Changes

;6b. Depth shift core data to logs: The depth correlation of log data to core data is frequently characterized by numerous abrupt changes in the amount to be shifted. Every [[trip]] with the core barrel is potentially a change in the relative core or log depth, even if continuous cores are taken. Zones flagged as lost core zones often are not where they were interpreted to be. Because of this, automatic depth shift procedures generally do not work when shifting core data to well log data. An overlay procedure is recommended where the core is segmented by core run and again where missing data occurs within a core. The core is then usually shifted by segments. Segments can separate or overlap. Separation is caused by incomplete or poor core recovery, and overlap can be caused by poor core-handling procedures. Review of the field [[core description]] can help clarify some of these problems. (For more information on cores, see [[Conventional coring]], [[Core handling]], and [[Core description]].)

;6b. Depth shift core data to logs: The depth correlation of log data to core data is frequently characterized by numerous abrupt changes in the amount to be shifted. Every [[trip]] with the core barrel is potentially a change in the relative core or log depth, even if continuous cores are taken. Zones flagged as lost core zones often are not where they were interpreted to be. Because of this, automatic depth shift procedures generally do not work when shifting core data to well log data. An overlay procedure is recommended where the core is segmented by core run and again where missing data occurs within a core. The core is then usually shifted by segments. Segments can separate or overlap. Separation is caused by incomplete or poor core recovery, and overlap can be caused by poor core-handling procedures. Review of the field [[core description]] can help clarify some of these problems. (For more information on cores, see [[Conventional coring]], [[Core handling]], and [[Core description]].)

:A core gamma ray can be a valuable aid in establishing depth correlations between core and logs. Boyle's law core porosity and core grain density can be used to construct a core bulk density curve to correlate with log bulk density to determine the amount of depth shifts required. Core bulk density usually correlates well with the density log because lithologic variations are eliminated, resulting in two similar curves being correlated.

:A core gamma ray can be a valuable aid in establishing depth correlations between core and logs. Boyle's law core porosity and core grain density can be used to construct a core bulk density curve to correlate with log bulk density to determine the amount of depth shifts required. Core bulk density usually correlates well with the [[density log]] because lithologic variations are eliminated, resulting in two similar curves being correlated.

:While interpolation is a necessary step in the depth matching of wireline logs, it is highly undesirable when shifting cores. Interpolation should not be done when a core segment is shifted. Also, core data should not be resampled if least squares correlations are planned for calibrating logs or developing porosity and [[permeability]] relationships. Linear resampling of permeability destroys porosity and permeability relationships and can make statistical inference incorrect when making core to core or log to core data correlations. It is recommended that in any of these correlations the logs be resampled, not the core data.

:While interpolation is a necessary step in the depth matching of wireline logs, it is highly undesirable when shifting cores. Interpolation should not be done when a core segment is shifted. Also, core data should not be resampled if least squares correlations are planned for calibrating logs or developing porosity and [[permeability]] relationships. Linear resampling of permeability destroys porosity and permeability relationships and can make statistical inference incorrect when making core to core or log to core data correlations. It is recommended that in any of these correlations the logs be resampled, not the core data.