Upload
daniel-viloria
View
231
Download
0
Embed Size (px)
Citation preview
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 1/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis
CHAPTER 2
CORING AND CORE ANALYSIS PROCESSES
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 2/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis
CONTENTS : CHAPTER 2
1. CORING AND CORE ANALYSIS PROCESSES 1
2. CORING 3
2.1 Core Types 3 2.1.1 Conventional Core 3 2.1.2 Sidewall Cores 3
2.2 Core Liners 2 2.2.1 Gel Cores 2
2.3 Sponge and Pressure Core Barrels 3 2.3.1 Pressure Cores 3 2.3.2 Sponge Cores 3
3. CORE ANALYSIS LABORATORY PROCESSES 6
3.1 Core Handling 6
3.2 Core Arrival 8
3.3 Core Gamma Ray Logging 8
3.4 Core Scanning 9
3.5 Core Plugging 10
3.6 Plug Measurements 12
3.7 Core Slabbing and Resination 13
3.8 Core Photography 13
4. PETROGRAPHIC TESTS 15
4.1 Thin Section Analysis 16 4.1.1 Selection and Preparation 16 4.1.2 Carbonate Phase Staining 16 4.1.3 Quantitative Nodal Analysis 17 4.1.4 Thin Section Descriptions 18 4.1.5 Sandstone Classification 18 4.1.6 Carbonate Classification 19 4.1.7 Example Thin Section 19
4.2 Scanning Electron Microscopy 23
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 3/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis
4.2.1 Background 23 4.2.2 Analysis Techniques 23 4.2.3 Analysis 23
4.3 X-Ray Diffraction 26 4.3.1 Theory 26 4.3.2 Whole Rock Preparation and Analyses 26 4.3.3 Extraction of the Clay Fraction and Analyses 27 4.3.4 Examples 28
5. CORE-LOG DEPTH MATCHING 31
6. REFERENCES 36
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 4/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 1
1. Coring and Core Analysis Processes
Figure 1-1 provides a flow chart for a typical core analysis process. The processes aresummarised below:
1. Normally nowadays, core is recovered in aluminium or fibreglass liners. Previously, core
was extracted from the barrel at wellsite. Following coring the core or core liners are
recovered and assembled at wellsite. The core lengths are marked with tramlines and
way up/depth markings. This aids re-assembly in the lab. Dean-Stark plugs may be
taken for later analysis in the lab. Preserved samples may be taken at regular or defined
intervals at wellsite or, if the core is recovered in liners, in the lab.
2. The core is boxed (or if recovered in liners, cut into suitable lengths) and shipped to the
laboratory.
3. In the lab, the core is removed from the boxes or liners, re-assembled and checked for
depth conformity.
4. The core sections are then passed through a gamma logger. This is essential for core-log
depth matching.
5. Two plug sets are taken: one for routine porosity, permeability and fluid saturations (the
RCA plug set) and the other plug set (usually taken from preserved samples) is scheduled
for special core analysis (SCAL). Both horizontal and vertical RCA plugs are taken for
analysis. On occasion, plugs are taken and preserved for later SCAL.
6. The plugs are cleaned and dried (fluid saturations may be measured) then subjected to
porosity and permeability analysis.
7. The remainder of the core is slabbed (probe permeability measurements may be made on
a surface-dried slab face) then photographed under white and UV light.
8. The slabs and plugs are then despatched to the operator’s core store and/or to partners and
regulatory authorities where they are archived. A thin slab may be resinated to help
protect the core from deterioration.
Each of the processes are described below and in later Chapters of this course. It should be
remembered that many stages in the process - cutting , recovering and handling the core; core
plugging, cleaning and drying, provides an opportunity for core damage – that is the in situ
core petrophysical properties are permanently altered. This is discussed in detail in thefollowing Chapter.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 5/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 2
ROUTINE CORE ANALYSIS PROCESSES
Dean-StarkPlug at wellsite
PreserveTest in lab
Helium Porosity
Air Permeabi li ty
Plug PreparationCleaning and Drying
Dean-Stark/RetortDetermine Fluid Saturations
Plugs
Cut H & V Plug sets
Resination
Core Photography
White Light and UV
Probe Permeability
Slabbing
Core
Core
RCA
Preserved Samples
SCAL
Gamma Ray LogDepth Matching
Core Removal & AssemblyLayout Core
Preserve Samples
Core Shipping Arrival in Laboratory
Core Recovery & Catching
Wellsite
Coring
Conventional/Liner
Well Archive
Figure 1-1: Coring and Core Analysis Processes
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 6/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 3
2. Coring
2.1 Core Types
2.1.1 Conventional Core
Figure 2-1 illustrates typical conventional core bits. Conventional coring techniques work
well in many reservoir formations. So long as the well-site geologist has adequately marked
both the core and its boxes, no particular problems are encountered in dealing with
conventional core. However, jumbled sections are not infrequently encountered.
Generally, the core is removed from the barrel at wellsite under uncontrolled conditions, then broken into 1m lengths and placed in wooden or reinforced cardboard core boxes for onward
shipment. The core should never be washed for wellsite inspection but should be wiped
down with a damp rag prior to inspection.
Preserved samples are taken at wellsite.
Figure 2-1 Conventional Core Bits
2.1.2 Sidewall Cores
Sidewall cores are taken to minimise coring costs or to obtain reservoir rock samples in an
interval which has either been cored and core recovery lost, or in an interval which has not
been cored.
There are two main types as illustrated in Figure 2-2 and Figure 2-3:
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 7/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 2
Percussion sidewall. This consists of a tool with a series of core “bullets” loaded with
explosive charges. Typical sizes are 1” by 1”. The core bit is similar to a chisel. The bullets
are loaded into the tool which is run in on wireline to the interval of interest. The gun is fired
and the explosive charge shoots the bullets into the formation. The mini core plugs cut by the bullets are retained within the bullets which are then brought back into the tool using a wire
chain and the tool returned to surface. The principal advantage with this method is that it is
cheap. There are few advantages with this method. The driving force required for the bullets
to penetrate the formation can cause stronger rock to fracture and weaker rocks to
consolidate. Porosity and permeability data measured on such samples will not be
representative. The prime application of percussion sidewalls is therefore restricted to
obtaining samples for lithological description, grain size, and palynology and paelontology,
although grain density measurements and particle size analysis measurements should be
possible.
Rotary sidewall. This was developed to overcome the problems with percussion sidewalls.
The tool consists of a series of mini core bits within a wireline tool. The tool is lowered to
the zone of interest and the core bit is extracted and pressed against the borehole wall. Mud
is circulated through the tool which causes the core bit to rotate and take the sample. On
completion, the bit is retracted into the tool and the tool is taken back to surface.
The quality of the plugs taken by this tool is far superior to the percussion sidewall.
Generally, reliable porosity and permeability data can be obtained on the plugs. However, it
is much more expensive and provides fewer plugs.
Figure 2-2 Percussion Sidewall Tool Figure 2-3: Rotary Sidewall Tool
2.2 Core Liners
Core liners were developed primarily to prevent damage to the core associated with
conventional core barrel assemblies. The core bit is similar, but the barrel has an inner liner
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 8/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 2
into which the core is retained as it is cut. The liners may be rubber, fibreglass (Figure 2-4),
or (aluminium) Figure 2-5. Liners were first developed for unconsolidated core but almost
every core caught in the North Sea recently is caught in liners.
Rubber sleeve coring was the first successful technique for coring unconsolidated or poorlyconsolidated formations. It suffered from restrictions, being especially prone to mechanical
failure and problems caused by inexperienced crews. Even in successful operations, cores
were frequently contaminated by drilling mud, and disrupted by "twist-off". Fibreglass (or
plastic) sleeves (liners) are more commonly used today: these can be utilised in standard
core barrels giving much larger diameter cores. With careful operation, core recoveries
approaching 100% can be achieved in poorly consolidated formations, even including Niger
Delta reservoirs, which a number of operators had previously written off as impossible to
core. The core liners are frozen, or the annular space between the core liner and the core is
filled with resin, to prevent damage to the core on shipment.
Unfortunately, wellsite geologists are only able to inspect the ends of each cut core section,so that wellsite lithological core description is limited. The liners are usually removed in the
laboratory, under better controlled conditions, and the lithology revealed. If the rock is
competent, the core can usually be easily pushed out of aluminium or fibreglass liners.
However if the rock is weak, extracting the core will result in unacceptable disturbance, so in
this case, plug samples are often taken through the liner prior to removing the core, and the
liners must be carefully cut open to reveal the core. In cores recovered in liners, preserved
samples are identified and selected under controlled conditions after the core is shipped to the
laboratory.
Figure 2-4: Fibreglass LinerFigure 2-5: Aluminium Liner
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 9/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 2
2.2.1 Gel Cores
The recent development of an encapsulated gel coring system holds promise in preventing
core damage during and after coring. For example, Baker Hughes1 Core Gels 3 and 4 are
non-toxic polypropyleneglycol-based zero spurt-loss gels which, when pre-loaded in theinner tube of the core barrel, protect core from drilling mud filtrate invasion and flushing
during and after the coring process (Figure 2-6). The purpose of Gel Coring is to help
preserve in situ saturations and rock properties of the reservoir sample and to improve core
recovery and reduce jamming.
As the core is cut and enters the inner tube, it displaces all but a substantial coating of gel
which remains on the core for protection. The Gels form a water-impermeable latex-like film
on the surface of the core. Core Gel 3 and 4 are high viscosity materials at ambient
conditions. This allows the gels to adhere to the core and also protect it while in transit to the
lab. Because the Core Gels come into direct contact with the core surface during and
immediately after coring, the core is protected from drilling mud filtrate invasion. This process helps provide the core analyst with an unaltered reservoir rock sample, e.g. with
formation fluid saturations that are largely undisturbed. In addition, the Core Gels provide a
stabilising material in the annulus space between the inner barrel and the core, helping to
support and protect the core on its trip to the surface and thence to the laboratory.
Baker Hughes claim great success for the system, but early job results were mixed.
Figure 2-6: Gel Coring System (Courtesy Baker Hughes)
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 10/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 3
2.3 Sponge and Pressure Core Barrels
As the core is brought to the surface, the hydrocarbon fluid will expand and, in an oil
reservoir, gas will be liberated when the oil is brought below the bubble point. Gas liberation
or expansion provides a force which will cause displacement of both the native fluids and the
invaded mud filtrate.
Gas evolution can cause mechanical damage to cores from loosely consolidated formations,
but this can be minimised by pulling the last few hundred feet of the core barrel string very
slowly.
The use of pressurised or sponge core barrels provides a means to prevent loss of oil from the
core on hydrocarbon expansion on core recovery. There primary use is to prevent loss of oil
from the core during filtrate invasion and gas evolution on core recovery. Both sponge core
and pressurised core barrels are often used to determine oil-in-place in depleted zones prior to
improved oil recovery project evaluation.
2.3.1 Pressure Cores
The core is maintained at the reservoir pore pressure within the barrel until arrival at the
laboratory. The barrel is then slowly depressurised and the volume of evolved gas
determined using a gas meter (Figure 2-7). From a knowledge of the PVT properties of the
oil, the volume of oil originally in the core can be determined. The whole core is then
cleaned and the pore volume determined. The volume of oil determined from gas evolution
and the pore volume are used to determine saturation.
The application of this techniques has been constrained by its expense, safety considerations,
and the availability of laboratories with the necessary equipment to handle pressured cores.
2.3.2 Sponge Cores
Sponge core involves the use of a polyurethane sponge liner in the annular space between the
barrel and the core. It is oil-wet, so that it adsorbs oil that bleeds from the core and holds it
opposite the formation from which it bled (Figure 2-8).
Sponge core analysis usually utilises whole cores.
The oil from the sponge adjacent to is extracted in a specially adapted soxhlet apparatus
(Figure 2-9). The oil content of the extracted oil/solvent mixture from the sponge is
determined from gas chromatography, and is added to the volume of oil extracted from theadjacent whole core by conventional cleaning. From subsequent pore volume measurements
on the whole core, the total oil saturation is obtained.
It is much cheaper and safer than pressure core barrels and can be successful under most
circumstances.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 11/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 4
Figure 2-7: Pressure Core Barrel
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 12/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 5
Sponge
Liner
Core
Figure 2-8: Sponge Core Barrel
Figure 2-9 Extractor for Core Sponge
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 13/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 6
3. Core Analysis Laboratory Processes
3.1 Core Handling
Inadequate core handling, storage and treatment cause the reservoir geologist as many
problems as do operational difficulties with coring. Many clients now specify the services of
a core analysis contractor to receive and box the core. This minimises error through coring
vendor company personnel inexperience. Wellsite core analysis personnel can also perform
wellsite core gamma logging which assists in the re-assembly of the core in the laboratory.
Core caught in liners can be transported to the laboratory in 30ft or 90ft lengths, or are cut to
3ft lengths. In the former case, it is essential that, at rigsite, the liners are supported during
handling and lifting, otherwise the core liners can flex (especially if fibreglass liners are
used) and cause mechanical damage to the core (Figure 3-1).
Figure 3-1: Core Liner Flexing Causes Core Damage
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 14/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 7
Cutting the liners into shorter lengths minimises this problem.
The sequence of events which a core will undergo after its arrival on the rig floor (or in the
lab if recovered in liners) is as follows:
• Removal from core barrel or liner (not always straightforward).
• "Way-up" marking.
• Depth marking
• Division into 1-m lengths
• Sealing in foil or plastic
• Crating
• Preserved sample selection
The opportunities for error are high at this stage: transposition of core pieces is not
uncommon and lengths may be reversed. In the case of conventional core, it may be
tempting to "fit in" rubble at the end of each core length.
The core is marked to ease its re-assembly in the lab and to prevent transposition errors.
Schema vary from company to company but should be consistent within a company. A
typical example is shown in Figure 3-2.
Depth
Figure 3-2: Typical Core Marking
The core sections are firstly marked with “Tramlines”. These are vertical continuous bands
of colour running the length of the core. Typical colour relationships are red on the right and
black or yellow on the left as viewed when looking at the core from the bottom up. Whatever
method is used, it has to be consistent. A circular depth marker is then marked on the core at
regular intervals, often with an arrow pointing to the top of the core.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 15/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 8
Core recovered in liners is not usually removed at wellsite. The core liners are often marked
and cut into 1 m lengths prior to shipment to the lab where the core is removed and re-
marked, as above.
On occasions, core plugs for Dean-Stark analysis are taken at wellsite. The processes aredescribed in a later Chapter.
3.2 Core Arrival
The lab is firstly required to carefully piece all the core back together in the correct sequence.
Every break in the core should be categorised by the laboratory (e.g. goodness of fit) to aid in
depth matching core and log data and to identify loss of core recovery, natural or induced
fractures, etc.
3.3 Core Gamma Ray Logging
A gamma ray log is almost always run in a conventional wireline or LWD log suite. The
principal reason for running a gamma ray log on a core is to match driller’s depth (core
depth) against wireline log depths, which is the common reference depth in a well. Both
depths are frequently different. Both total and spectral (uranium, thorium and potassium
ratios) core gamma logs can be run to assist in the depth matching process.
Core gamma ray response is normally provided in counts per second (cps) whereas wireline
logs are in API. It is possible to calibrate the gamma ray logger provided a suitable
secondary API standard is available, although cps and API are correlated linearly. Thus core
data and wireline data can be plotted on the same scale and where each trace overlaps thecore and log depths can usually be matched.
Typical equipment (Figure 3-3) includes:
• A time or speed controlled conveyor belt for core up to 4 m long.
• A sodium iodide gamma ray detector with lead shield
• A analyser system (multi-channel)
• A chart recorder or computer data acquisition system.
The core is laid out, measured and marked (Section 3.2) then placed in sections on the
gamma ray logger belt. Typical scan speeds are around 1 ft/min. Radioactive standards areoften used to determine the logger response. The output us total (or spectral) gamma counts
as a function of depth.
Core gamma ray logs can also be run at rigsite, using portable, handheld loggers. This
enables correct depth matching when the core is removed from the liners in the lab.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 16/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 9
Figure 3-3 Core Gamma Ray Logger
3.4 Core Scanning
Increasingly, technology is being applied to scan the core in its liner, prior to plugging. This
includes linear X-Ray and CT (Computer Tomograph) scanning. An example of a CT-
scanned core section is shown in Figure 3-4). In this representation, high density material
(heavy minerals, barites from the mud system) show up light grey/white whereas low density
material (and many fractures) appear dark. CT scanning is therefore use to select
representative plug locations, avoiding heterogeneous, invaded, or fractured intervals.
Figure 3-4: CT-Scanned Core Section (Courtesy Core Laboratories)
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 17/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 10
3.5 Core Plugging
Measurements of petrophysical parameters requires the preparation of representative
samples, normally right cylindrical plugs. In extremely heterogeneous formations – fractured
or vuggy carbonates for example, measurements are often made on whole or full diameter
core.
Normally two plug types are required (Figure 3-6):
1. Horizontal samples. These are plugs taken parallel to the apparent bedding plane features
not perpendicular to the long axis of the core. The objective is to sample the maximum
permeability in the formation which is normally parallel to bedding. Sample frequency is
typically 1 per ft (25 cm).
2. Vertical samples. These are plugs taken perpendicular to the bedding not parallel to the
long axis of the core. These sample the minimum permeability direction. Sample
frequency depends on lithology but are around 1 per 5 ft (1.25 m) to 1 per 10 ft (2.5 m).
One of the problems that arises, especially in the case of steeply dipping reservoirs or highly
deviated wells, is that plug orientation is not at a constant angle to bedding features. Samples
from sleeved cores must be taken 'blind' unless the sleeved core is first examined under a CT
or linear X-ray scanner.
The sampling may tend to be biased . The lab technician may move the sampling location to
avoid shale intervals, fractured or rubble zones, etc, or zones where making a measurement
on the plug would be difficult or time consuming.
In RCA programmes, 1” or 1.5” diameter plugs are taken at approximately 1 foot intervals.
The use of larger plugs is preferred since the errors involved in porosity and Dean-Starkmeasurements on small plugs can be large and can have a significant impact on the data. For
example, the pore volume of a 1” plug is about 4 times smaller than the pore volume of a
typical 1.5” plug of the same porosity.
The plugs should be ideally taken from the centre of the core to ensure that a non-invaded
samples is obtained (this is essential for Dean-Stark measurements and for SCAL plugs).
Figure 3-5: Core plug taken from centre of core
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 18/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 11
Core plugs are taken using a diamond-tipped, hollow cylindrical, rotary core bit mounted on
a drill press (Figure 3-7). A variety of tip lubricants are used, depending upon the fluid
content, core drilling mud, and lithology/mineralogy. Typical plugging fluids are:
• Brine (made up to same composition as formation water)
• Depolarised kerosene, base oil, or mineral oil (e.g. Blandol) which are used where brine-
rock incompatibility is expected or where cores are cut with oil-based mud.
Plugs should never be taken with tap water as the coolant as this can cause severe problems if
the core contains authigenic clays.
If there is any doubt about potential problems arising from water-formation incompatibility
then depolarised kerosene or mineral oil should be used.
Figure 3-6 Correct and Incorrect Horizontal and Vertical Plugs
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 19/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 12
Figure 3-7: Drill Press. Circulating fluid system tank shown
3.6 Plug Measurements
In many cases, two sets of plugs are taken: “hot-shot” and conventional plugs. The hot shot
plugs are required to provide immediate information, usually in 24 hours turnaround. These
are normally horizontal plugs that are cleaned and dried quickly to provide preliminary
information – initial log calibration, selection of perforation intervals, etc. The conventional
plug set are subjected to more rigorous, consistent and uniform testing procedures that must
not be compromised for client expediency.
Typical measurements on plugs principally include:
• Fluid saturations (Dean-Stark and retort)
• Porosity
• Air Permeability
These are discussed in separate Chapters.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 20/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 13
3.7 Core Slabbing and Resination
The core sections are slabbed after plugging, using a masonry saw, to provide a clean surface
for detailed description and for photography.
Slabbing is essential for adequate reservoir description, as it allows detailed observation of
sedimentary structures poorly displayed in the rough outer surface of the core. A trained
operator is essential, as mis-orientation can be confusing, and saw-marks may obscure 'real'
features. The number of slabs will depend on the number of partners and government
regulatory requirements. In the UK, cores are often cut into three sections (Figure 3-8). The
biscuit slab is normally preserved by resination in which the slab is immersed to just below
its top surface in epoxy resin.
Slabbing should be performed parallel to maximum apparent dip.
Slab 1
Slab 3
“Biscuit”
Slab
Figure 3-8: Typical Core Slabbing Arrangement
3.8 Core Photography
Large format photography is a valuable technique, as it
• Provides a permanent record of core plug sites, depths, etc
• Often reveals features which may later be rendered invisible by subsequent drying or
deterioration.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 21/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 14
The core photographs should be clearly labelled and should include plug numbers (even
porosity and permeability) and preserved sample intervals. Many labs now provide digital
images which can be incorporated into petrophysical and geological software packages.
However, unless special techniques such as infra-red or UV photography or X-ray methodsare used, the resolution of sedimentary structures in heavily oil-stained cores may not be
good, and detail shots are necessary to record specific bedding features. Nevertheless, it is
recommended that cores always be photographed and, even if cores remain in good
condition, these photographs should be reviewed during any subsequent reservoir study.
Both white light (Figure 3-9) and ultra-violet (UV) images are required. UV photography
(Figure 3-10) provides an indication of the presence of remaining oil (after flushing with
filtrate) through oil fluorescence. The value of UV photography is enhanced by the inclusion
of beakers containing different oils and/or oil-based mud filtrates which may fluoresce
differently from the native oil. Comparison of oil and OBM fluorescence helps identify oil-
bearing intervals.
Figure 3-9: White Light Photograph
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 22/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 15
Figure 3-10: UV Photograph
4. Petrographic Tests
Petrographical analysis techniques allow a much more detailed description of the rock
textural and cement properties that control petrophysical properties than is possible with the
naked eye or a binocular microscope. In particular, these techniques are used to identify
delicate grains and cements that might be easily damaged in the procedures used to prepare
cores for analysis: such as core plugging, cleaning and drying. There is little point in
measuring the rock properties in core analysis if the structure of the rock has been altered by
inappropriate preparation techniques. If sensitive minerals can be identified prior to the core
analysis programme starting, the core preparation procedures can be amended to suit.
The most common petrographic analytical techniques which the core analyst can employ are:1. Thin section analysis
2. Scanning Electron Microscopy (SEM).
3. X-Ray Diffraction (XRD)
These tests are normally included in the routine core analysis programme, though they may
be specified separately or , indeed, included in the SCAL programme.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 23/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 16
4.1 Thin Section Analysis
4.1.1 Selection and Preparation
Samples are selected from core plug end trims or adjacent core pieces, Sample pieces are
vacuum-impregnated with colour-dyed resin in order to facilitate identification and
illustration of the pore space, then cut and lapped to a standard 30 micron thickness. Sections
are then stained in:
a) sodium cobaltinitrite to reveal alkali feldspar and;
b) combined Alizarin Red-S/potassium ferricyanide to distinguish varieties of calcite and
dolomite.
The section is then cover-slipped, usually permanently.
4.1.2 Carbonate Phase Stain ing
In order to facilitate the identification of major carbonate mineral phases, thin-sections are
routinely treated with the combined stain solution formulated by Dickson2. The stain is
prepared by mixing together solutions in HCl of Alizarin Red-S and potassium ferricyanide,
in the approximate ratio 3:2. Prior to cover-slipping, the thin-section is lightly etched in
dilute HCI, then placed in the combined stain solution until sufficient stain is fixed. Final
treatment with Alizarin Red-S only (intensifier) may be used if necessary.
On the basis of the stain colours produced, varieties of calcite and dolomite are distinguished
as follows:
• Non-ferroan Calcite produces stain colours ranging from very pale pink, through shades
of orange-pink ("peach") to purples sometimes of fairly intense tone. Colours should lack
any blue hues.
• Non-ferroan Dolomite does not react and remains unstained.
• Ferroan Calcite typically produces the colour Turnbull's blue, a fairly dark shade of
Royal blue. With a weaker reaction, paler sky-blue colours may be produced. Pink-colour
AR-S pigment is usually visible beneath the blue and contributes to the overall colour
tone. Shades of mauve/violet with a weak blue component are thought to indicate lower
iron content.
• Ferroan Dolomite stains a fairly consistent turquoise blue or cyan, the colour tone
intensifying to reflect increasing immersion time or crystal solubility. Confusion with
ferroan calcite may arise in cases where the shade approaches a deep sky-blue, but with
dolomite the stain tends to be ragged and incomplete due to its poorer solubility in HCI.
Other optical properties are always considered in order to confirm identification.
Siderite may react very sparingly and take pin-points of blue stain. It is not uncommon for
coarse sparry calcite (eg. grainstone cement) to remain unstained. Such varieties are.
however, usually non-ferroan.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 24/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 17
4.1.3 Quantitative Nodal Analysis
Modal analysis is a technique used for determining the quantitative mineralogy of a thin
section. The data is presented in a tabulated form which lists the percentage abundance of
every recorded constituent. Results of the analysis can be used for:
• Classifying samples within a general scheme
• Comparison with other samples
• Use as a statistical data base
Typical equipment includes a microscope with an automatic camera and a motor driven
stage, together with a personal computer with point-count totaliser software with connecting
interface unit to drive attached stage.
Prepared thin sections are point counted using the Line Method. The line method involves
counting the number of constituents present in a given thin section sample. This is done bycounting individual components (for example. framework grains, clays, cements or pores)
encountered by the intersection of the cross-hairs along linear traverses spaced equidistantly
along the slide. The result of the method is a “number frequency” that simply shows how
often particular components were encountered during the count. This "number frequency'' is
automatically recalculated as a percentage figure by the point-count software. The percentage
figure obtained from these modal analyses is related to, but distinctly different from, the area,
volume or weight of any constituent present within that thin section.
Stage interval: The stage interval setting is used to vary the number of points observed during
one traverse of a slide. The point-count software allows the operator to select the desired
stage interval prior to each point count. Ideally, the stage should only land on an individualgrain once during a point count otherwise the sample will become biased due to an increased
probability of a larger grain being encountered several times during an analysis than a smaller
one. Consequently, the stage interval is set so as to advance the slide by a distance
equivalent to the average grain size of that sample. However, within the majority of samples
analysed, individual grains do not have the same cross-sectional area and consequently some
element of bias towards the coarser fraction is to be expected but can be kept to a minimum
by adjusting the stage setting.
At the end of each traverse, the slide carrier is pulled back to reset it ready for the next
traverse and the ratchet knob is turned to present a new section of the slide.
For general purposes 300 points is a good number to count in order to get the maximum levelof accuracy for the minimum investment of time. Below 300, the probable error increases
rapidly, whereas above 300 it decreases slowly. However, the particular type of investigation
being done determines the number of grains to be counted.
A thin section is placed with the point-count stage holder and set so that the point count will
start at the top left hand corner of the sample. The mineral beneath the cross-hairs is
identified according to its optical properties (using plane or doubly polarised light as
appropriate) as defined by Deer et al3 and Kerr
4 . The relevant channel button is pressed once
to record the occurrence of a particular component and is stored as a running total within the
automated computer point-count system. The stage traverses automatically as each button is
pressed, and the next mineral identification is made. When the edge of the thin section is
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 25/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 18
reached. the thin section holder is pulled back to reset it ready for the next traverse and the
ratchet knob is turned a small amount to present a new area of the slide.
When the target count has been reached, the point count program immediately displays a
percentage listing of the various recorded components and allows the operator to enter trace proportions of any rare components which were present within the slide but not encountered
under the cross-hairs during the counting procedure. These components are noted as traces in
order to distinguish them from completely absent minerals in the thin section.
When all the thin sections have been point counted, a print out of the modal table for all
samples may be obtained.
4.1.4 Thin Section Descriptions
Utilising the modal analysis data obtained during point counting, systematic descriptions of
samples are made. Three levels of thin section description exist:a) Standard
b) Intermediate
c) Detailed
A standard description lists major authigenic phases. porosity types and reservoir controls.
Intermediate format is similar but also has an additional section to deal with specific client
requests, for example a diagenetic sequence or information pertaining to fractures. Detailed
level descriptions include a systematic listing of all detrital components, authigenic phases
and porosity types present in a sample as well as a sample summary, a diagenetic sequence
and a section on porosity and permeability characteristics. In addition. each level ofdescription gives:
1. A visual estimate of grain size range and mean using graticule measurements which can
be converted to microns or the Wentworth size class (for example, fine sand, granule,
pebble) using the classification table above.
2. A visual estimation of sorting using charts for visual estimation of sorting based on
Pettijohn et al. al5 (Figure 4-1).
3. An estimation of grain packing (compaction) based on the grain contacts of framework
quartz grains (Allen6 ) Figure 4-2.
4. A note on the overall fabric of the thin section visible on a thin section scale. Forexample, horizontal lamination of micas, ripple cross-lamination, bioturbation or
fracturing.
5. Where requested by a client, an assessment of the sphericity of detrital framework grains
using images of representative grains (Figure 4-3) after Pettijohn et a/. (1973).
4.1.5 Sandstone Classification
The point count software automatically classifies sandstone samples according to a scheme
illustrated in Figure 4-4. This classification system recalculates the quartz, feldspar and lithic
components of a sample to 100% and the values obtained are plotted on a triangular graph.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 26/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 19
The sample plots in a position respective to these three calculated values, and the
corresponding area of the graph is identified to describe the basic sandstone classification.
This simple, three-variable spectral classification may then be qualified according to the
following conventions:
1. If the detrital matrix clay content is equal to or greater than 15%, then the suffix wacke is
used; if less than 15% the sandstone is classified as an arenite.
2. If any authigenic mineral is present in amounts equal to or greater than 10%, then an
adjectival prefix is added such as dolomitic, kaolinitic, illitic, siliceous, is used as
appropriate.
4.1.6 Carbonate Classification
The classification of carbonate lithologies is based upon a combination of the schemes of
Folk 7 and Dunham8. This approach is adopted to enable comprehensive naming of limestonesaccording to both composition and depositional fabric.
The scheme of Folk is used to describe the objective (spectral) composition of the limestones
and reference is made to allochems - bioclasts, ooliths, intraclasts, pellets, peloids - and
orthochems which comprises interparticle matrix or micrite and forms of sparry calcite or
sparite. Using a convenient form of abbreviation for allochem types, compositional rock
names are generated e.g. biopelmicrite, oobiosparite etc.
4.1.7 Example Thin Section
An example of a thin section from a North Sea reservoir sandstone is shown in Figure 4-5.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 27/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 20
Figure 4-1: Sorting Classification
Figure 4-2: Compaction Classification
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 28/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 21
Figure 4-3: Roundness and Sphericity
Figure 4-4: Sandstone Classification
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 29/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 22
Figure 4-5: Example Thin Section
This view shows a moderate to poorly sorted quartz arenite, Grain coating and pore lining residualhydrocarbons are developed on the grain surfaces. Note the presence of hydrocarbons coveringquartz overgrowth terminations indicating a post quartz overgrowth hydrocarbon migration.
Primary intergranular porosity (18.5%) is dominant and occurs evenly through the sample, averaging150 microns in size. Interconnectivity between pores is generally well developed with compactionaland silica overgrowth contacts, although abundant are not pervasive. Secondary porosity (2%) isformed from the often complete dissolution of unstable grains to give oversized pores (about 350
microns). The oversized pores add considerably to the interconnectivity of the intergranular porenetwork.
The high modal porosity coupled with the limited compaction, open pore network enhanced bysecondary dissolution and the absence of any significant pore occluding authigenic cement indicatessubstantial reservoir potential. The sample helium porosity is 17.6%, and the air permeability is 580mD.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 30/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 23
4.2 Scanning Electron Microscopy
4.2.1 Background
SEM analysis allows the detailed evaluation of rock specimens and can provide unique
insights into the nature of the rock pore structure and the location and morphology of
cements (especially clays).
Conventional light microscopes use a series of glass lenses to bend light waves and create a
magnified image. The Scanning Electron Microscope creates the magnified images by using
electrons instead of light waves. The SEM shows very detailed 3-dimensional images at
much higher magnifications than is possible with a light microscope. The images created
without light waves are rendered black and white. Samples have to be prepared carefully to
withstand the vacuum inside the microscope. Because the SEM illuminates them with
electrons, samples also have to be made to conduct electricity. SEM rock chip samples arecoated with a very thin layer of gold using a sputter coater. After the air is pumped out of the
column, an electron gun [at the top] emits a beam of high energy electrons. This beam travels
downward through a series of magnetic lenses designed to focus the electrons to a very fine
spot (Figure 4-6).
Near the bottom, a set of scanning coils moves the focused beam back and forth across the
specimen, row by row. As the electron beam hits each spot on the sample, secondary
electrons are knocked loose from its surface. A detector counts these electrons and sends the
signals to an amplifier The final image is built up from the number of electrons emitted from
each spot on the sample.
The SEM has a large depth of field, which allows a large amount of the sample to be in focusat one time. The SEM also produces images of high resolution, which means that closely
spaced features can be examined at a high magnification.
4.2.2 Analysis Techniques
Samples are prepared for SEM analysis by removing a small freshly fractured rock fragment
measuring less than 1cm diameter following air or critical point drying. Individual fragments
are cemented onto aluminium stubs with collodial carbon. Electrical conductivity is
established by applying a thin film of gold in a sputter coater and, where necessary, by
applying a coating of colloidal carbon around the base of the sample. Samples are analysed in
a scanning electron microscope fitted with an EDAX energy dispersive X-ray microanalyser
4.2.3 Analysis
All SEM analyses should be carried out by an experienced operator, with extensive
interpretation skills in clay mineralogy, texture and morphology. Clay and cement phases are
identified using a combination of morphology and qualitative EDS microanalyses, and
compared with in-house photographic records, published manuals and standard EDS mineral
traces.
Example of SEM photomicrographs are shown in Figure 4-7
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 31/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 24
Figure 4-6: SEM Schematic
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 32/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 25
c
d
a
b
SEM Photomicrograph (x 654):
This view shows occasional well-developed kaolinite (a) occasionally filling pores, with traces of calcite cement
(b). Note quartz overgrowths (c) and pyrite framboid (d). This is a clean, fine grained poorly cemented
sandstone. Detrital and authigenic clays are rare and the main causes of porosity reduction are via quartz
overgrowths. 16.2% Porosity: 150 mD Air Permeability
SEM Photomicrograph (x 425):
Clean, fine grained sandstone with well developed quartz overgrowths. Note fibrous illite bridging pores.
18.1% Porosity: 304 mD Air Permeability
Figure 4-7: SEM Photomicrographs
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 33/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 26
4.3 X-Ray Diffraction
4.3.1 Theory
Structural information about locations of atoms in a crystalline mineral can be deduced from
the sharp diffraction spots the crystal produces from the sharp diffraction pattern in a beam of
X-Rays. Put simply, the phenomenon of diffraction occurs when penetrating radiation, such
as X-rays, enters a crystalline substance and is scattered. The direction and intensity of the
scattered (diffracted) beams depends on the orientation of the crystal lattice with respect to
the incident beam. (Figure 4-8). Any face of a crystal lattice consists of parallel rows of
atoms separated by a unique distance (d-spacing), which are capable of diffracting X-rays. In
order for a beam to be 100% diffracted, the distance it travels between rows of atoms at the
angle of incidence must be equal to an integral multiple of the wavelength of the incident
beam. D-spacings which are greater or lesser than the wavelength of the directed X-ray beam
at the angle of incidence will produce a diffracted beam of less than 100% intensity.
Diffraction occurs at an angle of incidence equal to the Bragg angle, θ
θ λ sin2d =
Most diffractometers in petrographic applications utilise a powdered sample, a goniometer,
and a fixed-position detector to measure the diffraction patterns of unknowns. The powdered
sample provides (theoretically) all possible orientations of the crystal lattice, the goniometer
provides a variety of angles of incidence, and the detector measures the intensity of the
diffracted beam. The resulting analysis is described graphically as a set of peaks with %intensity on the Y-axis and goniometer angle on the X-axis. The exact angle and intensity of
a set of peaks is unique to the crystal structure being examined. A perfect crystal has well-
defined, single peaks (Figure 4-9), whereas a imperfect crystal has a broader distribution.
Amorphous material (such as liquid or glass) has poorly defined diffraction peaks. In a
multi-component mixture, confusion can arise when two or more components have a peak in
the same, or nearby, location on the X-axis. It is for sorting out these mixtures that a good
search/match engine or a search method becomes most important. The X-Ray diffraction
method is most useful for qualitative, rather than quantitative, analysis (although it can be
used for both).
4.3.2 Whole Rock Preparation and Analyses
Selected samples are gently disaggregated under alcohol in an agate mortar, and the resulting
slurry is transferred to a micronising mill (comprising agate segmented bars) and
alcohol-reduced for a period of ten minutes.
Samples are dried at 60°C down to 22°C in dry air, and then back-filled into an aluminium
holder (4g of sample) to produce a randomly orientated powder mount. Alcohol is used
during preparation to prevent damage to water-sensitive phases, and to reduce damage to the
lattice.
Two scans are run for each sample to determine major crystalline phases.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 34/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 27
1. After temperature stabilisation the random powder sample is analysed using CuKa
radiation at a scan speed of 1 °20m-1 over an angular range of 1.5 to 40.0°2O [Time: 40
minutes].
2. Upon completion of the 1st scan detailed analysis of each peak is made to determine thearea, intensity and position. The resulting data is stored in a digital format which
undergoes matrix flushing producing semiquantitative results.
4.3.3 Extraction of the Clay Fraction and Analyses
Initial disaggregation is conducted using a jaw crusher down to 4mm. The sample is washed
in distilled water to remove rock flour contamination, at this point all contamination (i.e.
organics and carbonate) is removed, and the sample is then stored in a large test tube with a
dispersing agent for 24 hours (cutting samples such as "Hot-Shots' may have drilling mud
removed using a screen and ultra sound). Secondary disaggregation is conducted under
distilled water in an agate mortar. Liberation of the clay component is achieved by standingthe tube of rock grains and dispersing agent (of known density) in an ultrasonic bath for 30
minutes. The >80 micron fraction is removed from suspension. All clay fraction sizes are
removed and concentrated using centrifugation. Known aliquots of the clay suspension are
then filtered under vacuum onto a membrane. The resulting clay film is transferred to a
known substrate by a membrane filter peel technique, giving a reproducible, preferred
orientated film of known thickness.
Four traces are run for each sample:
1. After drying the clay mount at room temperature and humidity. [Angular range 1 ° to 30 °
2(3 CuKa] .
2. After ethylene glycol saturation [24 hours at 25°C]or [~'Hot Shot"] 2 hours at 60°C.
[Angular range 1 ° to 20°26) CuKa]. This causes any expandable clays to expand and
increase their lattice spacing.
3. Immediately following heating to 375°C for 30 minutes [Angular range: 4 to 10 ° 2C
CuKa]. This causes any expandable clays to collapse.
4. After heating to 550°C for 14 hours [angular range: 1 ° to 30°2C CuKa].
Clay minerals are identified by using the four X-ray diffraction patterns of orientated
aggregates that enhance the basal [001 reflections (Z)]. This is because h.k.l peaks are very
similar in their X-Y directions. Mineral type is determined by peak position, intensity, shape
and breadth. Peak reflections are determined by Bragg's Law.
Illite and Glauconite Identification and Quantification
Pure illite and pure glauconite are unaffected by ethylene glycol and heating to 550°C.
Glauconite has a higher 001/003 intensity ratio than illite but determination is made on the
002 reflection as glauconite exhibits little or no peak at 5.00 A [18°2C CuKa] due to heavy
scattering from octahedral iron. Both glauconite and illite in most cases are quantified using
the intensity and crystallinity of the 10.1 A [9°20] 001 reflection.
Chlorite and Kaolinite Identification and Quantification
Chlorite has a basal series of diffraction peaks based on a first order reflection at 14.2 A, andkaolinite has reflections based on a 7.1 A structure. Even order chlorite peaks superimpose on
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 35/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 28
the members of kaolinite 001 series. So the distinction between chlorite and kaolinite is most
difficult. Chlorite is determined by an amplification peak intensity at 14.2 A[6°2CCuKa]
upon heating to 550°C and the presence of an 003 reflection 4.74 A [18.5°2C Cuka]. In cases
where chlorite masks the presence of kaolinite a further trace may be run using samplematerial that has been treated with ON HCL. This collapses the chlorite peaks allowing
verification but not quantification of kaolinite as intensity is reduced in the kaolinite.
Detailed identification of the chlorite and kaolinite can be determined by mathematical
manipulation of the diffractogram and comparing with standard patterns. Quantification of
pure chlorite is calculated on the corresponding unique peak of the chlorite present compared
to a known quantity of internal standard. Total chlorite is always measured on the 7.10 A
[12.5°2O CuKa] peak and kaolinite is quantified using 003 reflection. Mixtures of kaolinite
and chlorite are quantified using 003 reflections. If this is impossible a slow scan to improve
resolution at 24 [Kaolinite 002] and 25 1 °2(3 [chlorite 004] is used.
Smectite Identification and Quantification
Smectite is identified by a movement of the 001 reflection upon glycolation to 16.9 A
:5.2°2C CuKa]. Note that on the air-dried trace the presence of a broad peak at 15 A [6°2C
CuKa]. Quantification is taken on the glycolated trace at 16.9 A [5.2°2(] CuKa] if detected.
Illite-smectite Identification and Quantification
Illite-smectite is identified by a broad shoulder on the 001 illite reflection at 15 A [6°2C
CuKa] separating to a distinct smectite peak at 16.9 A - 17.2 A. Comparing 375°C trace with
the glycol trace gives direct quantification at 10 A [9°20 CuKa].
4.3.4 ExamplesAn example of whole rock and clay fraction diffractograms are shown on Figure 4-10 and
Figure 4-11.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 36/43
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 37/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 30
Figure 4-10: Whole Rock Diffractogram
Figure 4-11: Clay Fraction Diffractogram
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 38/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 31
5. Core-Log Depth Matching
Following assessment of the nature of the core and log data and its accuracy and reliability,
the next stage in core/log data integration and reconciliation is to ensure that there is a
reliable depth correlation between core and log data from the same well. This is essential
process for density log calibration, etc.
On most occasions the driller’s depth (core) will be different from the logger’s depth, since
both use different methods to calculate depth. An indication of any depth mismatch can be
obtained from analysis of data that can be conveniently measured on both core and logs: e.g.
gamma ray response and log density/core porosity. The key is not to match absolute values
(e.g. core and log gamma have different scale units and core is usually ambient values) but
the trends.
For example, Figure 5-1 compares the log gamma trace in a well (logger’s depth) with a core
gamma ray trace (driller’s depth) from measurements made in the laboratory. In these
relatively clean sands there is little activity on the gamma logs, but at about 17576 ft MD
there is an excursion in the core gamma towards high values associated with a shaly layer. A
similar excursion is seen on the porosity (density-calibrated) log at 17682 ft MD. This
suggests that the core depth is 6 feet too high compared to the log depth and so the core data
should be shifted down by 6 ft to obtain a match with log data. The log (density log) and
ambient core porosity comparison from the same well is shown in Figure 5-2. In this case,
the relative amplitude of the core data is greater than the log data, due to different
measurement scales and biased core sampling, but again, an excursion to low porosity(associated with the shaly layer on the gamma) can be seen on both the core and log plots.
This indicates a depth shift of about 5 ft, which agrees well with the gamma log. For reasons
discussed below, core gamma generally provides a better tool with which to match drilled
and logged depths, and should be run on all core analysis jobs.
The core porosity and log porosity match must consider the effects of biased sampling (cores
are usually selected from the best quality intervals, not strictly on a foot-by-foot basis) and
the scale of the measurement (about 25 to 80 cc) is much lower than logs (equivalent volume
about 100,000 cc). Prior to depth matching, the core data should be averaged, to “smooth
out” localised “peaks and troughs”. This accounts for the vertical section (volume) of rock
which the logging tools actually considers (about 3 ft). Typical weightings might be:
1 times φcore
(1 ft above subject depth)
3 times φcore
(at subject depth)
1 times φcore
(1 ft below subject depth)
This provides a mean weighted core porosity with which to compare the log porosity.
Figure 5-3, from the same data set, shows the point and smoothed depth-shifted core data
plotted against the log data. The smoothed core trend better fits the log trend.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 39/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 32
Core-log depth matching is often viewed as a “black art”. One of the problems is dealing
with lost core recovery and the stretch in the wireline. Ideally, the core-log depth shift is
constant over a given core run and a so-called “stretch shift” is applied. However, in the
industry we normally fit the core to the logs but in reality we should be fitting the logs to thecore since the core is not normally expected to contract or expand too much. Industry
practice (especially using automated software) therefore leads to so-called stretch shifts in
which the cores are expanded or contracted to fit the logs such that the depth shift is not
constant for a given core run.
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 40/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 33
17500
17520
17540
17560
17580
17600
17620
0.00 0.10 0.20 0.30
Porosity (-)
D e p t h ( f t M D )
Log
Ambient Core
core 5 ft higher
Figure 12: Core and Log Porosity Trends
Figure 5-1: Depth Matching using Continuous Log and Point Core Porosity
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 41/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 34
17500
17520
17540
17560
17580
17600
17620
0 20 40 60 80 100
Gamma Ray (API)
D e p t
h ( f t M D )
Log
Core
core 6 ft higher
Figure 13: Core and Log Gamma Ray Activity
Figure 5-2: Depth Matching using Continuous Log and Core Gamma Ray
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 42/43
Chap 2 Coring and Core Analysis Processes.DOC Special Core Analysis Page 35
Point
data
3 point
running
average
Point
data
3 point
running
average
Figure 5-3: Depth Matching using Continuous Log and Smoothed Core Porosity
7/22/2019 Chap 2 Coring and Core Analysis Processes.pdf
http://slidepdf.com/reader/full/chap-2-coring-and-core-analysis-processespdf 43/43
6. References
1 www.bakerhughes.com/inteq/Drilling/coring/gelcore/gelcore_tds.pdf
2 Dickson, JAD, 1966, “Carbonate Identification and Genesis as revealed by Staining”, Journal of Sedimentary
Petrology, 36, 491-505
3 Deer, W.A., Howie, R.A., and Zussman, J., 1966, An Introduction to the Rock Forming Minerals, Longman
Group, London.
4 Kerr, P.F., 1977, Optical Mineralogy, McGraw-Hill Book Company
5 Pettijohn, F.J., Potter, P.E., and Siever, R., 1973, Sand and Sandstone, Springer-Verlag, New York, 618 pp.
6 Allen, J.R.L, 1962, “Petrology, Origin and Deposition of the Highest Lower Old Red Sandstone of Shropshire,
England”, Journal of Sedimentary Petrology, 32, 657-697
7 Folk, R.L., 1959, “Practical Petrographical Classification of Limestones”, AAPB Bulletin, 43,
8 Dunham, R.J, 1962 “Classification of Carbonate Rocks According to Depositional Texture”, AAPG Mem 1