WELCOME TOCRAIN'S PETROPHYSICAL HANDBOOKPlease be fair to the author. Pay your Shareware Fee HERE and receive a copy of CPH by download.READING OR "PICKING" LOG VALUES FOR PETROPHYSICAL ANALYSISThis Page Boxing a Log Picking Logs Parameters ExamplesSee Also Response Equation Crains Rules Courses Site Map LOG PICKING - "BOXING THE LOG"In order to perform a log analysis, it is necessary to read or pick log values inthe various zones of interest, and other key locations, such as in shale orwater bearing zones. Picking log values and analysis parameters from logs isTHE most important step in quantitative log analysis. Mathematics cannotcompensate for poor selections. Few comments on this subject are found inservice company training manuals. As a result, beginners often find it difficultto start with valid data, or assume the task is easy and requires no thought orknowledge.Selections should be made on a consistent basis from day to day to assistreproducibility of results. When using digital log data, the digits themselves will be used by the computer program,but the analyst must still pick numerous values by observation of log curves, crossplots, or data listings.In computer aided log analysis, picks are made continuously with a digitizer or by reading magnetic tapes or discscreated when the logs were recorded. Such data tends to be more accurate than hand picked values. Accuracy canbe a hindrance on noisy logs, rounded bed boundaries, or in large or rough holes. Some editing or curve shapingmay be required prior to digitizing, hand picking data, or using existing digitally recorded data.To select a log value, it is helpful, especially for the novice, to "box the log". Draw horizontal lines at each bedboundary, at the inflection points on each curve. Draw vertical lines on each curve at the peaks and valleys, thustransforming the log into a series of individual beds with a single specific log reading. For thicker reservoir layers, anaverage value may be chosen if the log curve does not wiggle too much.Crain's Petrophysical Handbook - Picking or Reading Well Logs https://www.spec2000.net/01-readinglogs.htm1 of 9 02/07/2015 20:33Reading well logs is made easier by "boxing the log". Horizontal lines (bed boundaries) and vertical lines(log values or log readings) form boxes that bound the data to allow valid data picks. Be sure to honour the "peaksand valleys" rule in thin beds.With experience, it is possible to simply mark points at the peaks and valleys without drawing horizontal lines, asshown in the lower part of the example. Unless absolutely necessary, values should not be selected on slopes.Slopes indicate transition from one condition, such as porosity or hydrocarbon content, to another. Average values,halfway along the slope may be meaningful, but can also be misleading. Do not select values in thin beds unless youare also prepared to make bed thickness corrections.Be sure to pick the correct curve, its appropriate scale, and edit any noise or bad hole conditions prior to finalizingvalues.Write the log values picked into a table or preprinted form so that you will have a record of the data you are using.Note that very shaly zones are not usually interpreted. Therefore, this data can be left off the table or marked as shalewith no data values entered.SHALY SAND EXAMPLE Depths in Feet(Logs above were in meters)RAW DATA PICKSCrain's Petrophysical Handbook - Picking or Reading Well Logs https://www.spec2000.net/01-readinglogs.htm2 of 9 02/07/2015 20:33Shale Properties:Gamma Ray clean line (GR0) ___22___ api unitsGamma Ray shale line (GR100)___105___ api unitsSP clean line (SP0) ____14___ mvSP shale line (SP100)____78___ mvDensity shale line (PHIDSH)__0.03___Neutron shale line (PHINSH)__0.26___Sonic shale line (DTCSH)___NA___ us/mResistivity shale line (RSH) ___20____ ohm-mResistivity ofWater Zone (R0)___2.7___ohm-mLog Data Values for LayersA: Sh Sd B: Oil C: WaterLayer Top __3364__3499__3512__ feetLayer Bottom__3406__3512__3530__ feetDeep Resistivity RESD___20____100___2.7___ ohm-mNeutron PorosityPHIN__0.24____0.27___0.24_Density Porosity PHID__0.12____0.22___0.19_Sonic Travel Time DTC __NA______NA____NA__ usec/mGamma Ray GR__75______25_____30__api unitsSpontaneous PotentialSP __55______23_____15__mvPhoto Electric Effect PE__3.4___ __2.0_____2.0__ cuCaliperCAL__8.5____7.7_____7.7__ inchesWhen using computers, log data is usually digitized at an increment much finer than the tool resolution. Thusanswers are calculated even on slopes and in thin beds. Understanding such results usually requires some thought.Again, be sure to pick the correct curve from the data file.GENERAL RULES FOR PICKING LOG VALUESIn order to perform a log analysis, it is necessary to read or pick log values in the various zones of interest, and otherkey locations, such as in shale or water bearing zones. Selections should be made on a consistent basis from day today to assist reproducibility of results.In computer aided log analysis, picks are made continuously with a digitizer or by reading magnetic tapes createdwhen the logs were recorded. Such data tends to be more accurate than hand picked values. Accuracy can be ahindrance on noisy logs, rounded bed boundaries, or in large or rough holes. Some editing or curve shaping may berequired prior to digitizing, hand picking data, or using tape-recorded data.To select a log value it is helpful, especially for the novice, to "box the log". Draw horizontal lines at each bedboundary, at the inflection points on each curve. In thinner beds, draw vertical lines on each curve at the peaks andvalleys, thus transforming the log into a series of individual beds with a single specific log reading. Pick peaks orvalleys in thin beds to get the best possible values.On thicker beds, draw a line through the average value of the curve. It is necessary to create a new bed or layer eachtime the porosity changes by 2% (porosity units), or when resistivity or gamma ray change by more than 10%(relative units)The rule is to draw bed boundaries at the top and bottom of each clean zone, then draw boundaries at the porositybreaks within each clean zone. Finally draw any new boundaries needed to accommodate resistivity changes, whichusually represent oil or gas water interfaces. The major interval between the top and bottom boundary of a relativelyclean rock is called a ZONE. Each boundary inside a ZONE defines a LAYER of rock. Each layer will be analyzedseparately by picking log values for each layer, called "Reading the Log"..This concept is shown on the log at the right.For thick layers, pick average values. For porosity, gammaray, laterolog, and array induction logs, "thick" means morethan 6 feet (2 meters). For older style induction logs, "thickCrain's Petrophysical Handbook - Picking or Reading Well Logs https://www.spec2000.net/01-readinglogs.htm3 of 9 02/07/2015 20:33means greater than 15 feet (5 meters).In thick beds, pick average values (heavy black vertical lines)===>Old style induction log, layer roughly 15 feet (5 meters), pick peaks and valleys; other logs, pick averages ===>