Walkthrough

Core arrives at PTG
Barrels cut into 1m barrels , preserved samples taken
Barrels opened, core cleaned & measured. Box log and recovery %age
Hot shot plugs taken (preserves fluid content, wettablility etc

Core Gamma Ray or CT Scanning
Core plugged for RCA and SCAL Samples

Core Photographed under Core Slabbed Trim ends from core plugs sent for Thin section, BSEM, SEM Preparation
White Light and UV light Thin Section review, establishing of point cont classes
Core Description by a Geologist Thin Section Point Count
Core Resination
Lithology Thin Section Photomicrography and Description of mineralogy, diagenesis
and reservoir quality
Grain Size Sedimentary Structures Apparent dip of
Sorting beds, fractures and Description of SEM/BSEM Samples for mineral identification & effects of
Clay Content clay minerals or cements on reservoir quality
Grain Roundness other features CL, FI, Stable Isotope analysis
Ichnology, Ichnodiversity, Bioturba-
Borehole Image Log tion Index (Intensity)
Interpretations
Rock Colour

Diagenetic Features (cement,
nodules)

Interpretation of lithofacies scheme
(Lithofacies > Lithofacies Association > Depositional Subenvironment)

Interpretaiton of Depositional Environment and Conceptual Model

Vertical Stacking of Lithofacies and Seismic Data Correlation of Lithofacies or
Depositional Environments Depositional Environments to

Identification of any significant Reservoir Quality
trends, surfaces or horizons Identification of any secondary
(diagenetic) controls on Reservoir
Interpretation of sequence strati-
graphic intervals or horions Quality
Well Correlations
Definition of 3D Sandbody
geometries

Seismic Data

GEOLOGICAL
RESERVOIR MODEL

Established in 1988, PanTerra Geoconsultants have
a proven track record of quality, reliability and client
focus. Integrated by design, having a wide range of in
house, state of the art laboratory equipment, geoscience
and engineering expertise under one roof, PanTerra
Geoconsultants provides a truly integrated service. With
most of our senior staff having an international operator
background, our clients can rely in receiving focussed and
practical solutions.
PanTerra Geoconsultants, alongside our new partners
Azuren and EOR Laboratory Services (ELS), can offer a
suite of oil and gas subsurface services ranging from
core and fluid analysis, geology, geothermal evaluations,
petroleum engineering and (EOR) field development
planning
This manual gives a walkthrough of a geological analysis
of your core from when it is delivered to our laboratory to
when we send you the final report.

HOW DO I BUILD A GEOLOGICAL MODEL?

Building a reservoir model requires combining the results of several different analyses
into a single model. If one of the inputs to the model is inaccurate then the resulting
model will be inaccurate. Using PanTerra Geoconsultants for you analysis ensures that the
maximum information is extracted from each analysis.

Porosity Shape RESERVOIR MODEL
Permeability Length
Hydrocarbon Saturation (Sw) Width : Thickness
Sinuosity

Reservoir models are based on a grid of cells. Each cell requires a porosity, permeability and saturation
value which are derived from the RCA data and explained using petrography or SCAL analyses. The shape of
the relevant reservoir units is then interpreted based on the sedimentological interpretation

WHAT ARE THE CONTROLS ON RESERVOIR QUALITY ?

Reservoir quality analysis aims to combine the results from the sedimentology,
petrography, RCA and SCAL data together to define the controls on a particular reservoir.
These controls can be divided into two main groups. Primary controls are defined during

deposition whereas secondary controls overprint the primary controls.
PRIMARY CONTROLS ON RESERVOIR QUALITY

Facies are interpreted from the core description . The
next part of the analysis is to assign a value or a range
of porosity and permeability (and also saturation) for

each facies.
Primary controls on reservoir quality refer to detrital
elements. These are commonly a combination of grain
size and sorting of quartz grains and. clay content.
Usually this is a controlled by facies or depositional
subenvironments. Commonly, Porosity varies with
grain sorting as smaller grains sit between the gaps
made by the larger grains, and permeability varies
with grain size as larger grains result in larger, and less
numerous, pore throats which increases tortuosity and
hence permeability. Variations from these relationships
indicate that there are other factors affecting the

reservoir quality.
Usually cross plots are used to identify correlations or
trends between mineralogy, fabric, facies and porosity
or permeability. Porosity v permeability plots coded by

potential factors are also useful.
Porosity vs Depth trends are commonly difficult to
establish from single wells unless cores have been
taken at different levels in the well. This is because the
variation in porosity is unlikely to significant over the

depth of a cored interval..
SECONDARY CONTROLS ON RESERVOIR

QUALITY
Secondary controls on reservoir quality comprise
pose depositional (i.e. diagenetic) controls on
reservoir quality. Commonly this refers to the
dissolution of labile grains which provides material
to precipitate clays or cements in pore space which

reduces porosity or permeability.
Alternatively as the grain dissolve they leave holes

behind which become secondary porosity.
These diagenetic processes can completely
overprint the detrital controls on reservoir quality

CORE ARRIVAL AND OPENING

PanTerra Geoconsultants handle all sorts of core samples from reservoir sandstones to
shale gas or cap rock cores to shallow depth, unconsolidated cores. We pride ourselves
on offering an approach to the core material which maximizes the potential data
available. With over 25 years experience

CORE ARRIVES IN LEIDERDORP
Core arrives in 9 m barrels. Immediately on receipt
of core, Barrels are inspected to ensure there are no
discrepancies. The 9m barrels are them marked up
into 1 m sections and cut into smaller sections for
ease of handling.
Work starts as soon as the core is received at the
Laboratory.
CORE OPENING
Core is then removed from the barrels and cleaned
of any drilling mud using dry rags. Close attention
is paid to proper matching and orientation of core
fragments. Geologists are frequently consulted to
help identify that the core is correct.
Core is marked with red and yellow lines to preserve
the orientation (Red on the Right)
Pieces are then typically laid in clearly marked
wooden boxes for storage and protection

CORE GAMMA RAY LOGGING
Core Gamma Ray in principle is the same as a
wireline log gamma ray and provides a calibrated
measurement of the levels of Uranium, Thorium
and Potassium in the core at a much higher
resolution than wireline logs. This can be used
to help develop sample strategies and also to
correlate the core with wireline logs in the well.

CT SCANNING
CT Scanning (X-Ray Tomography) allows the
imaging of the internal structure of the core in a

non destructive way.
It can be used to inspect the core prior to
unpacking (e.g. to check that a shallow core
is consolidated enough for drilling or cutting)
but can also be useful to identify sedimentary
structures or fractures which may affect SCAL

Sampling.
Modern core scanning techniques can be used
on orientated core to create an image which
can then be directly compared to a Wireline

Borehole Image Log (e.g. FMI)

WHOLE CORE & CORE PRESERVATION
Whole core pieces are wrapped or undergo special treatment to keep them close to the natural “reservoir

state” for future analysis.
HOT SHOT SAMPLES

Hot shot (samples analyzed as quickly as possible) XRD or SEM samples are taken to investigate the possibility
of delicate clays or evaporite minerals (particularly important in the Dutch Offshore!) and the plugging or

cleaning methods are then adjusted accordingly

PLUGGING & REGULAR CORE ANALYSIS (RCA)

Regular core analysis covers measurements of porosity, permeability and grain density
under surface conditions. Core plug samples are taken so that the original core is not
affected. Samples are drilled using diamond matrix tipped drill bits with an appropriate
lubricant (Brine, mineral oil or air). Ends are trimmed and kept for geological analyses.

SAMPLE CLEANING METHODS
Samples need to be cleaned to remove any drilling
mud or oil from the sample. The most common
method of core plug cleaning is the Hot Soxhlet
method using Toluene at first to remove any oils
followed by methanol to remove any salts. Samples
are then oven dried in an oven. When the sample
weight shows no more decrease, the samples are
considered dry. If samples contain delicate clays,
constant immersion, warm cleaning maybe used.
This is takes more time but better preserves the clay
mineralogy. I
Heavy or waxy crudes or formation water salts can
be cleaned using flow through cleaning where solvents are flowed through the cores plug. Low flow rates
(lower than reservoir conditions) are used to minimise disruption of the sample.
If analysis of the clay is very important, or if clays are very fragile (e.g. fibrous illite - which can have a
massive effect of reservoir quality) samples can be cleaned using Miscible Cleaning and critical point drying.
HELIUM POROSITY DETERMINATION
Porosity is simply the percentage of the Bulk volume which is not part of the grain volume. Grain volume is
determined using a Porosimeter (If you know the volume of the chamber and the gas pressure, introducing
a sample (reducing the volume) will increase the pressure by an amount. The resulting pressure increase
can be converted to grain volume using Boyles Law) and Bulk Volume of determined using the Archimedes
Principal This is done using mercury and takes into account irregularities or vugs in the sample that would
not be measured by simple measurements of the plug.

100 - Grain volume / Bulk volume = Porosity (%)
Weight of sample / Grain volume = Grain Density (gcm-3)

PERMEABILITY MEASUREMENT
Ambient (400 psi) permeability is measured using a steady
state permeameter. Gas Permeability is inversely proportional
to the gas pressure used in the permeameter. This is due to
interaction of the gas molecules with each other as they
pass through the sample. More molecules (higher pressure)
means more interactions (lower gas permeability). This effect

is removed by the Klinkenberg Correction
Measurement of permeability is complex and can be affected
by several variables. These are more closely examined during

Special Core Analysis (SCAL)

RCA QUALITY CONTROL

PanTerra take the quality
control of RCA Data very
seriously. Several extra steps are
undertaken to make sure that
samples have been measured

properly
The first step is to plot porosity
values against Permeability.
Usually the results should line
up. Samples that plot outside of
the norm are re-examined and
checked for potential errors or

problems with the plug.
The grain density of quartz is
2.65 gcm-3 so another check is
to re-examine any samples with
values that deviate from this

value are also rechecked

UNCONSOLIDATED CORES 3 methods of sampling are available:
Cores cut at shallow depth can often be very
unconsolidated due to the lack of any cement.
This makes plugging and slabbing very difficult as •Plugging using compressed air (only useful on semi
the material can very easily fall apart making most consolidated cores)
further analysis redundant. •Plugging using a plunge cutter
•Plugging with liquid nitrogen on frozen core
Cores are commonly left in the barrels as long as Samples are then mounted
possible. Full CT Scans can be taken to help plan
where to take samples and windows can be cut in The decision on which method is used is based on
the barrel to inspect the area prior to plugging. cost and core quality.

SPECIAL CORE ANALYSIS

Following RCA, a selection of core plugs can be used for Special Core Analysis (SCA).
These analysis are important for the accurate estimation of hydrocarbon reserves and
well productivity. They may provide input to petrophysical equations or for geological or
engineering reservoir models.

ELECTRICAL PROPERTIES
Electrical Properties typically refer to the
coefficients used in the Archie Equation - a, m
& n.
Formation Resistivity Factor (FRF) is measured
at confining pressure and can be used to
calculate the porosity exponent ‘m’. Pore
volume reduction with increasing confining
pressure is also measured
Formation Resistivity Index (FRI) is often
performed in conjunction with Capillary
Pressure measurements and involves
measuring the resistance of the sample whilst it is undergoing desaturation. This can take place at
ambient or confining pressure. This is used to calculate the saturation exponent ‘n’.
Cation Exchange Capacity/Excess Conductivity asses the contribution of clay minerals (particularly
smectites) to electrical conductivity. This can be important when evaluating shaly formations.
MULTIPHASE FLUID ANALYSIS
RELATIVE PERMEABILITY refers to permeability of multiphase (in most
cases oil & water, gas & water and gas & oil). There are several methods
available which are commonly complex. Which method is used will
depend on the sample material and the aim of the investigation.
WETTABILITY reflects how mixtures of oil and water will interact with
the rock grains. It controls capillary pressure, relative permeability and
electrical properties and has a major effect on pressure, flow and fluid
distribution in the reservoir.
Coring and plugging will inevitably result in the invasion of non-native
fluids which will affect the wettability. Therefore cores or plugs need
to be re-saturated with reservoir fluids and stored (aged) at reservoir
conditions to re-establish the wettability.

RESIDUAL FLUID SATURATIONS are important to evaluate the quantity of hydrocarbons that can be
produced from the reservoir (Recovery Factor) and what drives may occur naturally or be needed. It is
controlled by the texture of the porous media (pore throat/pore body ratio and the clay content and
distribution). In oil reservoirs, Capillary pressure curves are used to calculate irreducible oil or water
saturation whereas gas reservoirs require special analytical methods.

PORE VOLUME COMPRESSIBILITY
As a reservoir is depleted, the pore pressure within the
reservoir drops. As the overburden pressure remains the
same, the increased stress on the rock may reduce the
pore space and porosity during production Knowledge of
this are important inputs to determinations of oil in place or

predictions of future pressure performance
SONIC VELOCITY

The velocity of elastic (Sound) waves in solids is a function of the
density and elastic properties of the materials. Compressional
(Vp) and shear (Vs) waves are propagated through the sample

at varying confining pressures and saturations
This allows the calculation of Poisson’s ratio, Shear modulus,
Young’s modulus and Bulk modulus. This allows the calibration
and interpretation of seismic amplitudes and lithology or

porosity log calibrations.
NMR ANALYSIS & CALIBRATION
NMR analysis on rock samples is primarily undertaken to calibrate downhole logs. IT can give information
on pore size distribution, capillary bound water, clay bound water and empirical permeability
CAP ROCK ANALYSIS
These analyses are used to evaluate the sealing
capacity of an overlying caprock. Threshold
pressures area converted into maximum
hydrocarbon column heights.
2 options are available
• Gas or Oil Injection - Stress is applied and
the fluid is pressurised to determine at what
pressure fluid is displaced from the plug (i.e. at
what pressure does the seal leak). This test is
accurate but time consuming and expensive
• High pressure mercury injection - provides
the air-mercury threshold pressure which then
needs to be converted to a subsurface system.
This introduces a level of uncertainty.
WHOLE CORE ANALYSIS
Dean Stark extraction (Sw & So), Helium porosity
& grain density, directional gas permeability,
Liquid permeability and FRF tests can be
undertaken on whole core pieces. This can
be useful if the sample is heterogeneous and
core plugs are not representative, for example
in fractured samples, interbedded samples or
samples with stylolites etc.

DETERMINING FLUID SATURATIONS

Oil, gas and water saturations are important parameters in the study of reservoirs.
Water saturation (Sw) is a key component to the STOOIP calculation. These analyses
allow this to be calculated with more accuracy than petrophysical calculations

DEAN STARK METHOD
Plugs are drilled immediately after the core is removed from the barrel
and placed in a Dean Stark Distillation apparatus. Water is boiled out of
the sample using toluene and collected. Oil dissolves in the toluene. The
amount of oil is calculated from the sample weight difference and the
amount of water
The sample is then cleaned with methanol to remove salt. The weight
change afterwards indicates the amount of salt. Pore volume is then
calculated using a Porosimeter (as with RCA Porosity).
The respective saturations of oil, gas and water can then be calculated (Sh
& Sw))
TRACER STUDIES
By adding chemical or radioactive tracers to the drilling mud during coring
it is possible to distinguish between formation water and invaded drilling
water. This requires preserved core samples and drilling mud samples.
The process comprises Dean Stark analysis alongside ultra-centrifuging of
samples and spectrometry analysis

PARTICLE SIZE ANALYSIS

SIEVE ANALYSIS
The Classical way to determine
grain size!
LASER PARTICLE SIZE
ANALYSIS
Sample sizes are measured using
the scattering of low-power
Helium-Neon Lasers. Particle size
ranges are calculated in 3 ranges
0.2-80 µm, -.2-180 µm and
0.5-600 µm. 32 size bands are
available within these ranges

Detailed grain size data can be important inputs to
•Engineering - well completion programs in friable/unconsolidated sand reservoirs (i.e screen sizes)
•Petrophysical - aiding interpretations of SCAL data and log responses
Geological - accurate determination of grain size and sorting help with reservoir quality interpretations
and facies analysis

CAPILLARY PRESSURE

Analysis of Capillary Pressure is one of the best ways of converting non-numeric
geological observations into numerical data which can be used in reservoir engineering

models

Capillary pressure is measured on core plugs and is used to
• Evaluate reservoir rock quality in terms of Pore throat size distributions and wettability
• Predict fluid saturations in the reservoir versus height
• Estimate thickness of fluid contact transition zones
• Quantify sealing capacity of overlying rock
For geologists, the main use is to quantify how , or if, pores of different sizes observed in thin section or
SEM are connected and how this may vary between different reservoir intervals. For example an interval
may have a very high porosity die to the presence of vugs. However, if these vugs are not connected then
the reservoir potential of the interval may actually be very low.

AIR-MERCURY
CAPILLARY PRESSURE
Mercury Intrusion and extrusion
curves can be generated on core
plugs and cuttings relatively quickly.
It gives capillary pressure data,
pore throat accessibility and pore
level heterogeneity. Theoretical
permeability, reservoir grade and
mercury entrapment can be derived

Macropore

Micropore

POROUS PLATE
Porous Plates are used to determine capillary
pressure curves and saturation relationships on core
plugs at ambient or reservoir conditions (Note that
the sample has to have a permeability of at least 1

mD to allow the sample to fully drain rock fluid
AUTOMATED ULTRA-CENTRIFUGE

This can be used to determine capillary pressure and wettability in low permeability rocks at ambient or
reservoir conditions. The confinement also allows it to be used on poorly consolidated samples. It can also

be done with samples returned to reservoir fluid conditions.

PREPARATION OF CORE FOR GEOLOGICAL ANALYSIS

When all the sampling has been completed, the core can be prepared for geological
analysis and storage. Geologists work on Slabbed Core as features show up much better
on the fresh surface. As the core ages and dries out in can begin to disaggregate or even
discolor and change as minerals are oxidised in the air. Several methods are available to
minimise the effects of so called “core store diagenesis”.

CORE SLABBING
Core is slabbed using a saw to cut cores in
a lengthwise direction. Slabbing takes place
perpendicular to the inclination of planar
structures (usually lamination). The slabbed
surface then shows the maximum dip of the
inclined structures
CORE RESINATION & STORAGE
The penultimate stage of core preparation
is to create resinated slabs. The core is laid,
slabbed side down in a plastic tray marked
with Core Depths and a depth scale, sample
locations and numbers and any other well
details. A transparent epoxy resin is the
poured around the core and allowed to
set. The top of the core is then removed by
slabbing the core parallel to the resin tray.
The top section is then packed in plastic and stored in boxes
Advantages of Resin Slabs
• A 1cm thick core slab is preserved along with all core and sample identifcations. This provides a much
easier way to Preserve and access the information. Paper tags left in core boxes inevitably are lost or
destroyed in storage or when new people access the core
* If the core is analysed by several different contractors or companies, pieces can be mislaid or
mishandles. Resin slabs provide a permanent record of the rock and its original sequence
* The resin slabs are very useful for exhibitions or presentations and are also great for mini-permeametry

CORE PHOTOGRAPHY
Core slabs are digitally photographed at high resolution and high quality using white light and UV light.
These can be imported into the Core Description software and displayed next to the description allowing
the actual rock to be visualised next to the interpretation. UV light shows is very useful for indicating
hydrocarbons and can help identify beds containing hydrocarbons that may be missed by wireline logs or
petrophysical methods. This is particularly important for thin bed evaluation.

CORE DESCRIPTION

Core Description forms the basis for the geological analysis of the core. The aim is to
record the position of features in the core accurately against depth so that a person who
has not seen the core can interpret it as though he was in front of it. Core description,
development of facies and upscaling them to environment level intervals is one of the 2
pillars of deterministic geological modelling.

Cores can be described in-house or away. Cores are commonly decribed in WellCAD software but
description programs can be designed based on the nature of the core and the aims of the project.
Textural features (controls on reservoir quality) recorded are:
-- Grain Size
-- Sorting
-- Grain Roundness
-- Clay content
Sedimentological Features (interpretation of depositional environments):recorded are:
-- Lithology and lithological accessories
-- Sedimentary Structures
-- Bioturbation & Trace Fossils
-- Diagenetic features
-- Apparent dip of features
Fractures

UPSCALING DESCRIPTION TO INTERPRETATION

The next stage of the interpretation is to take the fine scale deposition and “lump” the
features into intervals with the same, or similar, features - Facies. These then form the

building blocks to build up into a geologic interpretation.

LITHOFACIES LITHOFACIES DEPOSITIONAL Regoional
ASSOCIATION SUBENVIRONMENT Palaeo-
Mmr LFA-3 - FLOODPLAIN geography
(Al, Ar) Rooted shales with Blocky, sheet-like, Overlying &
interbedded thin non reservoir, underlying
Al sandstones beds
Ar LFA-2 POINT BAR Ichnology,
Ax Low energy Lobate shaped mineralogy,
C sandstones reservoir, laterally Biostratigraphy
discontinuous etc.
LFA-1
High energy
sandstones with
erosive base

The minimum bed thickness that can be recorded on a 1:50 Scale core description is 5cm (1mm thick on
the chart).
Beds are grouped into repeated intervals based on sedimentological features. Usually approximately 10-
15 facies are assigned in a particular reservoir interval - These are called Lithofacies (cm-scale)
These Lithofacies are then grouped into intervals based on observed features. These may be be based on
lithology, bedding style, ichnofacies, wireline log or grain size trends or in some cases mineralogy from
point count or XRD data - These are called Lithofacies Associations (m-scale)
Lithofacies are then used to interpret the depositional subenvironment (m-scale to 10s m-scale

10s metres (DFSlSaEut-SpTrFa)tidal
(DInSEte-IrTtFid)al Flat
bainr(ftDoeDrbSmeEee-dsDpd(TDTeCiSddhcEath)-aiwTldanCcBinadthh)Ilhnsaamoennllunasntde(eeDldlsdSy(EtDi-dSITaECl-ShT) Ch)

10 - 100s kilometres

BOREHOLE IMAGE LOGS

Borehole Image (BHI) Logs are the best way of expanding the outputs from the core
description to the whole reservoir interval in a particular well. BHI data can help to
interpret sedimentology in uncored intervals or can form the basis for analysis into a
fracture network. When integrated with core, BHI Logs are a very powerful tool.
SEDIMENTOLOGY

Unlike many BHI specialists, PanTerra work
with all the available well data to produce an
image interpretation that is fully integrated
with the core, petrography and, where present,
pressure and production data. Images are
interrogated to extract the maximum amount
of sedimentological information obtainable
which can then be fed into the well correlation
or sequence stratigraphic model. This allows
interpretations made in the cored interval to
be extended over the whole reservoir interval
providing unparalleled input to the geological
reservoir model.
Palaeocurrents can be derived from cross bedding
which can provide sandbody orientations

STRESS INDICATORS
These refer to the orientation of borehole
breakouts or induced fractures. These occur
due to the present day stress on the wellbore.
Breakout occurs when the borehole “expands”
winhtehreeadsirinecdtuiocendofframctinuirmesumoccsutrrepsesr(pσe3nodricuShla)r,
to that in the direction of maximum horizontal
stress
FRACTURE STUDIES
Borehole image logs are one of the only ways
to examine a fracture network in the well.
Fractures can be identified, classified and given
an orientation. These can be used to plan
production from the fracture network, plan
artificial fracturing or form an input to a fracture
model.

FACIES CORRELATION & SEQUENCE STRATIGRAPHY

Well Correlation is the first step in turning a series of 2D well sticks into a 3D model. There
are several methods or concepts to consider when correlating wells. The best option will

depend on the data available and the aim of the project

Several methods of correlation can be employed to correlate wells, for example:
• Lithostratigraphic Correlation - Simple correlation of the sand layer in Well A with the sand layer in Well

B. It may not lead to an accurate geological correlation but may be useful for reservoir analysis
• Sequence stratigraphic Correlations - The concept of correlating rocks of the same age or which were

laid down at the same moment. For example, the river channel supplying a delta from which turbidites
flow into deep water are all deposited at the same point of geological time, but obviously have wildly
different environments and lithologies. Important sequence stratigraphic surfaces occur where there
are breaks in facies trends.
• Biostratigraphic Correlation - Correlations are based on ages derived from fossils or palynomorphs. This
may be closely linked to sequence stratigraphic
• Chemostratigraphic Correlations - Well correlations based on chemical signatures measured from rock
samples. Depending on the chemical markers, this can either result in a sequence stratigraphic or
lithostratigraphic correlation
PanTerra can supply correlated data sets in ODM or Petrel formats depending on client needs or supply

more conceptual illustrated panels to show interpreted features more clearly

SAMPLE SELECTION

The initial Sample Selection forms the basis for all following analysis and is therefore very important.
Usually after the core sampling has been finalised, the core is slabbed, resinated and packed away
making taking further samples difficult (and for some analyses, no longer possible). As soon as the core
barrels are opened and the core exposed to non reservoir conditions, changes may occur in the pore
space which can lead to unexpected changes in the rock.

RCA plugs tend to be taken at regular intervals (e.g. 25cm)
which is usually sufficient to cover every facies. However,
petrographic analyses usually need to be more targeted.
Petrographic samples are chosen based on the RCA data,
facies described in the core and to help explain features
observed in the core during core description e.g. cemented
beds, nodules etc.
Care must be taken to get the correct balance between
identification of small or discontinuous features and defining
the controls on reservoir quality.
When the sample locations are defined, the analyses can be
planned. Usually it is necessary to do thin section analysis on
all samples, however budget constraints may limit the number
of samples used for advanced diagenetic analysis. Note that
while it can be tempting to spread analyses approaches
around each sample it is often better to concentrate all
approaches on a few samples in order to fully describe each
sample. It is usually more accurate to then interpolate these
features into other samples using thin sections

THIN SECTION PETROGRAPHY

Thin sections are described to determine and illustrate the
texture and mineralogical composition of the reservoir. This
is done to determine the depositional and post-depositional
alterations that influence reservoir quality and to allow the
prediction of reservoir quality in other areas.

PETROGRAPHIC DESCRIPTION

Fabric (sedimentary structure, Wentworth grain size, sorting
and shape) is described using a comparator. It is used to define
depositional controls on reservoir properties and possibly about
the environment of deposition
Composition is described alongside XRD data (where possible).
It is split into detrital composition (e.g. Quartz, Feldspar, rock
fragments) and authigenic composition (e.g. clays or cements).
Detrital composition is used to derive a Pettijohn classification
for the sample. If a provenance study is required, the Dickinson
classification scheme is used.
The effects of compaction are analysed and described qualitatively.
These can be estimated by examining grain contacts to give grain
rearrangement, deformation or pressure solution. The level
of compaction can be used to estimate how thew reservoir will
change with depth
The paragenetic sequence of the minerals is then defined. This is
the relative succession of diagenetic events, determined on the
relationship between the grains, clays and cements. CI and FI (see
opposite) can also be used.
The Porosity is then described to establish the type of porosity
(primary, secondary or microporosity), the size of the pores and
the pore throats, how the pores are connected, does the point counted porosity vary significantly from the measured
porosity and how does the porosity compare to the capillary pressure data?
Finally the reservoir quality is summarised based on the features described above

POINT COUNTING

Thin sections are point counted using computer
controlled microscope stages. Classes are defined
beforehand based on what is observed in the rock and
the aim of the project. It is important to keep classes
consistent between wells and projects of different
ages in order to allow comparisons between wells.
Commonly 300 points are counted (100 points for
grain size analysis)
PanTerra can design point counting and other
petrographic analyses to integrate with Touchstone
Modelling Datasets.

ADVANCED DIAGENETIC ANALYSIS

Advanced diagenetic analysis are used alongside thin section analysis to further define
the controls on reservoir quality. This can involve improving mineral identification, better
defining the paragenetic sequence or the timing of mineral precipitation.

SCANNING-ELECTRON (SEM) & BACKSCATTER- SCANNING-ELECTRON (BSEM) MICROSCOPY
SEM microscopy gives a 3D view of the grains, pore network, cements and clays in the sample and is a
powerful tool to assess how mineral morphology will affect reservoir quality. It is also a very useful support
for petrographic study of then sections as Energy Dispersive Analysis of X-Rays (EDAX) can be used to
directly identify minerals
BSEM analysis is performed on a 2D, polished surface and is similar to petrographic analysis although with
far higher resolution and with the aid of EDAX.
Combinations of both approaches are recommended to help define the mineralogy, paragenetic sequence
and effect of authigenic minerals on reservoir quality
X-RAY DIFFRACTION (XRD) MINERAL ANALYSIS
Samples are ground to powder and formed
into samples. X-rays are then passed over the
sample. The spacing between the crystal layers
can be calculated from the variation in X-rays.
Different minerals have different minerals affect
the wavelength in specific ways providing a
means of identification. The abundance of the mineral can be calculated from the number of counts.
Commonly, whole rock (everything in the rock) samples and Clay Fraction (everything under 2μm sized) are
analysed)

CATHODOLUMINESCENE (CL) MICROSCOPY
CL microscopy is used to identify distinct cement generations that may otherwise appear indistinguishable.
or can reveal properties in detrital grain populations that may be useful for tracing sediment provenance.
Analysis involves viewing the intensity of light emitted when a polished rock surface is bombarded with
a critically focused beam of charged gas molecules. Different crystal structures will luminesce differently

which allows chemically similar (optically identical) to be differentiated.

CL Images of quartz (from the blue & brown colours,
likely metamorphic quartz, and zoned dolomite

STABLE ISOTOPE ANALYSIS
Most stable isotope geochemistry is performed
on the light elements Oxygen, Carbon, Hydrogen,
and Sulphur. Analyses can be performed on
whole-rock samples, carbonate and evaporite
mineral components, or reservoir fluid samples.
These analyses require highly specialised
equipment and are performed at a university or
a commercial isotope lab. Information can be
obtained about the source of the element (for
example, the relative contribution of organic
matter or fresh water) and the temperature of

mineral precipitation.
FLUID INCLUSION ANALYSIS

Fluid Inclusion microthermometry measurements can provide quantitative numerical estimates of the
temperatures at which mineral cements were precipitated during diagenesis. This can then be compared
with burial history curves to estimate when cements were formed or the conditions during deformation
events. Hydrocarbon inclusions can be used to examine the petroleum migration history as well as

properties such as API gravity and GOR
Aqueous fluid inclusions contain a bubble which can be observed under high power microscopy. As the
sample is heated and cooled the bubble expands and contracts. The temperature at which the gas in the
bubble fully returns to solution is the temperature the inclusion was formed at. The salinity of the fluid can

be determined from the freezing temperature of the liquid.