) with DST temperatures, from the same wells, that are believed to represent true formation temperatures. Equations developed previously using data over different depth ranges from Malaysia and Mexico yield fair results when applied to the North Sea. However, a better fit to the Danish data was obtained using new equations that are similar to those published in the earlier studies. The correction method here is based principally on time since end of circulation (TSC), but it also includes a small dependence on depth. In this study the true subsurface temperature (T_true) (°C) is given by $\hspace*{10pc}T_{\rm true} = T_{\rm surf}+f\cdot(T_{\rm meas} - T_{\rm surf})$ where the correction factor f = 3.07·TSC^(−0.09)/(0.47·Z^(0.175)), T_surf is the seafloor temperature in °C, T_meas is the measured log temperature in °C, TSC is in hours, and Z is the depth below seafloor in meters. When TSC is not known, maximum probable, minimum probable, and most likely values can be estimated from the observed trend of TSC with depth.

An estimate of the uncertainty in the corrected temperature can be obtained from the equation $\hspace*{10.5pc}\sigma=0.183{\rm \, exp}(-0.048{\rm TSC})$ where σ is the standard deviation of the error in the correction factor f. This approach can be modified to include the additional uncertainty associated with unspecified TSC.