Using Well Logs (e-logs)
in the Petroleum industry
Earth Science World
Context
• First exposure to well logs - petroleum industry.
• Well logs are proxies for stratigraphic sections.
• Identify lithology.
• Represent stratigraphic sections.
• Lithostratigraphic correlation.
• Concepts
• Porosity & permeability re-visited.
• Rocks contain fluids – salty water or petroleum.
• Density stratification of fluids in rocks.
• Depositional models paramount in petroleum industry.
• Practicality of sedimentary geology ---> jobs.
Materials
• Borowski - SERC on-line materials.
• 1-inch well logs purchased from a log broker, Cambe.
Exercise components
• Lithology via well logs – shale vs. unconsolidated sand.
• Formation fluids – brine vs. petroleum (oil).
• Reinforces concept of porosity and permeability.
• Lithostratigraphic correlation using well logs as proxy.
• Concept of oil/water contact – density stratification.
• Calculate elevation of strata – datum, sea level.
• Structural cross section & structural trap.
• Test concept of horizontal oil/water contact.
• Necessity for geologists and use of depositional models
in petroleum industry.
Let’s run through the exercise!!
Chandeleur Sound Block
25 Field
• St. Bernard Parish, LA
• BB Sand - deltaic EODs
• 40 MMBO
Venice
BB Sand (Tex. “L”)
~ 9 Ma
Hyne (2000)
John et al. (2003)
Base map
Interpreting logs
• SP –spontaneous potential
• Passive tool
• Voltage between sensors
SP
shale
baseline
deep
High (+) – impermeable - shale
shallow
Low (-) – permeable – non-shale
• Shale baseline – connect farright readings
• Readings left of baseline
• Admixtures of coarser
grains w/ shale
• Interbedding of sand &
shales – different thickness
amplified
• Resistivity
• Deep- & shallow-reading
curves (sensor spacing)
SP
• Amplified curve
• Rock fluid content
deep
shallow
• Water in pore spaces with
increasing residence time
- increase [ dissolved ions ]
• Brines – high conductivity
– low resistivity
Low resistivity – brine – left
High resistivity – HC? – right
amplified
• Depth
• 1 inch = 100 feet
• 10 divisions each 100 feet
• each division = 10 feet
• Interval 5123’ – 5200’ MD
• Dominant lithology =
unconsolidated sand
• Interbeds of sand & shale
• SP curve kicks left away
from shale baseline
• Interval 5123’ – 5200’ MD
• Deep (left) and shallow
(right middle) read low R
(high conductivity)
• Consistent with brine
Defining the reservoir sand
•BB sand & BB marker (EODs)
Atlantic
S/L 4542 #14
Marine shale – delta avulsion
Sheet sand
– delta subsidence
Crevasse splay
Delta lobe
Distributary mouth bar
Matching
well log
curves
• Find the
correlation
in the #4
well, using
information
from the
#14.
curves
• Do this
correlation
for all wells
in the cross
section.
Identifying brine and petroleum
• High resistivity zone at top of BB Sand
• Higher resistivity is inconsistent with brine, but
consistent with oil
Atlantic S/L 4542 #4
(low conductivity).
• Bottom of high R
is at ~5285 MD.
• Brine occurs below –
low resistivity =
high conductivity.
• Top = 5272 MD,
bottom = 5285 MD
Thickness = ~13’ oil
High
R
Color-coding lithology and fluid content
Atlantic
S/L 4542 #4
sand
shale
oil
water
yellow
brown
green
blue
Correlating the reservoir across the field
• Well log correlation mimics lithostratigraphic correlation.
Correlated cross section
Petroleum patterns
• Oil always occurs atop salty water in the field wells.
• This occurs within pore spaces of the reservoir rock due
to density difference between oil and water
r oil < r water
Atlantic
S/L 4542 #4
Petroleum patterns
• Pore space in rocks is filled either with water, cement
(i.e., calcium carbonate, CaCO3), or petroleum.
• Almost always water is first within pore spaces and must
be displaced by migrating oil.
• This process can be modeled by thinking of an upsidedown bowl within a tub of water that is injected with
salad oil.
• Thought
experiment or
demonstration?
Horizontal
oil/water contact
Calculating elevation IRT sea level
Kelly bushing (KB) – measured depth = elevation IRT
elevation
target
sea level
Atlantic 4542 #4
25’
–
Now determine the bottom
of oil in each well and
determine its subsea
elevation, showing your
calculation at the bottom
of each well log on the
cross section.
5287’
= 5262’ subsea
O/W contact
Testing a hypothesis
• Given our experiment, the oil/water contact should be
horizontal within Chandeleur Sound Block 25 Field.
• This means that the contact should be at the same
elevation.
• Is it? Look most closely at the 4542 #4 well and compare
its oil/water contact to that of the #3 and 4545 #4 wells.
• Give plausible reasons why this is or isn’t so.
Testing a hypothesis
• Look most closely at the 4542 #4 well and compare its
oil/water contact to that of the #3 and 4545 #4 wells.
4542 #4
~ -5262’
4542 #3
~ -5270’
4545 #4
~ -5272’
oil/water
contact elev.
• Elevation of oil/water contacts differs by at least 10’;
maximum is ~ 33’
(#1 : #4 wells).
Testing a hypothesis
• Give plausible reasons why the contacts may be
different.
• E.g., different compartments with sand reservoir.
Need for geology & geologists in the petroleum industry
• The vertical (stratigraphic) and lateral distribution of
permeable reservoir rock is dependent upon the
depositional environment of that rock.
• Use depositional models to assess and
predict:
• reservoir quality (f & k)
• reservoir thickness
• reservoir compartments.
All figures after Reading (1978)
Summary
www.icdp-online.org
• Serves as an introduction to well
logging, proxy interpretation, & the
petroleum industry.
• More advanced exercise concerning
detailed e-log interpretation loaded
for this workshop.
• My Petroleum Geology materials
also available as handouts.
• Please do hesitate to contact me
concerning improvements
(w.borowski@eku.edu).
www.logwell.com
Completed cross section - left
Completed cross section -right
Completed cross section