Petrophysics and Exploration Targeting: Best Practice and

ApplicationsMike Dentith, Cameron Adams

The University of Western Australia

Barry BourneTerra Resources

Petrophysics – rock physical properties• Links the geologist’s view of the world with the

geophysicist’s viewChemistry → mineralogy → lithologyDensity/magnetism/conductivity etc → geophysical response

• Significance – what is the geophysics telling us?Mineral system concept has suggested many new geophysical targets: fluid/metal sources, fluid pathways, fluid reservoirsExploration under cover will use more geophysics but its interpretation will have less geological control

Petrophysics – a way forward?• Datasets are often too small and lack

adequate geological contextLithology is not the primary control in many cases → geology ≠ geophysicsPetrophysical consequences of alteration

• Predictive capability is generally poor• Particular need to understand seismic

and electrical conductivityMagnetotellurics, passive seismics

Petrophysics – a conceptual framework

Texture(Geometric Relationships Between Grains)

Bulk(Overall Mineralogy)

Grain(Amount, Size, Shape of Particular

Mineral Species )

Velocity

Paramagnetism

ElectricalConductivity

Ferromagnetism

Density

Lithology can be important, but this is not necessarily so

Porosity is a major control on all physical properties

(except magnetism)ElectricalPolarisation

Carbonate-hosted base metals -• Mississippi valley-type, Irish style• Traditionally considered a poor

geophysical target Mineralisation is poorly conductive and non-magnetic and does not contain radio-elementsGravity and seismic responses are of low amplitude and must be recognised against a noisy backgroundMay be an alteration halo of Fe-sulphides

FrSV

Tv

DaNEA ETDMTD

OLBVT

UMDKILOMETRES

0 500

Harper and Borrok (2007)

Dolomitised rock behind

dolomitisationfront

Carbonate-hosted base metals • Map prospectivity indicators undercover using velocity

(passive seismic) and/or conductivity (magnetotellurics)?N America: deposits associated with a regional dolomitisation frontIreland: deposits associated with ‘hydrothermal’ dolomite

ETD – East Tennessee DistrictMTD – Middle Tennessee DistrictOLB – Old Lead BeltUMD – Upper Mississippi DistrictDa – DavisNEA – NE ArkansasVT – Viburnum TrendTv – TimbervilleFr – FreidensvilleSV – Sinking Valley

MVT Deposits/Districts

Carbonate physical properties• Simple mineralogy

Calcite, dolomite (anhydrite, aragonite)• But chemically very reactive

Dolomitisation (calcite ↔ dolomite)Prone to dissolution and cementation

• Very complex porosity-permeability characteristics

Porosity is a major control on all physical properties (except magnetism)

Carbonate seismic properties• What are the key controls on the

distribution?

1.5 2.0 2.5 3.01000

2000

3000

4000

5000

6000

7000

Density (g/cm3)

Velo

city

(m/s

)

1.5 2.0 2.5 3.01000

2000

3000

4000

5000

6000

7000

Density (g/cm3)

Velo

city

(m/s

)

Calcite Dolomite

Anhydrite

Aragonite

Calcite Aragonite

Anhydrite

Dolomite

Data from 7 published sources

DolostoneLimestone

Carbonate seismic properties• What are the key controls on the

distribution?

1.5 2.0 2.5 3.01000

2000

3000

4000

5000

6000

7000

Density (g/cm3)

Velo

city

(m/s

)

1.5 2.0 2.5 3.01000

2000

3000

4000

5000

6000

7000

Density (g/cm3)

Velo

city

(m/s

)< 5%5-10%10-15%15-20%20-25%25-30%30-35%35-40%40-45%45-50%50-55%

Calcite

Dolomite

Anhydrite

Arag’

Calcite Aragonite

Anhydrite

Dolomite

Porosity()

Carbonate seismic properties• What are the key

controls on the distribution?

0 20 40 601000

2000

3000

4000

5000

6000

7000

Porosity (%)

Velo

city

(m/s

)

Calcite

Dolomite

0 20 40 601.0

2.0

3.0

Porosity (%)

Den

sity

g/c

m3 )

CalciteDolomite

0 20 40 601000

2000

3000

4000

5000

6000

7000

Porosity (%)

Velo

city

(m/s

)

Calcite

Dolomite

DolostoneLimestone

Carbonate seismic properties• What are the key controls on the distribution?

1.5 2.0 2.5 3.01000

2000

3000

4000

5000

6000

7000

Density (g/cm3)

Velo

city

(m/s

)

MoldicInter-particleIntra-particleMicroDensely cemented

1.5 2.0 2.5 3.01000

2000

3000

4000

5000

6000

7000

Density (g/cm3)

Velo

city

(m/s

)

DolomiteCalcite

Dolomite

Calcite

Reference

Stiff Pores

Cracks

Developed from diagrams in Anselmetti & Eberli (1997) and Xu & Payne (2009)

Carbonate seismic properties• What are the key controls on the

distribution – predictive capability?Velocity

DolomiteCalcite

Increasing porosity

Increasing density

Velocity

DolomiteCalcite

Increasing porosity

Increasing densityDeposition

(high carbonatesediment)

Early compaction& diagenesis

Sucrosicdolomitisation

Dissolution(moldic)

Cementation

Dolomitisation(no change to

Early compaction& diagenesis

Dissolution

Intense dolomiticcementation

Dissolution(intra/interparticle)

Developed from diagrams in Anselmetti & Eberli (1997)

Carbonate seismic properties• Theoretically you can map dolomitisation

fronts using seismic methodsAcoustic impedance – seismic attributesVelocity – passive or tomographic seismic

• Other options? Xiao et al. (2016)

Carbonate electrical properties• Formation factor and the Archie equation

Conductive Pore Fluidin Intergranular Porosity

Conductive Pore Fluidin Secondary Porosity

Low Formation Factor

High Formation Factor

Increasing Complexityof Current Flow Path

Pore Fluid orConductive Minerals

in Fractures

This

This ima

This image ca

This image cannot currently be displayed.

This image cannot currently be display…

Th

Th

101

102

103

10-2 10-1 100

Porosity ()

Form

atio

n Fa

ctor

(F)

10-3

100

Inter-Granular

Planar

Vuggy5

6 7

8

12

3

4

Saturated rockPore-fluid

F =am= a, m - empirical parameters

- fractional porosity

Carbonate electrical properties• Matrix plays no part• Electrical properties correlated with the type of

porosity/permeabilityDifferent values for different types of porosityRecognisably different populations for dolostone and limestone – because of their different types of porosity

Data replotted from Ragland (2002)

1.0Cementation Factor (m)

3.02.0 4.0

20

10

0

15

5

Dolostone

1.0

30

Cementation Factor (m)3.02.0 4.0

20

10

0

35

25

15

5

LimestoneMoldic/vuggyInter-particle/crystallineNon Fabric Selective

25

Carbonate electrical properties• Navan deposit, Ireland• Halo of hydrothermal dolomite reaches 20%

VE 20:1

KILOMETRES

0 500

Section A

Section B

Meath Fm

Stackallen Mbr

Vuggy porosity

Limits of dolomitisation

Conglomerate Group oreU/1 lens ore5 lens ore

Faults

A

BKILOMETRES

0 1

Ashton et al., (2015); Braithwaite & Rizzi (1997)

Conclusion• Petrophysical data are potentially very useful for

both testing and generating ideas about exploration strategy

More than just a guide for modelling geophysical responses

• Uncover and mineral systems: a need to understand what controls physical properties and develop a predictive capability

Think beyond lithology - consider alteration and porosityNeed plenty of data and of the right kind - scanners