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OrganicGeochemistryforUnconventionalExplorationandProduction
JeremyDahlStanfordUniversity
UnconventionalExploration/LandEvaluation
Crucialfactorsaffectingtheeconomicsofatight-shaleplay
• Carboncontenti.e.,TotalOrganicCarbon(TOC)• Liquidsgenerationpotentialoftheorganicmatter• Determinationofhow muchgasandoilhavealreadybeengeneratedfromthekerogen
• Determination oftheextentofoilcracking(thermalconversionofliquidstogas)
• Mineralogy,StructuralGeology
CommonmethodsusedtodetermineGenerativePotential
• LECO(Combustion)TOC,H/CRatio• Rock-Eval• Arethereconventionalwellsinthearea
MaturityImmobileOrganicMatter
• Rock-Eval• Vitrinite Reflectance
MaturityMobilePhases• GC,GC-MS(Biomarkers,Diamondoids)• GasIsotopesandWetness
Most steranes in petroleum originate from sterols (1) in the lipid membranes ofeukaryotic organisms. Stereochemistry is indicated by open (a) or solid (ß) dots, whichindicate that hydrogen is directed into or out of the page, respectively. The biologicalconfiguration [14a,17a(H)-20R] imposed on the sterol precursor and its immediatesaturated product (2) by enzymes in living organisms is unstable during catagenesisand undergoes isomerization to geological configurations (e.g., 3, 4, 5).
Figure 2.31
Diagenesis
Sterol in Eukaryotic Organism
X = H, CH 3, or C2H5
Sterane in Sediment (BiologicalConfiguration) αα20R
HO
1720
X17
20
X
X X X
ββ20S
Steranes in Source Rocks and Crude Oils(Geological Configurations)
3ββ20R αα20S
4 5
αα20R1 2
Biomarkers
Biomarker maturation parameters respond in different ranges of maturity and can be used to estimate the maturity of crude oils or source-rock extracts relative to the oil window.
BiomarkerMaturityRatios
δDMethane (â )
-350 -300 -250 -200 -150 -100
δ13 C M
etha
ne (â
)-80
-70
-60
-50
-40
-30
-20
Gas Wetness (% C2+)0 5 10 15 20 25 30
-80
-70
-60
-50
-40
-30
-20
Post-MatureWet Gas
Microbial GasCarbon Dioxide Reduction
MicrobialGas
AcetateFermentation
Microbial GasCarbon Dioxide Reduction
Microbial GasAcetate Fermentation
Mixed Gas
Mature GasFormed With Oil
Mixed Gas
Mature GasFormed With Oil
Post-MatureWet Gas
Post-MatureDry Gas
PostMatureDry Gas
Modified after Schoell, 1983
DeterminingGasThermalMaturity
OilCracking:HighlyImportant• Generatesreservoirpressure• Generatesgastosolubilize andcarryliquidstowell-head
• Crackslargemolecules(asphaltenes andresins)whichcanplugnanopores tosmallmolecules
• Createsporosityandpermeability• Iftoomuchcrackinggetdrygas• Sweetspot =Intermediateamountofcracking
1.GasIsotopesandWetness2.Directdeterminationofoilcracking(thermalconversionofliquidstogas)
• Diamondoids
11
Theyarenanometer-size,hydrogen-terminateddiamonds
AdamantaneDiamond CVD Higher Diamondoid
0.5-2nm
Micro-cyrstalline
Whatarediamondoids?
Whathappenstodiamondoidswhenwecrackanoil?
Oil
Methane + Pyrobitumen
ActivationEnergy
FreeEnergyΔG
(Temp)
Methane
50%OilCrackingDoublestheDiamondoid Concentration
1ml5ppm
0.5ml10ppm
0.5ml10ppm
Methane
0.25ml20ppm
CrackAnother50%oftheOilandtheDiamondoid ConcentrationDoublesAgain
Methane
0.25ml20ppm
0.125ml40ppm
AndSoOnAccordingtotheequation:%Cracking=[1- (Co/Cc)]X100
OilGenerationMovieIllustratingImportanceofTOC,Generation
Potential,MaturationandOilCracking
Dahl,Castagna,Skinner,Goergen andLemmen
StanfordUniversityandFEI
FractureOrientation
• Arefracturespredominantlyparalleltobeddinglikeinthemovie?
• Ismigrationpredominantlyparalleltobedding?• Aresourcerockswithverticalfracturesintectonically-activeareasmorepronetoexpulsionandthereforemorepronetogenerateconventionalplays?
• Aresourcerocksonstablecratons morelikelytoretainhydrocarbonsmakingthembetterunconventionalplays?
Low
Low Diamondoids
Low
High
High
Low
Low
Low
Diamondoid ConcentrationsShowLong-DistanceLateralMigration
20
Low Diamondoids
CORE Migration
(SweetSpot)
Low
UnderstandingPost-HydraulicFractureProduction
Zone1
Zone2
GeochemicalProductionAllocation:GCPeakHeightmethod.NeedEndMembers
Zone1
Zone2
GC Peak Height method (Mark McCaffrey)In the commingled oil, a particular peak height (concentration of this chemical component) is made up of contributions from the end members β1X1 +β2X2 +...βmXm.
β1X1 +β2X2 + β3X3
If there are 3 commingled Zones (3 end-members),then the peak height is:
β1X1 contribution
β2X2 contribution
β3X3 contribution
UnderstandingPost-HydraulicFractureProduction:ManyZones
Depth
Biomarker/Isotope/Diamondoid values
UnconventionalFracturedsection
CoreExtractAnalyses
1. Analyzeandplotcoreextractstomakeatemplate.
PlotOilValuesDe
pth
Biomarker/Isotope/Diamondoid values
UnconventionalFracturedsection
CoreExtractAnalyses
1. PlotproducedoilsonTemplate Oil1
Time1Oil2
Oil1Time2
Lossoflightends
Generallyduetopoorsamplehandlingandevaporation.
Lossofheavyends
ProductiontoofastorReservoirhasgonebelowbubblepoint.
MonitoringProductionofUnconventionals
Unfractionatedoilsfromthesamesourceplotonmaturity/crackingline
4- +3-methyldiamantanes:LargerDiamondoids
1-+2-methyladamantanes:SmallerD
iamondoids
