petrophysics of thin beds - lps.org.uk · thin beds –conventional petrophysics struggles to...
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Geological Outcrop examples of Thin Beds
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Deepwater Thin Beds, Carboniferous Ross Fm, Ireland Triassic Blomidon Fm, Bay of Fundy, Nova Scotia
E. Jurassic, Bridport Sands, West
Bay, Dorset, UK
Quiz - Scale of Wireline Logs sampling volume
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Question 1/
The Bulk Density wireline tool samples a volume of rock
similar to the volume of what household item ?
A B C
Quiz - Scale of Wireline Logs sampling volume
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Question 1/ The Bulk Density wireline tool samples a volume of rock similar to the
volume of what household item ?
Answer/
A
Quiz - Scale of Wireline Logs sampling volume
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Question 2/
The Induction Resistivity wireline tool samples a volume of rock similar to the volume of what household item ?
A B C
Quiz - Scale of Wireline Logs sampling volume
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Question 2/
The Induction Resistivity wireline tool samples a volume of rock similar to the volume of what household item ?
Answer/
C
Thick Beds are well quantified byPetrophysics 1)
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5 ft 5 ft
Core Oil
Stain Flag
Net
Reservoir &
Net Pay
Flags
Core
Porosity
Core
Permeability
Water
Saturation
curve
(shaded to
show
hydrocarbons
Thick Beds are well quantified by Petrophysics 2)
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2 ft
Tight or shaley
interval
indicating no
fluorescence
in UV light i.e.
No oil stain.
Computed
Shale Volume,
VSH, log curve
Oil-leg N.B. Thick sands
showing UV
fluorescence
i.e. oil stain)
Core photos can even detect Oil-Water contacts in Thick beds
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5 ft
Tight or shaley
interval
indicating no
fluorescence
in UV light i.e.
No oil stain.
Water-leg N.B. Lack of UV
fluorescence
(i.e. oil stain)
Gap in core
Oil-leg N.B. Thick sands
showing UV
fluorescence
i.e. oil stain)
Thin Beds – Conventional Petrophysicsstruggles to quantify due to resolution
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1
2
50 ft
Wireline logs struggle to resolve and quantify Thin Beds e.g. < 2 feet thick.
Thick Beds are well quantified byPetrophysics – although still resolution issues 1)
12Tailwind Energy Ltd. Ltd.Generally Net Reservoir & Net Pay shows good agreement to Oil Staining in core over Thick beds. However,
Petrophysics not flagging some Net Pay (Oil Stain) due to log vertical resolution issues.
Core Oil Stain
Flag
Petrophysics
Net Flags
2 ft
Thin Beds – Conventional Petrophysicsstruggles to quantify due to resolution 2)
13Tailwind Energy Ltd. Ltd.However, over Thin beds Petrophysics can underestimate Net Reservoir & Net Pay (in this example by 3.5 feet)
compared to Oil Staining as indicated by core photos.
Core Oil Stain
Flag over Thin
Beds
Petrophysics
Net Flags
2 ft
Thin Beds – Conventional Petrophysicsstruggles to quantify due to resolution
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Core Oil Stain
Flag over Thin
Beds
2 ft
Here, for computed Net Pay to match core oil stain footage a strict VSH cut-off of <=0.40 is required
Thin Beds – Conventional Petrophysicsstruggles to quantify due to resolution
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2 ft
In previous work, these oil-stained Thin Beds were totally underestimated by Petrophysics.
A more relaxed VSH cut-off of <0.65 is required to match core oil stain footage
Special Case: Glauconitic interval & Thin Beds
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Interval looks shaly but is due to Glauconitic sand (mica). ALWAYS CHECK COMPOSITE LOG (Cuttings)
& CORE DESCRIPTIONS. Also, useful to generate a Density/Neutron Separation log to detect thin sandy beds
1) Section appears to be
entirely shaley. However, the
interval was tested and gave
good rates. Presumably he
interval was tested based on
core/cuttings indicating sands.
Also Deep Res > Shallow Res
indicating moveable
hydrocarbons.
1) Core indicates sand units with
oil stain, totaling 23 feet of oil
stained sands. These sands
contain Glauconite – giving the
shaley appearance on logs.
Density/Neutron separation
closing up indicates sandy units.
To match core and well test
result petrophysics requires
increased RHOsh parameter.
10 ft
Thin Beds – Petrophysical Findings
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• Standard Wireline & LWD logs may not be able to detect / quantify ThinBeds e.g. less than 2 ft (60 cm) thick.
• It is always good practice to refer to lithology cuttings and coredescriptions / core photographs for evidence of thin beds and unusualmineralogy. e.g. Micas such as Glauconite.
• Modern day technology can help detect and quantify Thin Beds e.g.Image logs, NMR, Multi-array Resistivity, Forward modelling.
• Core oil stain footage can be replicated by Petrophysical evaluations bycomparing/contrasting to Petrophysical cut-offs when computing NetPay footages.
• Petrophysical Cut-offs may need to meet more relaxed or stricter criteria,depending on the nature of the geological setting.