Introduction to Facies Concepts, continued: interpreting facies

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Introduction to Facies Concepts, continued
interpreting facies Ideally, excellent
exposure on multiple outcrop faces or modern
environment with lots of exposure/many
samples. Reality, no exposure, limited
geophysical data, rarely a core. Learn to make
interpretations with limited data
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Well Logs (Wireline Logs) used since
1920s originally (and for some people still)
just used for correlation between locations now
much more sophisticated, multiple tools used to
give details of units being logged, and of the
fluids in them most common are open hole logs
run after the drill string is pulled out, before
setting any pipe in the hole Cant go back and
log old wells.
http//www.bakerhughes.com/bakeratlas/about/log4.h
tm
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Measure electrical (spontaneous potential,
resistivity), acoustic, and nuclear properties
(gamma ray).
http//www.bakerhughes.com/bakeratlas/about/log_in
dex.htm
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Common Logs Spontaneous Potential (SP
self-potential) logs electric log is a
record of naturally occurring electrical
potentials in a well hole as a function of depth.
The idea behind this log is very simple and it
was one of the first collected. One simply
measures the voltage between a point of
investigation (at some depth in the hole) and the
ground surface.
http//mac01.eps.pitt.edu/courses/GEO1413/SPLogs/S
P.html
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Uses of SP logs Differentiate potentially
porous and permeable reservoir rocks from
impermeable shales. Define bed
boundaries. Give an indication of shaliness
(maximum deflection is clean sand minimum is
shale). Determine Rw in both salt and fresh
muds.
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SP millivolts negative (sand) on left
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Resistivity Log electric log measures the
bulk resistivity (the reciprocal of conductivity)
of the formation. Resistivity is defined as the
degree to which a substance resists the flow of
electric current. Resistivity is a function of
porosity and pore fluid in a rock. porous rock
containing conductive fluid (such as saline
water) will have low resistivity. A non-porous
rock or hydrocarbon-bearing formation has high
resistivity. This log is very useful for
determining the type of fluids in formations and
is frequently used as an indicator of formation
lithology. important because one of the only
logs that measures several feet into the rock
beyond the borehole.
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often three resistivity curves collected at
once. collected by sending a current of known
intensity between two electrodes--one in the
hole and the other either at the surface or at
a distance in the hole.
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Resistivity ohm-meter2/meter
(ohm-m) higher values on right slightly
more difficult to interpret than SP--in
general is mirror image, but depends on fluid
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Gamma Ray (GR) Log nuclear log record the
amount of natural gamma radiation emitted by the
rocks surrounding the borehole. The most
significant naturally occurring sources of gamma
radiation are potassium-40 and daughter products
of the uranium- and thorium-decay series.
clay- and shale-bearing rocks commonly emit
relatively high gamma radiation because they
include weathering products of potassium feldspar
and mica (and tend to concentrate uranium and
thorium by ion absorption and exchange). can be
recorded in cased wells. tool is just a
scintillation crystal, a few inches long. GR log
provides good vertical resolution.
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Gamma Ray Log API units higher values on
right mimics SP, can be used as a substitute
for SP
http//mac01.eps.pitt.edu/courses/GEO1413/gamma/ga
mma.html
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SP, Gamma
Resistivity
Many more
http//strata.geol.sc.edu/log-suites.html
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Dipmeter Logs a well log from which formation
dip magnitude and azimuth can be determined.
resistivity dipmeter includes three or four
(sometimes eight) micro-resistivity readings made
using sensors distributed in azimuth about the
logging sonde and a measurement of the azimuth of
one of these a measurement of the hole deviation
or drift angle and its bearing and one or two
caliper measurements. The microresistivity curves
are correlated to determine the difference in
depth of bedding markers on different sides of
the hole.
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Dip log gross patterns only no details of
sedimentary structures
http//www.spwla.org/library_info/glossary/referen
ce/glossd/glossd.htm
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Seismic Data traditionally low-resolution,
deep-penetration industry scale data cannot
be used for much environmental interpretation
industry most interested in deep-penetration,
more regional analysis a pay-off between
resolution and penetration higher-frequency
sound waves have a higher chance of reflecting
off more subtle acoustic interfaces therefore,
the higher-frequency sound does not penetrate
far always improving and changing, especially
with 3D and attribute analysis
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Low-resolution seismic example (offshore South
Africa)
http//strata.geol.sc.edu/exerices/seismic/pg49_in
terpreted-samll.jpg
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High-resolution seismic line (offshore
Galveston-West Louisiana)
High-resolution seismic techniques can be used to
bridge the gap between traditional reflection
seismic techniques and outcrop-scale facies
analysis.
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http//www.rr-inc.com/Frame20Pages/Papers/gwpc/Re
flection20Survey.GIF
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Very-high-resolution methods 3.5 kHz
subbottom profiler penetration only a few tens
of meters resolution in decimeter
scale hull-mounted continuous collection,
normal cruising speeds, deep-water
3.5 kHz data from Pine Island Bay
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Chirper systems very-high-resolution,
decimeter scale towed system, close to the sea
floor
Chirp image from Ross Sea, including side-scan
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GPR (ground penetrating radar) somewhat
equivalent to these last two very- high-resolutio
n systems for onshore use only cannot
penetrate clay-rich deposits or below the water
table
GPR with 1 GHz antenna
http//www.terraplus.com/gprdetails.htm
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High-resolution techniques Uniboom electrome
chanical transducer emits sound waves 400-1400
Hz resolution to 0.5 m penetration to about
100 m, water depth to about 300 m
Uniboom record of Holocene reef, offshore Puerto
Rico
http//geology.uprm.edu/Morelock/GEOLOCN_/7_image/
prfcore.gif
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Sparker system high-voltage electrical
charge emitted from a towed array vaporizes
the water in vicinity of charge, creating an
air bubble which is the acoustic source sound
is in the 50-5000 Hz range resolution is a few
meters initial bubble pulse obscures upper
tens of meters
Sparker record from offshore California, from a 1
kjoule sparker
http//seis.natsci.csulb.edu/dfrancis/SEISPV.HTM
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Water guns and small air guns high-pressure
air (2000 PSI) is expelled into water column
to create energy source larger guns (measured
in cubic inches of air chamber) create greater
acoustic energy and lower frequencies air
guns provide greater energy per size water guns
have higher frequencies can tow an array of
air guns to gain energy and maintain high
frequency
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Water gun setup
The water gun is divided into two chambers, the
upper firing chamber, which contains compressed
air, and the lower chamber, which is filled with
water. When the gun is fired, the compressed air
forces the shuttle downward and this expels the
water from the lower chamber. The shot of water
leaving the gun creates a void behind it and the
collapse of water into this void creates an
acoustic wave.
http//woodshole.er.usgs.gov/operations/sfmapping/
seismicsources.htm
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Air gun setup
http//woodshole.er.usgs.gov/operations/sfmapping/
seismicsources.htm
On command from the seismic recorder, the air gun
releases a specified volume of high pressure air
into the water. The explosive release of air
produces a steep fronted shock wave followed by
several oscillations resulting from the repeated
collapse and expansion of the air bubble.
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Airgun record (210 in3) from Pine Island Bay
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References used throughout for Class 2 332
notes from Anderson Exxon training manual UT
Austin training manual Schlumberger Log
Interpretation Principles/Applications (1989)