Minute Mark

Key Takeaways

• Logging speed limits for nuclear measurements are determined by the statistical counting requirement of the detector system: the gamma ray tool must be moving slowly enough that the measurement window (the integration time of the photomultiplier tube detector) corresponds to a depth interval small enough to resolve the formation features of interest, with the standard gamma ray tool having a vertical resolution (half-amplitude bed thickness) of approximately 0.6 meters at the standard logging speed of 275 to 365 meters per hour (900 to 1,200 feet per hour); faster logging degrades the vertical resolution because the measurement window spans a greater depth interval and the recorded value is an average over that larger interval, smoothing out thin bed responses; the density and neutron porosity tools have additional speed constraints because their dual-detector spine-and-rib algorithms for borehole correction require that both the short-spaced and long-spaced detectors measure the same formation interval simultaneously, which places a maximum logging speed of approximately 275 to 550 meters per hour (900 to 1,800 feet per hour) depending on the tool geometry; the minute mark allows immediate verification on the log print that these speed limits were not exceeded, without requiring access to the depth-time recording from the surface acquisition system.
• The minute mark is generated by the surface logging unit's depth measurement system, which triggers a brief signal to the downhole tool or the surface recorder at each 60-second interval: in older analog logging systems (pre-1980s), the minute mark was produced by briefly interrupting a designated curve (typically a resistivity or gamma ray track) to create a visible gap in the ink trace on the paper log print, with the depth counter value at each interruption recorded in the log header; in modern digital logging systems, the minute mark may be encoded as a discrete flag or timing marker in the digital data file that is displayed as a small symbol on the log plot, with the time-depth table from the acquisition system also preserved in the log header for independent verification of logging speed and depth positioning; the transition from analog to digital recording means that some service company presentations no longer show a visible minute mark on the color log plot delivered to the client, but the equivalent time-depth information is always present in the raw digital data (LAS file, DLIS file) for quality review.
• Logging speed inconsistencies detectable from minute marks include stick-slip (the tool momentarily stopping due to friction against the borehole wall and then jumping upward as tension in the cable builds, producing a compressed depth interval followed by a stretched depth interval between consecutive minute marks), cable slippage on the surface sheave (producing a systematic speed error if the sheave wheel encoder slips rather than rotating at the same rate as the cable), and deliberate slow logging over critical intervals (the engineer reducing logging speed over pay zones to improve statistical precision at the cost of longer logging time, visible as wider minute mark intervals in those zones); the stick-slip artifact is particularly significant in deviated wells where the logging tool drags on the low side of the borehole, producing depth intervals between minute marks that alternate between too small (while tool is stationary) and too large (during the catch-up lurch), creating a tool speed that oscillates around the nominal logging speed and producing artificial thin-bed-like responses on nuclear measurements over the affected intervals; stick-slip intervals can sometimes be identified retrospectively from the minute mark pattern even after the well has been logged and the logging unit has left the location.
• The minute mark is one of the primary quality control indicators reviewed during the log verification process, alongside the calibration records (pre-run and post-run), the repeat section comparison, and the environmental correction validity checks: when the log is delivered to the operating company, the quality review should include confirming that the minute mark depth intervals are consistent with the specified logging speed throughout the run, that any speed variations are documented and explained in the job log or log header, and that the speed over the critical pay intervals was within the tool's specified operating range; logs with documented speed violations (minute mark intervals indicating logging speed above the maximum for that tool and measurement type) require a note in the formation evaluation report that the affected measurements may have degraded vertical resolution and should be interpreted with appropriate uncertainty bounds on thin-bed identifications; the API recommended practice for gamma ray logging (API RP 33) and the service company operating manuals are the authoritative sources for the maximum logging speed specifications for each tool family.
• The time-depth relationship recorded in the minute mark sequence has applications beyond simple speed verification in specialized logging operations: in the computation of tool acceleration and velocity profiles for stick-slip correction algorithms, the minute mark provides a coarse time reference that is combined with the high-frequency depth encoder signal to reconstruct the actual tool position as a function of time; in formation pressure testing using wireline formation testers (MDT, RDT), the time-depth record allows the test sequence timing to be correlated precisely to the depth at which the tool was set; in long-duration logging runs in deep wells (greater than 5,000 meters) where the cable stretches significantly from temperature and weight, the minute mark intervals are used to compute a cable stretch correction that adjusts the depth scale of all logs to the more accurate corrected depth, reducing the depth uncertainty from the uncorrected cable stretch of 0.5 to 1 meter per kilometer to less than 0.1 meter per kilometer after correction.