The spontaneous potential (SP) log measures naturally occurring electrical voltages in formations and is used to determine lithology and permeability. Specifically, SP response is created by differences in salinity between mud filtrate and formation waters in permeable beds. SP can be used to detect permeable beds, determine formation water resistivity, and calculate shale volume. Key factors that influence the SP measurement include the ratio of mud filtrate to formation water resistivity and the presence of hydrocarbons or shale.

1. SpontaneousPotential

2. Michel Heeremans, IG, UiO
General
•SpontaneousPotentialis oneoftheearliestlogsused and still in use
•Primarilyused for determininggross lithology, i.ereservoirvsnon-reservoir(permeable vsnon- permeable)
•Correlation
•Severalotherequallyimportantuse

3. Michel Heeremans, IG, UiO
TheSP Log
•A recordofDirectCurrent(DC) voltage(or Potential) thatdevelopsnaturally(spontaneous)betweena moveableelectrodein thewelland a fixedelectrodelocatedat thesurface
•Measuredin millivolts(mV)
•Mnemonics: SP
•SP responsecreatedby electricvoltagesarisingfrom elctrochemicalfactorsin theboreholeand adjacentrocks
•SP Readingsarecausedby differencesin salinitiesbetweenmud filtrate and formationwaters in permeable beds
•Salinityofa fluid is inverselyproportionalto itsresistivity, so in practicemud filtrate salinityis indicatedby mud filtrate resistivity(Rmf) and formationwater salinityby formationwater resisitivity(Rw)
•Conductivefluid necessaryin boreholeto createSP response, so SP cannotbe used in nonconductive(e.g. oilbased) drilling muds or in air-filledholes

4. Michel Heeremans, IG, UiO
TheSP Log
•Usuallydisplayedin thelefttrack(correlation)
•Used to
–Detectpermeable beds
–Detectboundariesofpermeable beds
–Determineformation-waterresistivity(Rw)
–Determinethevolumeofshalein permeable beds
–Detectionofhydrocarbonsby thesuppressionoftheSP curve

5. Michel Heeremans, IG, UiO
Introductionto theterm SSP
•SSP:
–StaticSpontaneousPotential
–MaximumSP thata thick, shalefree, porousand permeable formationcanhave for a given ratio betweenRmfand Rw
–Determinedby formulaor by chart
–Necessaryfor determiningaccuratevaluesofRwand volumeofshale

6. Michel Heeremans, IG, UiO
WhatinfluencesSP measurement
•SP influencedby:
–Bed thickness
–Bed resistivity
–Boreholediameter
–Invasion
–Shalecontent
–Hydrocarboncontent
and, most important
–Ratio ofRmfand RwNormal SPReversedSPFreshwater
SSP: Max deflectionpossiblefor given Rmf/Rw
SP: SP responsedue to presenceofthinsbeds and/or gas presence
PSP: PseudostaticSP; SP whenshaleis presentRmf= freshRw= saltRmf= saltRw= fresh

7. Michel Heeremans, IG, UiO
WhatinfluencesSP measurement
•Bed Thickness
–Thinformations(<3m) themeasuredSP is less thantheSSP.
–Narrow, pointedSP curve; correctionfor bed thicknessrequired
•Bed Resistivity
–HigherresistivitiesreducethereflectionoftheSP curve
•Boreholeand Invasion
–Usuallyverysmall and can, in general, be ignored
•ShaleContent
–Presenceofshalein a permeable formation, reducestheSP deflection
•HydrocarbonContent
–In hydrocarbon-bearingzones, theSP curvesdeflectionis reduced: Hydrocarbonsuppression.Onlyqualitative, not possibleto determineShc

8. Michel Heeremans, IG, UiO
ShaleBaseline
•ShaleBaseline: –TherelativelyconstantSP responseofshales–Assumedto be zero–Permeable zonesindicatedwherethereis a deflectionfrom theshalebaseline•SP readingsaremeasuredrelative to theshalebaseline–Candrift over longdistances. Oflittleconsequencefor single formationsPermeableZonesImpermeable ?

9. Michel Heeremans, IG, UiO
Shape oftheSP curve
•Shape and Amplitude oftheSP deflectionoppositea permeable bed dependson:
–Thickness, h, and true resistivity, Rt, ofthepermeable bed
–Resistivity, Rxo, and diameter, di, ofthezonecontaminatedby mud filtrate invasion
–Resistivity, Rs, oftheadjacentshaleformation
–Resistivityofthemud, Rm, and thediameter, dh, oftheborehole

10. Michel Heeremans, IG, UiO
FormationWater Resistivity(Rw) from SPnmtwntwntwRRaRRFRRS1110⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ ××=⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛×=⎟⎟⎠ ⎞ ⎜⎜⎝⎛ = φ Remember: •Procedure: –Necessaryinfo: Rmfand Rm(at a given T), Tsurf, TD, BHT–DetermineFormationTemperature, Tf(calculateor chartGen-6) –CorrectRmand Rmffor Tf(chartGen-9) –DetermineSPfrom log: Max SP –ShaleBaseline SP–CorrectSPto SSPfor thinbeds (chartSP4-6) –DetermineRmf/Rweratio and from thatRwe(chartSP1) –CorrectRweto Rw(chartSP2-3)

11. Michel Heeremans, IG, UiO
Identifya zoneonthelog thatis clean, wetand permeable. Pickthemax. valueofSP in thatzone
CalculateformationT at thedepthofSP
BHT –BottomHole Temperature
AMST –AnnualMeanSurfaceTemperature
FD –FormationDepth
TD –Total Depth
Tf–FormationTemperature
ConvertRmffrom surfaceT to formationtemperature
Rmf–Rmfat formationtemperature
Rmfsurf–Rmfat measuredtemperature
Tsurf–MeasuredtemperatureofRmf
Findtheequivalentformationwater resistivity, Rwe, from theSP and Rmf
Rwe–equivalentRw
ConvertRweto Rw(at formationT)
FormationWater Resistivity(Rw) from SPAMSTFDTDAMSTBHTTf+⎟⎠ ⎞ ⎜⎝ ⎛×− = ()BHT133.061SPmfwe10RR⋅+×= () ()⎥⎦ ⎤ ⎢⎣ ⎡ −⎥⎦ ⎤ ⎢⎣ ⎡ +×− ×+ = 8.50BHTlog0426.0we29.19BHTlog1wew10R5.010131.0RR

12. Michel Heeremans, IG, UiO
CalculationofShaleVolumewithSPSSPSPSSPPSPVorSSPPSPVshaleshaleshale− − = −=0.1
Volumeofshaleis used in theevaluationofshalysand reservoirsand as mappingparameter for bothsandstoneand carbonatefaciesanalysis

13. Michel Heeremans, IG, UiO
Summary
Used for
Knowing
Quantitative
Rw
Rmfand T
Vsh
SSP and ShaleBaseline
Qualitative
Permeabilityindicator
ShaleBaseline
Facies(shaliness)
Clay/Grainsizerelationships
Correlation
PrincipalusesoftheSP log

14. Gamma Ray

15. Michel Heeremans, IG, UiO
General
•Gamma Ray(GR) toolmeasuresthenaturalradioactivityin formations
•Used to identifylithologies, correlationand Vshcalculation
•Shale-freesandstonesand carbonateshave lowconcentrationsofradioactivematerial and givelowGR readings
•Increasingshalecontent(= increasingcontentofradioactivematerial) causeincreasingGR readings
•Be aware, cleansst withK-feldspar, mica, glauconite, or U-richwaters mayalsoproduceshighGR readings
•Usuallydisplayedin thelefttrack(Correlationtrack)
CurveName
Mnemonics
Units
(Total) Gamma Ray
GR
API
U-freeGR
GRS, SGR, KTH
API
Potassium
POTA, K
Percent
Uranium
URAN, U
Ppm
Thorium
THOR, TH
ppm

16. Michel Heeremans, IG, UiO
SpectralGR
•ThespectralGamma Ray(SGR) log recordsnot onlythenumberofgamma raysemitted, butalsotheenergyofeach•Processingthatinformationgivescurvesfor thepresenceofTh, K and U in theformation•HighK, HighGR responsemayindicatea feldspathic, glauconiticor micaceoussst

17. Michel Heeremans, IG, UiO
() ()[] () rocksOlderLarionovVClavierIVSteiberIIVRocksTertiaryLarionoVGRGRGRGRIGRGRIshGRshGRGRshIshGR)1969( 1233.0)1971( 7.038.37.1)1970( 23)1963( 12083.022127.3minmaxminlog−= −−−= ×− = −= − − = ⋅ ⋅
CalculationofShaleVolumewithGR
•First stepis to calculatetheGR Index(IGR)
•Successively, severalnon- linearrelationshipsmaybe applieddependingonfomationage or otherlocalinformation

18. Caliper

19. Michel Heeremans, IG, UiO
Caliperlog
•Interpretationgoals:
–Indicationofhole diameter and volume
–Qualitaticeindicationofpermeability
–Correlation
–Log qualitycontrol
•Units: inches, cm
•Mnemonics: CAL, CALI

20. Michel Heeremans, IG, UiO
Examples•DeteminetheRwfrom theSP log•Given: –Rmf= 0.51 ohmmat 135°F (BHT) –Rm= 0.91 ohmmat 135°F (BHT) –SurfaceTemperature= 60°F–Total Depth(TD) = 8007 ft–BottomHole Temperature(BHT) = 135°F•From thelog: –FormationDepthat SPmax= 7446 ft–Bed Thickness= 7450 –7442 = 8 ft–Short normal resistivity(Ri = Resistivityoftheinvadedzone) = 33 ohmm

21. Michel Heeremans, IG, UiO
Examples
•DetermineVshfrom GR•From thelog: –GRmin= 14 API at 13593 ft–GRmax= 130 API at 13577 ftand 13733 ftDepth(ft)GRlogIGR13,534320.1613,570280.1213,700550.3513,53413,70013,570

22. Michel Heeremans, IG, UiO
Examples
Shale Volume
Depth GRlog IGR
Linear
Larionov
(older
rocks)
Steiber
13,534 320.160.160.080.06
13,570 280.120.120.060.04
13,700 550.350.350.230.16
DetermineVshfrom GR

23. Exercise