A statistical method for correcting log-derived temperatures

Abstract

Statistical analysis of subsurface temperatures in the Malay Basin has been carried out in order to (1) determine appropriate correction methods and correction factors for temperatures obtained from wireline logs and (2) investigate the confidence levels associated with those corrections. DST temperatures were taken as ‘ground truth’ for these corrections. For individual log-derived temperatures the correction factor f S , which is applied to the difference between the measured temperature and the surface temperature, is given by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="block" overflow="scroll"> <mml:msub> <mml:mi>f</mml:mi> <mml:mtext>s</mml:mtext> </mml:msub> <mml:mo>=</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.1462</mml:mn> <mml:mtext>Ln</mml:mtext> <mml:mo stretchy="false">(</mml:mo> <mml:mtext>TSC</mml:mtext> <mml:mo stretchy="false">)</mml:mo> <mml:mo>+</mml:mo> <mml:mn>1.699</mml:mn> <mml:mo stretchy="false">)</mml:mo> <mml:mo>/</mml:mo> <mml:mo stretchy="false">(</mml:mo> <mml:mn>0</mml:mn> <mml:mo>.</mml:mo> <mml:mn>572</mml:mn> <mml:mo>⋅</mml:mo> <mml:msup> <mml:mi>Z</mml:mi> <mml:mrow> <mml:mn>0.075</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mo>,</mml:mo> </mml:math> where TSC is the time since end of mud circulation in hours and Z is the depth in metres. For temperatures that have already been corrected by extrapolation using Horner plots, f HP is given by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="block" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>f</mml:mi> <mml:mrow> <mml:mtext>HP</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.1321</mml:mn> <mml:mtext>Ln</mml:mtext> <mml:mo stretchy="false">(</mml:mo> <mml:mtext>TSC</mml:mtext> <mml:mo stretchy="false">)</mml:mo> <mml:mo>+</mml:mo> <mml:mn>1.52</mml:mn> <mml:mo>,</mml:mo> </mml:mrow> </mml:math> where TSC is the maximum time since circulation stopped (hours) in the Horner plot set. Uncertainties in f S decrease markedly as TSC and depth increase. Uncertainties in f HP decrease as maximum TSC and the number of consistent temperature measurements at a given depth increase. Although these correction factors were developed using data from a single basin, our experience suggests that they can be used with reasonable confidence in many or most other geological provinces. Additional local calibrations would help test and refine this hypothesis.