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Cuttings Descriptions Clastic
Description order – memorise this!!
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Rock type (% and modifier, if required)
Colour or colour range
Hardness
Fracture and texture (Break)
Grain size: Range and Dominant size
Sorting
Angularity or Roundness
Sphericity
Matrix
Cementation: Degree, Percentage of each cement and composition
Accessories and Fossils: Type and Percentage of rock
Effective Visual porosity, type(s) and amount
Hydrocarbon indications – shows description (separate module)
Rock Name
Arenaceous Siliclastics
Ü Arenaceous rocks may be clastic but generally they are resistate (i.e. without clay),
comprising predominantly quartz, minor feldspar and other detrital accessories (rock
fragments).
Ü Little useful information can be obtained about the quartz mineralogy at the wellsite
although the physical condition of the grains may tell you some information. Like?
Ü The type, condition and abundance of minerals other than quartz will be of help in
interpreting the environment and rate of sedimentation and may help in isolating the
source and history of the sediment.
Ü It will also help the identification of the sediment for later correlation. Identification of rock
mineralogy may also be important in selecting matrix properties for the interpretation of
porosity and other wireline logs.
Ü A guide to proper naming of the rocks is shown in the next slide.
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Lithology Definition – after Folk, 1974
Examples
80% Q, 16% F, 4% R =
Sub-Arkose Sandstone
74% Q, 7% F, 21% R =
Litharenite Sandstone
50% Q, 40% F 10% R =
Arkose Sandstone
50% Q, 24% F, 26% R =
Feldspathic Litharenite
Ü By using this naming method, it is immediately obvious to the reader what type of
arenaceous rock is being described.
Ü The FOLK method is primarily useful when describing sidewall cores (SWC and RCOR –
rotary side wall cores) and conventional core chips as you can see the original rock
textures which has not been totally destroyed by the drilling action of PDC bits.
Ü However, you CAN use this as part of a drilled cutting description i.e. Litharenite or
‘Quartzite’ Sandstones, these are quite easy to identify.
Ü If used, be careful to be correct (the WSG may well be asked to explain his findings in a
conference call with town).
Ü As stated in the first slides - It is best practice when unsure of naming a rock to follow the
rock name with a ? if not sure i.e. Lithic Arkose?: pinkish grey, etc.
Argillaceous Rocks – Reference text
Ü Argillaceous rocks and much of the matrix and secondary mineralisation in rudaceous
(coarse grained) and arenaceous rocks a production of hydrolysis, e.g. clay minerals,
hydrous micas, hydroxides and some oxides. It is important to realise the subtle though
significant difference between hydrolysate sediments and the other so called “chemical”
sediments.
Ü Hydrolysate minerals result from the chemical weathering of the parent minerals at the
point of weathering and throughout the period of transport and sedimentation.
Ü True chemical sediments are produced by crystallisation or precipitation at the place of
sedimentation and may show no direct relationship to the parent, or parents, or the means
of weathering and transport.
Ü The five most significant minerals present in argillaceous rocks are the sheet silicates: illite,
montmorillonite, vermiculite, kaolinite (all clay minerals) and chlorite. (Note: each of these
mineral names encompasses a range of varying composition, i.e. a group of minerals
related by a common structure.
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Ü For your reference - The term “smectite” is commonly used to describe the montmorillonite
group, sometimes to include vermiculite.
Ü Clay minerals are usually the products of weathering and hydrothermal alteration of parent
rocks, the latter probably being of lesser and possibly not quantitative importance.
Ü Acidic rocks, deficient in calcium, magnesium and sodium tend to yield kaolinite, whereas
Alkaline rocks generally yield montmorillonite.
Ü Illite may result from either rock type when potassium and aluminium concentrations are
high.
Ü Chlorite is often detrital in sediments but may form from the degradation of
ferromagnesian minerals.
Ü Vermiculite may result from the degradation of micas and is also present in a mixedlayered form with detrital or secondary chlorite.
Ü In addition to the sheet silicates, fractions of accessories include unaltered parent minerals
and resistant material, e.g. Quartz.
Ü Reworked, previously compacted and re-weathered clay minerals may also be present.
Ü The presence or absence of these in quantity gives clues to energy and activity of the
environments of weathering, transport and sedimentation.
Ü Since the physic-chemical weathering process is continuous, conditions within the
environments of weathering, transport and sedimentation have as large, if not larger effect
on the mineral product as the parent.
Lithology Definition - General WSG Field
80-20
Sandstone
50-50
50-50
Sa
nd
yS
ilts
ton
e
Sand /
Silt /
Clay
20% silt, 30% clay, 50% sand =
Argillaceous Silty Sandstone
y e
nd
Sa ston
y
Cla
19% clay 81% sand =
Sandstone
Sil
ty
49% clay, 51% sand =
Argillaceous Sandstone
20-80
s
ou
ce
illa ne
Arg dsto
n
Sa
Sa
nd
sto
ne
Examples
20% clay, 80% sand =
Argillaceous Sandstone
80-20
20-80
10% silt, 30% clay, 60% sand =
Argillaceous Sandstone
Siltstone
20-80
Argillaceous
Siltstone
50-50
Silty
Claystone
Claystone
80-20
If a rock has 20 – 50% of a minor
constituent then the name of the lithology
MUST have a modifier.
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Lithology Percentages
No easy way to do this. Practice and experience helps. TIP: Geoprolog have a good chapter
in there Field Handbook that discussed percentages and the apparent differences of light on
dark cuttings and vise versa.
Colour
Ü GSA Rock Colour Chart
Ü Published by the Geological Society of America, this chart contains 115 colour chips for
identifying the range of rock colours. The chart is based on the Munsell colour system.
Ü The Munsell system consists of three independent dimensions which can be represented
cylindrically in three dimensions as an irregular colour solid: hue, measured by degrees
around horizontal circles; chroma, measured radially outward from the neutral (grey)
vertical axis; and value, measured vertically from 0 (black) to 10 (white).
Ü Colour estimations should NOT be made without the aid of the colour chart.
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Ü Ascertaining accurate colours is a critical part of the cuttings description e.g. slight colour
changes can reflect facies, depositional environment and mineralogical changes and can
vital in aiding correlation with offset wells.
VERY IMPORTANT: DESCRIBE THE COLOUR AND EVERYTHING ELSE WHEN THE
CUTTINGS ARE WET, AND STRESS THE PREDOMINANT COLOUR!
How is this done correctly?
Ü Firstly select a suitable cutting of the LITHOLOGY you wish to described, OR a number of
cuttings if they are small and have a tendency to stick together (or there is a big colour
range between cuttings).
Ü The cutting/s should be placed on the colour chart square eyeball the cutting/s first (in
visible light) to ROUGHLY determine which page of the colour chart you will need, and
roughly which colour square your Lithology lies in the range of the cutting i.e. colour chips
in the range of olive grey to greenish grey.
Ü Then, place the colour chart WITH the cutting placed on top of the colour chip square
under the binocular microscope. The WSG must then look down the microscope to
ascertain the colour using the microscopes light source.
Ü Using this method you can easily move the cutting onto different colour squares. The
cutting lies on top of the colour square so it is a direct comparison and it is EASY to see.
Use this method to determine colour
Ü Some other useful descriptive terms for colour, the WSG can use before the colour in the
description; varicoloured, banded, iridescent, speckled, spotted, scattered, disseminated,
variegated, mottled.
Ü Its more accurate that just ‘dim mudlogging unit lighting,’ it produces consistency and it is
easier to determine the colour down a microscope AND even IF the light source strength
(too high/too low) changes then the colour squares appearance will ALSO change.
Ü As the cutting is directly next to the colour square then you ALWAYS get and accurate
color/colour range.
Ü Also if ALL WSG use this method, when you look at an offset well – the colours described
should be the same!
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TIPS
Ü Try and pick out clean well formed cuttings.
Ü If drilling with PDC bits normally there is one flat clean ‘CUT’ surface – use that side.
Ü Depending on the mud system that is being used, the mud is liable to stain the cuttings
(particularly if they are at all porous).
Ü Take this into consideration and when the cutting/s are placed on the colour square break
it open to find and nice clean surface with NO mud staining.
Staining
Ü Staining is important and can originate from a variety of colouring agents:
Ü Carbonaceous or Phosphatic material plus Iron Sulphide and Manganese oxide can range
from grey to black or even brown lignite.
Ü Glauconite, Ferrous Iron, Serpentine, Chlorite and Epidote are green colouring agents.
Ü Red or orange mottling can be derived from surface weathering or subsurface oxidation by
circulating waters. Haematite or Limonite (hydrated ferric oxide) gives red, brown or yellow
shades.
Hardness/Induration
Ü This cohesive strength should refer to individual cuttings or chips and not to individual
grains.
Ü How is this done correctly?
Ü Use the forceps or the steel pointed ‘prodder’ provided by ALL mud logging companies.
Ü Pressure should be applied to the cutting/s and the WSG must determine from how much
pressure is applied what the hardness of the rock is.
Ü Please NOTE: due to the shearing cutting action of PDC bits the original rock fabric is lost
by this cutting action.
Ü This will affect the apparent cutting hardness dramatically. i.e. a well consolidated, very
hard siliceous Sandstone after being drilled by a PDC bit will appear in the cuttings as
amorphous soft rock flour OR very fine silt accretions which are friable and soft.
Loose/Uncon
-solidated
Particles are discrete and non-coherent, unsonsolidated sands.
Friable
Coherent, but crumbling under slight pressure.
Soft
Clays, marls and silts which can be deformed by slight pressure
Plastic
Pliant clays that show putty-like deformation
Firm
Compact, breaks under slight pressure.
Moderately
Hard
Grains can be detached using knife. Small chips can easily be broken by
hand.
Hard
Solidly cemented or lithified. Does not break under slight pressure, but
can be scratched with knife blade.
Very Hard
Can not be scratched with a knife blade, usually siliceous in nature.
Brittle
Moderately hard, but breaks easily with firm pressure. Generally applies
to shale with platey fracture, coal or certain limestones.
Dense
Commonly used to indicate a fine grained, well lithified tight rock (usually
limestone) with sub-conchoidal fracture.
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Texture and Fabric
Ü After you have applied pressure with the ‘prodder’ breaking the cutting (if it is not too hard),
next you describe the surface fabric, habit and fracture – or the ‘break’ of the cutting.
Ü Texture is defined by the size, shape and arrangement of the component particles of a
rock and will have be described under the headings of grain size, shape and sorting. Other
textural descriptions fall under the terms fabric, habit and fracture.
Ü The nature of the break is indicative of internal rock stresses and composition e.g. angular
break, conchoidal, crumbly, fissile, hackly (rough or jagged), splintery, and earthy.
Ü Fabric - Several descriptive terms are used to describe the type of fabric, commonly as a
result of cleavage or bedding, seen in argillaceous and carbonaceous cuttings. These
include:
Fracture & Break
Blocky
Used to describe claystone, marl and limestone in which fractures are
developed at approximately right angles, so that small blocks are formed.
Sub blocky
Commonly used to describe PDC drilled cutting that are not quite 100%
blocky with clean breaks not perfect right angles and not perfectly angular.
Angular
Used to describe well lithified formations that break chips with angular and
surfaces, generally as limestones, and siliceous hard formations.
Conchoidal
Commonly seen in dense rocks such as chert, argillite and flint and or coal.
The term refers to the concave and convex surfaces developed on fractures.
The fracture of hard limestone produces somewhat less strongly developed
curved surfaces and the fracture has been called "sub- conchoidal".
Flaky
The rock fractures into small flakes or chips. Common in some marls and
occasionally in metamorphic rocks.
Platy/Fissile &
Used to describe shale and marl in which fissility is well developed. The rock
breaks in parallel sided thin plates. This is commonly caused by fracture
along bedding planes, or along cleavage directions.
Sub Fissile
Splintery
Used to describe shales in which the fissility is not strongly developed, but
exists sufficiently to cause irregular surfaces and edges, like a board broken
across the grain.
Ü Example of blocky break – cuttings breaks in half with slight pressure (moderately hard),
approximately right angles, so that small blocks are formed
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Surface Texture & Fabric
Amorphous
Cuttings with no distinct shape.
Homogeneous
Composed of parts or elements that are all of the same kind.
Heterogeneous
Composed of parts or elements of different kinds; having
widely dissimilar elements or constituents.
Sucrosic
Surface breaks have a sugar like crystalline appearance
(limestones and some siliceous siltstones).
Vesicular
Characterized by or consisting of vesicles
Earthy
Of the nature of earth or soil/unglazed pottery commonly used
in conjunction (together) with gritty as a textural term.
Smooth/rough
As stated.
Etched Frosted,
Pitted,
As stated (sandstones and limestones).
Striated
Surfaces marked with striae; furrowed; striped; streaked
common on flat cut surfaces of PDC drilled cuttings.
Lustre
Ü Together with surface texture the lustre of clean cuttings or free mineral grains, chipped
surfaces can also be used:
Ü Definition: The quality and intensity of light reflected from the surface of a mineral (or in
our case drilled cuttings). This property must be observed first-hand and cannot be
demonstrated in a photograph.
Ü Metallic
- strong reflection, shines like metal, may be very shiny (like a chrome car
part) or less shiny (like the surface of a broken piece of iron);
Vitreous
- glassy, bright (shines like glass);
Resinous - a resin-like shine (resembling amber for example);
Greasy
- a dull sheen, has the appearance of being coated with an oily substance;
Pearly
- a whitish iridescence (resembling pearl for example);
Silky
- a sheen like that of a fibrous material, e.g. silk;
Adamantine - a brilliant lustre such as that of diamond;
Earthy
- like the surface of unglazed pottery.
Shale Swelling
Ü After a Claystone cutting has been broken and the fracture/break interpreted, place a
small sample in a porcaline spot tray – add water to determine the hygroturgid (swelling
nature) of the Clays.
Ü Marked slaking or swelling in water is characteristic of montmorillonites and distinguishes
them from kaolinite and illite.
Ü Drilling with OBM. Cuttings may have a film of oil coating the cuttings. In these cases look
for clean break surfaces, add some dilute HCL break the oil film.
Ü Using the binocular microscope, watch the clean surfaces for speed of the swelling
(hydrating) reaction.
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Swelling Descriptive terms
Ü
Ü
Ü
Ü
Ü
Non-swelling: does not break up in water even after adding 1% HCl
Hygroturgid: swelling in a random manner
Hygroclastic: swelling into irregular pieces
Hygrofissile: swelling into flakes
Cryptofissile: swelling into flakes only after adding 1% HCl
NB: If reaction in distilled water is inhibited by traces of oil add droplet of HCl to break oil film.
Udden-Wentworth Scale
Ü The scales used to define grain sizes in sediments and sedimentary rocks are grade
scales; that is, they are created by imposing arbitrary subdivisions on a natural continuum.
The terminology which is most familiar to us is that of the Wentworth Scale, which includes
the major classes: gravel, sand and clay, with their numerous subdivisions. Because the
range of grain sizes found in nature is so large, a logarithmic scale, such as the UddenWentworth scale shown to the left, is more practical than a linear scale.
Ü The phi scale, devised by Krumbein, is computed by the following equation:
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Grain Size – with shaker screen sizes
U.S. Standard Sieve
Mesh Number
Grain size (mm)
Use Wire Squares
Microns
Phi (φ)
Wentworth Size Class
256
-8
Bolder
64
-6
Cobble
16
-4
Pebble
5
4
-2
10
2.0
-1.0
Granule
18
1.0
0
Very Coarse Sand
35
0.5
500
1.0
Coarse Sand
60
0.25
250
2.0
Medium Sand
120
0.125
125
3.0
Fine Sand
230
0.0625
625
4.0
Very Fine Sand
Analysed using pipette
or hydrometer
0.031
31
5.0
Coarse Silt
0.0039
3.9
6.0
Medium Silt
Fine Silt
Very Fine Silt
G
R
A
V
E
L
S
A
N
D
M
U
D
After FOLK 1974
Always use a grain size comparator. The best type are the translucent plastic
comparators as they can be placed on the sample tray. This eliminates the
need to retrain your eye when the zoom on the microscope is adjusted.
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Grain Size Comparator
Ü Numerous times (like the colour chart) have I entered the mudlogging unit to find a pristine
unused grain size chart – or on some TEPI operations NO grain size chart at all. Discuss.
Ü If you don’t carry your own (I DO) and Geoprolog don’t provide one then have them order
some immediately. It is very important.
Ü IF for some ‘crazy’ reason there isn’t a grain size comparator at hand in the mudlogging
unit, AND the WSG does not posses his own then…
Ü By using this simple method of using the tip of a propeller pencil (0.5 = medium) you can
make a rough estimation of grainsize.
Sorting
Ü
Ü
Ü
Ü
Ü
Very well
90% of grains in one grain size class.
Well
90% of grains in two or grain size classes.
Moderate 90% of grains in three grain size classes.
Poor
90% of grains in four or more grain size classes.
Very Poorly 90% of grains in five or more grain size classes.
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Very Well Sorted
Distribution
Well Sorted
Moderately Sorted
VF
F
M
C
VC
Grain Size
Angularity or Roundness
"The degree of abrasion of a clastic particle as shown by the sharpness of its edges and
corners can be expressed as the ratio of the average radius of curvature of the several edges
or corners of the particle to the radius of curvature of the maximum inscribed sphere (or to
one-half the nominal diameter of the particle.)"
Wellrounded
Original faces, edges, and corners have been destroyed by
abrasion and whose entire surface consists of broad curves
without any flat areas.
Rounded
Round or curving in shape; original edges and corners have
been smoothed of to rather broad curves and whose original
faces are almost completely removed by abrasion. Some flat
areas may remain.
Subrounded
Partially rounded, showing considerable but not complete
abrasion, original form still evident but the edges and corners
are rounded to smooth curves. Reduced area of original faces.
Subangular
Somewhat angular, free from sharp edges but not smoothly
rounded, showing signs of slight abrasion but retaining original
form. Faces untouched while edges and corners are rounded off
to some extent.
Angular
Sharp edges and corners, little or no evidence of abrasion.
Subangualsubrounded
A term sometimes used when one can not decide which to
choose.
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Angularity or Roundness
It is important that the description given should be of the original detrital grain. If the grain is
affected by authigenic overgrowths, this should be noted and the concepts of angularity
abandoned.
Sphericity
Ü Grains can also be described according to their shape, either low, medium or high
sphericity.
Ü Alternately they may be described as elongate, sub-elongate, sub-spherical and spherical.
When choosing your preference stick to that way of describing – remember
CONSISTENCY.
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Matrix and / or CEMENT
Ü Cement is deposited chemically and matrix mechanically.
Ü Should be described by type (silt, clay, etc) and proportion (%) of overall rock.
Ü In cuttings, clay is always described as matrix as it is not possible to determine its mode of
origin by use of a binocular microscope.
Abundant
15-20%
Common
10-15%
Minor
5-10%
Rare
1-5%
Trace
≤1%
Matrix
Ü Silt acts as a matrix, speeding cementation by filling interstices, thus decreasing the size
of interstitial spaces
Ü Clay is a matrix material, which may cause loss of porosity either by compaction, or by
swelling when water is introduced into the formation.
Ü Argillaceous material can be evenly distributed in siliciclastic or carbonate rocks, or have
laminated, lenticular, detrital or nodular form.
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Cement
Ü Identified by type and effectiveness of the cement (calcite, quartz, dolomite etc.).
Adjective
% of Pore Space
Filled
Well
70-100
Moderately 30-70
Poorly
0-30
Ü The order of precipitation of cement depends on the type of solution, number of ions in
solution and the general geochemical environment.
Ü Several different cements, or generations of cement, may occur in a given rock, separately
or overgrown on or replacing one another.
Ü Chemical cement is uncommon in sandstone which has a clay matrix.
Ü The commonest cementing materials are silica and calcite.
Ü Silica cement is common in nearly all quartz sandstones. This cement generally occurs as
secondary crystal overgrowth deposition.
Ü Opal, chalcedony and chert are other forms of siliceous cement. Dolomite and calcite are
deposited as crystals in the interstices and as aggregates in the voids.
Ü Dolomite and calcite may be indigenous to the sandstone (the sands having been a
mixture of quartz and dolomite or calcite grains) or the carbonate may have been
precipitated as a coating around the sand grains before they were lithified.
Ü Anhydrite and gypsum cements are more commonly associated with dolomite and silica
than with calcite.
Ü Additional cementing materials, usually of minor importance, include pyrite (generally as
small crystals) siderite, haematite, limonite, zeolites and phosphatic material.
Cement Interpretation TIPS - Calc vs. Silica
Ü Quite often you will not be able to see cutting aggregates to determine what the nature
and amount of cementation is. i.e. PDC drilling destroys rock fabric.
Ü When this happens you have to use your well tuned WSG detective skills.
Ü To a sample of bit crushed Quartz add HCL acid and look for reaction (calcite/dolomite or
even a proportion of each.
Ü If no reaction and drilling of the formation was relatively slow over that depth interval, you
can safely assume there is some siliceous cementation – look closer for any Quartz
overgrowths.
Determining Silt detritus Content of CLST & SLST’s
Ü I devised this method as a fairly accurate way to determine silt detritus content of
claystones and siltstones. i.e. remember rock naming – over 20% of a constituent requires
a modifier (Silty Claystone).
Ü Place a cuttings sample of the lithology in a white porcelain spot tray as in the below
picture.
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Ü Add either water of some dilute HCL to the spot tray (if you add acid you can combine the
2 test at one time – saving time).
Ü Crush the cutting/s with the bottom of a test tube or the other side of your ‘prodder’ – like
this.
Ü This will give you the first indication of Silt content – i.e. is the cutting gritty against the
glass – you will also be able to hear a grinding noise.
Ü Then look down the microscope with the test tube displacing the liquid and you will be able
to clearly distinguish any silt / or sand detritus.
Look down the
microscope through
the test tube glass
to look at the silt
content.
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Ü From what you see you can describe the nature of the silt i.e. Quartz, detrital, and even
hazard a guess at the minerology, glauconite, apatite, etc.
Ü Also you can pour off the fluid with the clay dissolved in it leaving the detritus in the spot
tray as it is more dense.
Ü From the size of the original cuttings vs. what is left you can give a fair estimation
(percentage wise) of the SILT/SAND content of the bulk lithology being described.
Ü Using this method – granular break clean claystones that look like siltstones can easily be
identified.
Common Accessory Minerals
Ü Identified by
Type: carbonaceous, pyritic, feldspathic,
micaceous, fossiliferous, cherty, glauconitic.
Ü Amount – Trace Appearance
Scattered, speckled, disseminated, floating.
Ü Additionally colour, hardness, form (prismatic,
tabular, globular, euhedral, anhedral, cubic,
fibrous, rhombic, etc) can also be described..
Common Accessory Minerals
Pyrite
Ü Pale brass yellow
Ü Hardness of 6.5.
Ü Cubic crystalline structure
Ü GR = 0API
Ü Can act as a cement or be found as aggregates of crystals or disseminated, common also
replacement mineral.
Calcite
Ü Colorless, White, Pink, Yellow, Brown.
Ü Hardness of 2.5
Ü GR = 0API
Ü Can occur as clear or milky white crystal, veins, fibrous or be amorphous.
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Dolomite
Ü Variable: pinkish, brown, yellow, colourless, white, yellow, black.
Ü Hardness of 3.5 - 4
Ü GR = 0 API
Siderite
Ü Yellowish brown colour
Ü Hardness of 3.5 – 4.5
Ü GR = 0API
Ü Sideritic carbonates usually give a dull orange mineral fluorescence when viewed in UV
light and have a slow rate of effervescence with dilute HCl. Can easily be mistaken for
dolomite.
Glauconite
Ü Varying shades of green, blue green, yellow green.
Ü Hardness of 2
Ü High in potassium
Ü GR = 78.31 API
Ü Generally fairly glassy BUT can occur as pellets, or may be very soft and amorphous
(mushy) – not to be confused with chlorite.
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Chlorite – can look very much like Glauconite
Ü Varying shades of green, rarely red, yellow and white
Ü Hardness of 2-2.5
Ü Vitreous pearly lustre
Ü GR = 180-250 API
Ü Chlorite is widespread in low grade metamorphic rocks such as slate and schist, in
sedimentary rocks, and as a weathering product of any rocks that are low in silica
(especially igneous rocks).
Chlorite and hematite
Othoclase KAlSi3O8
Ü Variable, Pinkish white, off-white, yellow, or shades of red, orange to brown
Ü Specific gravity - 2.6
Ü Transparency - Translucent to opaque (rarely transparent)
Ü Hardness of 6
Ü Lustre - Vitreous
Cleavage/fracture - Perfect in two directions, seldom twinned
Ü High in potassium
Ü GR = ~200 API
Ü Orthoclase is a member of the feldspar group and is a framework silicate. Orthoclase, also
known as alkali feldspar or K-feldspar, is one end-member of a solid solution between
orthoclase and albite.
Ü Orthoclase is found in silica-rich igneous rocks such as granite, and in high grade
metamorphic rocks.
Plagioclase CaAl2Si2O8 (anorthite), NaAlSi3O8 (albite)
Hardness - 6-6.5
Specific gravity - 2.6-2.8
Transparency - Translucent to opaque (rarely transparent)
Colour - Usually white, grey or colourless
Lustre - Vitreous
Cleavage/fracture - Perfect in two directions,
Ü Crystal habit - Prismatic, tabular
Ü GR = ~200 API
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Ü Plagioclase consists of a solid solution between the albite and anorthite end-members,
and together with quartz is the most common of the rock forming minerals.
Ü The twinning in plagioclase produces stacks of twin layers that are typically fractions to
several millimetres thick. These twinned layers can be seen as striation like grooves on
the surface of the crystal and, unlike true striations, these also appear on cleavage
surfaces.
Chert (microcrystalline quartz) (SiO2) includes chalcedony, agate, jasper and flint.
Ü Variable colour
Ü Hardness of approximately 7
Ü Conchoidal fracture
Ü Can be clear to opaque and may be mistaken for dolomite as calcareous inclusions may
occur which will effervesce slowly. Check the hardness to identify if it’s chert. Inform the
company immediately on finding chert as it will ‘kill’ a PDC that is rotating at high RPM bit
very fast.
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MINERAL
DENSITY
(S.G.)
HARDNESS
(MOH’S)
OBSERVABLE FEATURES
OCCURRENCE
VERY COMMON
Grossular
3.594
_
Pale green-yellow: some times
white
Detrital from metamorphosed
impure calcareous rocks
Andradite
3.859
_
Golden yellow-black
Detrital from metamorphosed
impure calcareous and calcic
igneous rocks
Uvarovite
3.9
_
Dark Green
Detrital from Serpentines
Hydrogrossular
3.13. to 3.594
_
Red/brown: dodecahedral
crystal form or as spherical
masses or grains: weakly
magnetic
Detrital from all igneous and
metamorphic rocks
Hornblende
3.02 to 3.45
5 to 6
Dark green-black, good
cleavage: weak to moderately
magnetic
Detrital from many igneous
and metamorphic rocks
Ilmenite
4.70 to 4.78
5 to 6
Black: rarely with red/brown
tinge: sub-metallic lustre:
embedded masses or
irregular-hexagonal plates;
difficulty soluble in acid:
moderately magnetic: may be
distinguished from magnetite
by presence of greyish white
alteration product, Leucoxene
Detrital from many igneous
and metamorphic rocks
MINERAL
DENSITY
(S.G.)
HARDNESS
(MOH’S)
OBSERVABLE FEATURES
OCCURRENCE
Limonite
2.7 to 4.3
4 to 5.5
Yellow/brown-dark
orange/brown: earthy:
occasionally vitreous “varnish1ike”coating: slowly soluble in
hydrochloric acid: yellow
streak
1. Alteration product of ironbearing minerals
2. Biogenic deposit
Magnetite
5.2
6
Black-dark grey: opaque
brittle: fine -dull metallic lustre:
grains lacking structure:
strongly magnetic
1. Detrital from many small
igneous rocks
2. Thermally altered
sediments
Muscovite
2.77 to 2.88
2.5 to 3
Colourless-pale brown/green:
high lustre, strong cleavage:
may be difficult to distinguish
from Biotite if colour is not
discernable
1. Detrital from acid igneous
and associated metamorphic
rocks
2. Low grade phyllites and
schists
Pyrite
4.95 to 5.03
6 to 6.5
Brassy yellow: occasionally
black metallic lustre:
conchoidal-uneven fracture:
cubic or pyritohedral crystal
form
1. Hydrothermal veins
2. Detrital from metasomatic
phyllites
3. Biogenic and diagenic in
muds
Zircon
4.6 to 4.7
7.5
Red brown yellow-grey green:
tetragonal crystal form
Detrital from sodium rich
plutonic rocks. May survive
several cycles of weathering
and deposition
VERY COMMON
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MINERAL
DENSITY
(S.G.)
HARDNESS
(MOH’S)
OBSERVABLE FEATURES
OCCURRENCE
COMMON
Actinolite
3.02 to 3.44
5 to 6
Grey-bright green: opaquetranslucent: vitreous lustre:
may occur as tibrous growths
Detrital from contact and
regional metamorphic rocks
Andalusite
3.13 to 3.16
6.5 to 7.5
Pink: may be white-rose/red:
subtranslucent: brittle
splintery
Detrital from metamorphosed
argillites
Augite
2.96 to 3.52
5 to 6
Dull green-brown/black:
presence of opaque black
from weathering products will
distinguish from hornblende
1. Detrital from gabbros,
dolerites and basalts
2. Detrital from metamorphosed
Limestones
Cassiterite
3.98 to 4.02
9
Red/brown-black:
adamantine lustre: slowly
dissolved by acids
Detrital from tin-bearing acid
igneous rocks
Chromite
5.09
7.5 to 8
Red, brown, black, green:
high lustre; pithy, rarely of
megascopic size
Detrital from basaltic and
ultramafic igneous rocks
Corundum
3.98 to 4.02
9
Dark blue/grey: smoky:
adamantine-vitreous lustre:
translucent-opaque, grains or
shapeless lumps
Detrital from alkaline and silicapoor metamorphic rocks
Enstatite
3.21 to 3.96
5 to 6
Grey or green, yellow-brown:
similar to Augite but iron-poor
1. Detrital from ultra basic
igneous rocks
2. Detrital from medium grade
metamorphosed argillites
MINERAL
DENSITY
(S.G.)
HARDNESS
(MOH’S)
Epidote
3.38 to 3.49
6
Olive-yellow green; opaquetranslucent: vitreous lustre,
bundles of bladed prisms or
needles, slow reaction with
acid
Detrital from
metamorphosed basic
igneous rocks
Glaucophane
3.08 to 3.30
6
Lavender-deep blue: similar
to Hornblende: distinguished
by colour
Detrital from highly
deformed meta-sediments
e.g. greenschists, metagreywackes
Gypsum
2.30 to 2.37
2
White or colourless:
occasionally with red or blue
tinge: white precipitate with
barium chloride:
distinguished by density and
hardness
1. Dehydration of sea water
2. Groundwater alteration of
calcium carbonate
Anhydrite
2.90 to 3
3 to 3.5
Covered in separate
section
Kyanite
3.53 to 3.65
5.5 to 7
White-pure blue: vitreous or
pearly lustre: bladed crystals
or columnar masses
OBSERVABLE FEATURES
OCCURRENCE
COMMON
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Detrital from
metamorphosed
sandstones
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MINERAL
DENSITY (S.G.)
HARDNESS (MOH’S)
OBSERVABLE FEATURES
OCCURRENCE
COMMON
Monazite
5.0 to 5
35
Yellow-red/brown: spherical
masses or grains
1. Detrital from granitic rocks
2. Detrital from dolomitic marble
Rutile
4.23 to 5.5
6 to 6.5
Red/brown: may be black,
violet green: fine needle-like
crystals in shale
1. Detrital from granite pegmatites
and quartz veins
2. Detrital from metamorphosed
argillites
3. Maturation of clays and shales
Staurolite
3.74 to 3.83
7.5
Blood red-yellowish brown:
stout thick crystal: commonly
associated with garnets
Detrital from medium grade
metamorphosed argillites grits and
carbonates
Titanite
3.45 to 3.55
5
Colourless, yellow, green
brown: rhombic cross section
1. Detrital from intermediate and
acid plutonic rocks
2. Detrital from Impure calcsilicate metamorphic rocks
3. Possibly (?) digenetic in
sandstones
Topaz
3.49 to 3.57
8
Colourless, rarely yellowbrown or white: brittle with
uneven fracture
1. Detrital from acid igneous rocks
2. Detrital from metamorphosed
bauxite
Tourmaline
3.03 to 3.25
7
Black: very rarely green,
brown, red: opaque: glassy
dull lustre, long thin prisms
with curved triangular cross
section
1. Detrital from granitic rocks
2. Detrital from metasomatised
basic igneous rocks
3. Secondary mineral growth on
detrital grains in sandstones
4. Replacement in Limestones
Porosity
Ü Porosity estimation is very SUBJECTIVE. Different WSG have different ideas on what is
good and what is bad porosity.
Ü Visual porosity is a difficult, but a critically important parameter to evaluate.
Ü Generally one cannot see the pore spaces under the binocular microscope, except in
cases of high porosity - the observer must rely on other features for apparent porosity
estimations.
Ü NOTE: Porosity does not systematically vary with the size of the particles making up the
rock. Rocks with a fine grain size may be more porous than those with coarse grain size,
since porosity is defined as the percentage of pore space to the total volume of the rock.
Ü Factors such as sorting, packing/compaction, cementation and other effects determines
ultimate effective porosity.
Excellent
20% and greater
Good
15 - 20%
Fair
10 - 15%
Poor
5 - 10%
Nil (Tight)
0 - 5%
Porosity
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Ü In general, if you can see the porosity, it is very good to excellent.
Ü If you cannot see pores, there is a high percentage of matrix, the cuttings are smooth
textured and the interval drilled relatively slowly, then the rock is likely to have poor
porosity.
Ü The fair to good grades of porosity lie between these two described cases and experience
will guide the observer. A useful technique is to describe cuttings of an offset well and to
“calibrate” the descriptions of porosity with the wireline offset or RT LWD data.
Inferred Porosity
Ü Poorly cemented sandstone cuttings will often arrive in the sample tray as loose quartz
grains.
Ü The wellsite geologist needs to search for clues as to what the real ‘in-situ’ porosity is.
Ü When this is done it is usually referred to as inferred porosity.
The constraints are:
Ü ROP: The faster the ROP, the better the porosity? Hmm, not necessarily with modern
PDC bits and deviated holes.
Ü Cement: Observe for cementing minerals such as calcite and silica. Well developed
quartz overgrowths or angular ‘broken grains’ will generally indicate harder drilling and
greatly reduced porosity, while well rounded grains are generally indications of better
porosity. But not if you have a lot of…
Ü Matrix: Observe for “mushy” argillaceous material that may be associated with the sand
where argillaceous material is more likely to originate from the matrix of a sand rather than
a separate Claystone lithology.
Ü Other minerals: the cleaner the sand the less likely that growth of authigenic matrix such
as Illite will develop from the decay of unstable minerals such as feldspar and mica.
Fossil Identification in Cuttings Samples
Ü The destructive action of any drill bit will almost completely destroy the vast majority of any
fossils contained in the original rock.
Ü Therefore, most commonly known macrofossils (i.e. those that can be normally seen by
the naked eye) such as ammonites, bivalves, gastropods, echinoids, corals etc. will
become almost unrecognisable in cuttings samples.
Ü However, fragments of such fossils may be observed and, in some rare cases, extremely
small specimens may be preserved whole. In the latter case, this can apply particularly to
gastropods and bivalves (in which case they are referred to in literature as
"microgastropods" and "microbivalves").
Ü Another group of fossils that can be observed whole in cuttings samples (i.e. unaffected by
the drilling process) are microfossils, specifically foraminifera, ostracods, diatoms,
radiolaria and sponge spicules.
Ü Other familiar "microfossils" such as palynomorphs (spores, pollen and dinoflagellates)
and calcareous nannofossils are likewise preserved whole, but are much too small to be
observed even with a higher-powered geological binocular microscope.
Ü Even those microfossils mentioned are quite small with the most common sizes ranging
from 0.2mm – 0.5mm, and therefore even they may be difficult to spot using a normal
microscope.
Ü The identification of fossils or fossil fragments cuttings sample, even at a relatively nonspecific level, can often provide much useful information concerning the depositional
environment of the original sediment.
Ü Several drilling factors can affect the likelihood of observing fossils in cuttings samples.
The most important factor in this respect is bit selection.
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Ü Cuttings generated by rock bits and most PDC type bits on "traditional" or rotary-steerable
assemblies tend to yield relatively good numbers of fossils and fossil debris.
Ü PDC bits when combined with downhole (mud) motors generally yield only moderate fossil
recovery. When PDC bits are coupled with a downhole turbine, almost all fossil evidence
is destroyed by the high RPMs (and consequent thermal attrition) associated with such
assemblies.
Ü Mud type is also a factor in that oil-based-muds may also have a detrimental effect on
fossil recovery.
Ü Microgastropods, look like very small versions of their "normal" size counterparts.
However, they can also easily be confused with certain types of foraminifera (a
microfossil).
IDENTIFICATION - "Microgastropods"
Ü The simplest comparison to make for gastropods is that they look like snails or certain
types of sea shells such as whelks or periwinkles. The shell is coiled - either in a high,
cone-like appearance similar to a whelk, or in a lower, more globular fashion similar to a
periwinkle or land snail.
"Microbivalves"
Ü Microbivalves also look like very small versions of their counterparts - bivalves. As the
name suggests, these are comprised of two similar-size half-shells which lock together
along a hinge line. They are vaguely similar in appearance to a pair of castanets and tend
to be somewhat circular in outline.
Foraminifera
Ü Foraminifera are a very common component of marine sediments and therefore may be
expected to be found in most cuttings samples from marine sediments. Foraminifera
("forams") are single-celled animals and have a bewildering variety of different shapes.
They can range in size from <0.2mm up to several centimetres, although the vast majority
are between 0.2mm - 0.5mm in size.
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Three main groups of Foraminifera (Forams)
Generally
(but not
always) have
a golf-balllike
punctated/
reticulated
shell wall
PLANCTONIC - those
that live by floating in
oceanic waters and
form their shells by
secreting calcium
carbonate
CACLACEOUS
BENTHONIC - those
that live on the sea bed
and also form their
shells by secreting
calcium carbonate
AGGLUTINATED - those
that also live on the sea
bed but form their
shells by sticking
detrital grains (normally
sand or silt) onto their
naked bodies.
Generally
(but not
always) have
a smooth
and shiny, or
sometimes
smooth and
dull, surface
texture
Generally
(but not
always) have
a sugary-like
or "gritty"
surface
texture
It is sometimes possible (if conditions are good enough),
to determine which of the three groups a specimen
belongs to under the normal geological microscope
Benthonic foraminifera
A sub-group of the calcareous benthonic foraminifera known as the "porcellanous"
group because of their walls' resemblance to white porcelain are typically recorded
from shallow, warm, tropical waters (see below) The below picture shows several
types of benthonic foraminifera .
This large, brown
specimen near the
bottom right corner
is an agglutinated
foram and has a
distinctly "grainy"
surface texture. The
specimen in the top
right corner is also
agglutinated.
All the other specimens
are calcareous
benthonic forams. They
have generally smooth
glassy or opaque walls
although the ovoid
specimen near the top
middle of the picture has
longitudinal striations
on the surface.
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Planktonic Foraminifera
Ü This SEM (Scanning electron Microscope) illustration below shows several types of
planktonic foraminifera. The golf ball-like texture can be seen on most of the specimens
though spinose ones (top right) do occur.
Osctarcods
Ü Ostracods are occasionally observed in unprepared cuttings samples but, like bivalves,
are comprised of two similar-size half-shells which lock together along a hinge line.
However, in many cases the two ostracod half-shells will have become separated. Unlike
bivalves, ostracods generally tend to be more elongated in outline and have a vaguely
"potato" shaped appearance. The surface may also be variously ornamented with ribs,
reticulation and pustules.
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Diatoms
Ü Diatoms are single-celled algae with a siliceous shell and are also only rarely observed in
unprepared cuttings. However, they are often preserved as pyrite moulds which causes
them to stand out from the background rock cuttings. They are almost invariably either
disk-like, often resembling a pill-box or aspirin tablet, or flattened triangular in shape. The
photos below are somewhat atypical in that the detailed surface features shown are
almost never observed in fossil specimens .
CASE STUDY:
Diatomite.
What is it?
Russia/WSG
roll
Nanopaleontologist
Radiolaria
Ü Radiolaria are, like forams, single-celled animals, but they construct their shells using
silica (like Diatoms) rather than calcium carbonate, and also build their shells in a slightly
different way. In appearance that tend to resemble planktonic foraminifera in that they also
display a golf-ball-like surface texture.
Ü However, being siliceous rather than calcareous, they will not of course react to acid
(although replacement by calcite has been known to take place occasionally). Radiolaria
tend to be either spherical, lens-like or bell-shaped, although the spherical forms are likely
to be more commonly observed. In certain formations such as they, they are often found
as pyritised moulds.
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Sponge Spicules
Ü Sponges are metazoans (multicellular animals) which inhabit the sea floor. They are built
by many thousands of interlocking, siliceous rods called "spicules." They are very delicate
and are not commonly seen in cuttings samples. Some spicules can be subspheroid or
ovoid in shape and typical of these types is a form called "Rhaxella“ which resembles a
very well rounded, frosted quartz grain with a slight dimple on one side giving it the vague
appearance of a glassy kidney bean.
Shell Fragments (very common in samples)
Ü General shelly material is often found in cuttings samples although it can be difficult to
determine its origins. The most likely origin for most cuttings-size shelly material is
probably from bivalves although gastropod and echinoid origins cannot be ruled out
without specialist scrutiny.
Most of the ("normal size") shell
fragments in this picture are of bivalves
although a gastropod (blueish colour
near bottom right corner) can also be
seen.
It is not unusual to find horizons within
formations with abundant shell
fragments/shell debris – know as “shell
beds.”
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Inoceramus Fragments
Ü Inoceramus is a particular kind of Cretaceous bivalve (often achieving very large size of up
to 1 metre across), small plate-like fragments of which are commonly recorded from
chalks and marls. They tend to have a pale orange or brown colouration and appear
somewhat "chunky". The Inoceramus shell is composed of calcite prismatic hexagonal
rods and therefore the broken surface of an Inoceramus fragment when viewed side-on
may resemble columnar basalt in appearance if not in size.
Ü Also, Inoceramus fragments have frequently been recorded erroneously (wrongly) by
some WSG as "vein calcite".
Echinoids
Ü Echinoid (starfish, sea urchins etc.) debris can often be indistinguishable from general
shell debris without specialist knowledge.
Ü However, echinoids often possess spines and these can sometimes be identified. The
spines can range from long and thin spikes which are often fragile and completely
destroyed by drilling, to short stubby spikes which can sometimes be observed.
Ü A typical echinoid spine will often have a more bulbous knob on the end which originally
formed the point of attachment to the main body of the animal although in some species
the bulbous knob is at the distal end of the spine (see photo).
Ü The spine itself is often striated in appearance rather than being completely smooth.
Echinoid spines can be commonly found in some chalks.
These echinoid spines are from largesize specimens but those found within
cuttings samples are similar in overall
shape and appearance.
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These echinoid spines – a common feature
within chalk samples – are
characteristically bulbous at the distal end.
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Ammonites
Ü Ammonite fragments are only very rarely observed in cuttings, and unless certain
characteristic features can be observed at that scale (i.e. suture lines) the fragments may
easily be mistaken for something else.
Ü This ammonite specimen clearly shows numerous complex, florid suture lines which may
be observable in some cuttings-sized fragments.
Charaphytes
Ü Charaphytes are the remains of part of the reproductive mechanisms of a specialist group
of freshwater algae.
Ü In appearance they are of similar size to the microfossils (0.2mm – 1mm) and are
generally globular or ovoid in shape. Characteristically they have a spiral groove-like
structure covering the entire surface.
Ü However, they are only extremely rarely recorded in cuttings samples as they originate
from fresh to slightly brackish water settings – an environment which does not "preserve"
well in the sedimentary record.
Ü This illustration shows the Charaphyte plant, together with the reproductive cells which are
the only parts found as fossils.
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Significance of Fossils in Cuttings - reference
Ü With the exception of charaphytes which have a fresh water origin, the vast majority of
fossils described above are recorded from the marine realm. However, it is possible to
derive at least some palaeoenvironmental information from any data observed.
Ü Most of the groups live in habitats found at the sea floor (benthonic organisms). The
exceptions to this are the planktonic foraminifera, diatoms and radiolaria (planktonic
organisms).
Ü These planktonic organisms are, in most cases, restricted to true oceanic environments,
or to shelf seas which have good open marine connections to oceans. Planktonic forams
and radiolaria are particularly sensitive to reductions in salinity and therefore their
presence in a cuttings sample is usually a good indicator of open marine conditions with
water depths of no less than 30 metres and little or no fresh water influences.
Ü Planktonic forams and radiolaria are extremely abundant in the surface waters of the open
ocean and can form foraminiferal and radiolarian "oozes" as deep oceanic sediments –
discussed previously Diatomite/Diatomaceous Ooze.
Ü Benthonic organisms tend to be more sensitive to local environmental conditions and can
vary widely from place to place.
Ü The majority of marine benthonic organisms tend to occur on the shelf and upper parts of
the continental slope, although benthonic foraminifera (both types) can be found in very
deep waters.
Ü Agglutinated forams and radiolaria, since they have no calcium carbonate in their shell
structure, can withstand conditions (anoxic or dysaerobic), therefore they can be found
down to water depths of 6000m plus.
Ü Agglutinated forams are also often found thriving in marshy or shallow brackish water
conditions also so their presence cannot alone be relied upon for exact
palaeoenvironmental determination without specialist knowledge.
Ü The presence of types of forams known as the "porcellanous" group can be useful to
identify warm (tropical), shallow, clear water environments.
Ü They are common in many limestones. Care should be noted however, as some of the
"porcellanous" forms are also recorded from oceanic sediments beneath waters several
thousands of metres deep.
Ü Sedimentary context of the cuttings samples, will enable to geologist to differentiate
between the two environments.
Calcareous / Domomitic nature of Clastic Rocks
Ü As well as carbonates, Argillaceous rocks should be tested with HCL for calcium
carbonate and dolomite composition.
Ü Arenaceous Siliclastics should be tested to determine the cement and matrix composition.
Ü I usually place the calcareous comment as the last item on a Claystone and Siltstone
description.
Ü Arenaceous Siliclastics descriptions should denote when describing the matrix and/or
cement.
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Calcareous Rocks Classification
Rock type
% Calcareous
Material
% Clay Material
Calcilutite
(mudstone)
100 - 80
0 - 20
Argillaceous
calcilutite
80 - 60
20 - 40
Marl
60 - 40
40 - 60
Calcareous
claystone
40 - 20
60 - 80
Claystone
20 - 0
80 - 100
NOTE: carbonates will be covered in a separate module
HCL test
Rock Type
10% HCL reaction
Limestone
Violent effervescence; frothy audible reactions;
specimen bobs about and tends to float to the surface
Dolomitic Limestone
Brisk, quiet effervescence; specimen skids about the
bottom of the container, rises slightly off the bottom,
continuous flow of CO2 beads through the acid
Calcareous Dolomite
Mild emission of CO2 beads, specimen may rock up and
down, but tends to remain in one place
Dolomite
No effervescence; no immediate reaction; slow formation
of CO2 beads, reaction slowly accelerates until a thin
stream of fine beads rises to the surface – heat to
increase speed of reaction.
Rock Type
Dolomite
50% HCL reaction
Violent effervescence; frothy audible reactions.
TIPs
Ü To save time describing samples and if dolomite is suspected I tend to forget about the
10% and test directly with the 50%. This will give an immediate vigorous reaction.
Ü As I sated earlier I tend to combine the HCL test with my SILT/SAND test to save time.
Ü Some people may not agree with me BUT (like with shows) if a lithology being described
is non calcareous then state so in your description.
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Humic Coal
Ü Humic Coal: woody, plant tissue dominant (gas-prone source rock). Further divisible by
rank i.e. on the decreasing proportion of volatile constituents (primarily water) ie. peat →
lignite → sub-bituminous → bituminous → semi-bituminous → anthracitic (decreasing
water).
Ü Distinguished by appearance and texture - laminated, friable in part, jointed, fibrous, bright
‘jet’ like layers, variable lustre, hardness/brittleness.
Lignite
Bituminous coal
Anthracite
Sapropelic Coal
Ü Non-woody, comprises spores, algae and macerated plant material (oil-prone source rock).
Ü Distinguished by massive unlaminated glassy appearance, conchoidal fracture, firm rather
than hard.
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Coal
Ü Check coals for fluorescence, cut and crush cut fluorescence.
Ü Coals are clearly definable on wireline logs, particularly density-neutron. Neutron porosity
is high due to the high hydrogen content of coal.
Δt
(μsec/ft)
∅N
COAL TYPE
ρ
(g/cm3)
Lignite
0.70 - 1.50
140 - 180
>50
Sapropelic
Coal
0.90 - 1.25
Bituminous
Coal
1.24 - 1.50
110 - 140
>50
Anthracite
1.40 - 1.80
90 - 120
>50
>50
Bituminous Rocks
Ü Dark shales and carbonates may contain organic matter in the form of kerogen or bitumen.
Ü Dark, bituminous shales have a characteristic chocolate brown streak which is very
distinctive.
Ü The reverse side of a porcelain spot dish makes a handy streak plate for testing this.
Mud Additives
Ü A variety are used in drilling operations for various reasons. Reference samples should be
kept in the logging unit like the below picture.
Be aware of what is
being added to the
mud and what it
looks like in a sample
tray, these are “raw”
examples and very
often change when
added to the mud
system! Discuss.
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Calcium Carbonate AKA Baracarb
Ü Is used as a fluid loss additive when drilling through reservoirs. Very fine to medium sized
clear to translucent calcite crystals. Often mistaken for sand. Add 10% HCl to identify.
Ü If graded calcium carbonate has just been added to the mud system, and is flooding the
samples making it hard to identify the presence of sand, do the following:
Ü Take a small amount of sample and place it on a separate sample tray and apply acid to
dissolve the calcium carbonate.
Ü Whatever is left is the real formation sand minus any calcite cement of course – be aware
of that.
Common Mud Additives
Ü LCM material to control drilling fluid losses:
Ü Nut plug: Black very hard, sometimes brown, woody, doesn’t look like any formation –
easy to distinguish.
Ü Mica: LCM material. White mica is generally used, often graded into fine, medium and
coarse.
Ü Barite: orange brown material used to weight up the mud, often mistaken for silt to very
fine sand, high density. Be careful when drilling with heavy muds (high barite content).
Numerous geologist have described barite as Quartz sand!
Ü Ilmenite: Recently barite has been replaced in some counties (for environmental reasons)
for ILMENITE. This is a black powder and unlike Barite it is easily distinguished in samples.
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Oil Shows – Fluorescence & Show descriptions
Ü
Ü
Ü
Ü
All cuttings sample lithologies should be checked for oil. Not only is oil (hydrocarbons)
found in Sandstones and Limestones (50% of the world reservoirs are Limestone), but
also in tight Siltstones and Claystones too!!
Tight Siltstones and Argillaceous Siltstones with zero visible porosity can frequently have
oil shows, lignite & source rock Claystones and Carbonaceous Claystones can also be
packed with kerogens and oil.
When testing tight Sandstone, Siltstone and Claystone lithologies, the lack of
permeability in the rock means simple solvent cut test with will not give results even if the
lithology is exhibiting quite a strong direct fluorescence (DF) (occasionally rare pinpoint
diffuse CF may be seen from broken cutting surfaces).
When testing these lithologies it is CRITICAL the CRUSH cut test is performed –
discussed in later slides.
Fluorescence
Oil fluorescence is brought about by the excitation of electrons by ultraviolet light from their
ground state to a higher energy level and the subsequent return of the electrons to their
ground state accompanied by the emission of a quantum of energy perceived as colors.
Which is a fancy way of saying a photon is emitted at a different energy level.
What does the fluorescence colour tell us?
The fluorescence color observed depends on the API gravity of oils.
Dry gas
no fluorescence
Gas/condensate
white to blue-white, frequently "spotty"
35-45º API
blue-white to light yellow
25-35º API
light yellow - dark straw yellow
15-25º API
dark straw yellow - orange brown
less than 15º
orange brown - no fluorescence
Mineral Fluorescence
Mineral fluorescence is distinguished from hydrocarbon fluorescence by the lack of cut
fluorescence – in most cases. The diagnostic natural fluorescence colours are shown below:
Mineral
Colour of Fluorescence
Amber
bright yellow to white (occasional cut)
Dolomite
subtle purple-white
Calcite
variety of colours from dull yellow and dull
brown to distinctive orange
Limestone
generally little or no fluorescence
Feldspars
variable bright yellowish white to white
when partial weathering to Clay may
occasionally exhibit a slight cut caused by
the clay dissolving in the solvent.
Lignite
blue-white
Chert
dull brown/yellow
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Oil Show Description Procedure (WBM Systems)
Reagent Cut Test
Any samples exhibiting fluorescence should be treated with a solvent such as Trichloroethane
(now illegal as it is carcinogenic), or more commonly Iso-Propanol. Discuss.
The colour resulting from the addition of the solvent to a dried sample is known as the “cut”
when viewed in natural (white) light. When viewed in ultraviolet light (UV), the colour is
described as “cut fluorescence”.
It is very important that lithology and percentages are stated & if a stain, cut or ring is
invisible, say so, rather than not saying anything
Sometimes WSG’s are known just to write a show description for a specific cutting
sample depth, without reference to what lithology or giving a percentage of the
lithology that contains oil show
An example of how a correct show should be described is:
70% SANDSTONE: medium light grey to light olive grey, etc…
SHOW in SANDSTONE: 80 to 90% with etc…
Oil Shows should be described in 7 distinct stages.
1) Smell the sample
Get your nose into the sample tray and describe any hydrocarbon odour
This may range from heavy, characteristic of low gravity oil, to light and penetrating as for
condensate. Describe as weak, moderate/light, strong/heavy or no odour
2) Cuttings in white light (visible staining)
The amount by which cuttings and cores will be flushed on their way to surface is largely a
function of their permeability. In very permeable rocks the drill cuttings retain only a small
amount of oil. Often bleeding oil and gas may be observed in cores, and sometimes in drill
cuttings, from relatively tight formations.
Using the binocular microscope search the tray and described as visible, with colour and form,
or invisible.
Give percentages of the tray that contains oil staining.
Examples of this would be:
SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible
brownish black oil stain…
OR
SHOW in (70%) SANDSTONE: 20% with spotted visible black free globular oil…
OR
SHOW in (50%) SANDSTONE: 100% with even pale brown visible oil stain…
OR
SHOW in (70%) SANDSTONE: No visible oil stain…
3) Cuttings under UV light
Place the whole sample tray under the fluoroscope for examination. Describe fluorescence,
colour, intensity and form. Also, this is important, please refer to the percentage of the tray
exhibition UV shows.
Examples of this would be:
SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible
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brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct
fluorescence (DF).
OR
40% bright milky yellowish white spotted to locally patchy DF…
OR
100% Even dull orange brown DF…
OR
Trace (1-2%) pinpoint very bright straw yellow DF…
OR
No DF…
4) Solvent Cut under white light
Select some suitable cuttings where visible light oil staining is evident or UV DF. Place
aggregates in white spot tray and add drops of solvent. Describe cut as visible, with colour
and speed of cut, or no cut.
**The speed of the solvent cut coming from a cutting aggregate is
permeability of the formation**
an indication of the
Examples of this would be:
SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible
brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct
fluorescence (DF), instant dark brownish black cut/tea…
** You may have seen or heard this expression before? ‘TEA’ is used
to describe the colour of a solvent cut in white light**
OR
thick black flashing tea (cut)…
OR
slow blooming (or steaming) pale brown tea cut…
OR
very pale brown diffuse cut…
OR
NO cut/tea…
Solvent Cut under white light TOTAL colour chart
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5) Solvent Cut under UV light
Examine an aggregate in the fluoroscope for cut fluorescence, also examine an aggregate
that has been dried then crushed Reservoirs with low permeability may not show a cut
fluorescence but will show a crush cut fluorescence. Describe fluorescence, intensity and
speed of cut fluorescence/crush cut fluorescence or say no CF.
Examples of this would be:
SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible
brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct
fluorescence (DF), instant dark brownish black tea cut, instant flashing bright yellowish white
cut fluorescence (CF)…
OR
moderately bright slow blooming (or pinpoint steaming) yellowish green CF…
OR
very slow pale diffuse milky bluish white CF…
OR
trace diffuse moderately bright milky white CF, instant flashing moderate milky white crush cut
fluorescence (CCF)…OR
no CF/CCF
Blooming vs. Streaming
Note: keep a reference
sample of the solvent –
some exhibit slight direct
fluorescence
6) Ring under UV light
Allow the solvent to evaporate and describe any residual ring fluorescence. Describe intensity,
thickness of the residual ring and colour.
Examples of this would be:
SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible
brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct
fluorescence (DF), instant dark brownish black tea, instant flashing bright yellowish white
solvent cut (SC), moderately bright thick solid yellowish gold residual UV ring…
OR
moderately bright thin veneer to locally spotted residual UV ring…
OR
pale fine spotted milky white to yellowish white pinpoint residue…
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OR
no residual UV ring
7) Residue under white light
Allow the solvent to evaporate and describe residue. Describe as visible with colour or
invisible.
Examples of this would be:
SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible
brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct
fluorescence (DF), instant dark brownish black tea, instant flashing yellowish white solvent cut
(SC), moderately bright thick solid yellowish gold residual UV ring, thick even brownish black
residue…
OR
thin moderate brown veneer residue…
OR
trace pale brown to light tan ring residue…
OR
no residue…
Dead Oil
Ü
Ü
Ü
Ü
There has been much confusion, inconsistency and misunderstanding concerning the
usage of this term.
It has been used to describe oils that are either very waxy and solid, non-producible or
immobile. All of those definitions are misleading and deceptive.
In addition, it has never been clear whether or not so-called “dead oils” exhibit
fluorescence and cut fluorescence.
In view of the above the term “dead oil” should only be used to describe thermally dead,
solid hydrocarbons that DO NOT fluoresce. Whenever the term is used, qualifying data
should be given.
Oil Show Description Flowchart
Sm e ll t he sa m ple
t ra y - not e t he odour
N ot e Pe rc e nt a ge of
lit hology fluore sc ing
Cut
N ot e c olour, form
a nd int e nsit y of
fluore sc e nc e
N ot e c olour a nd
spe e d in of c ut in
visible light
N o Cut
Crush som e dry
sa m ple - spot t ra y
or m ort a r a nd pe st le
ADD SOLV EN T
N ot e c olour a nd
spe e d in of c ut in
U V light
Add solve nt N B: c rush c ut ring F should
be se e n on blot t ing pa pe r but
for quic k look int e rpre t a t ion c rushe d DRY sa m ple in spot
t ra y w ill suffic e
N ot e c olour of c ut
fluore sc e nc e a nd
ring fluore sc e nc e
N ot e c olour of
re sidue in w hit e
light
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re pe a t t he proc e ss
for sa m ple s
e x hibit ing c ut
Re c ord a ny c ut a s
‘c rush c ut ’ in
de sc ript ion
N ot e c olour of c rush
c ut fluore sc e nc e
a nd ring
fluore sc e nc e … e t c
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Give reference to Oil distribution inside the rock
Ü
In your description it is very useful to comment on where the oil/visible light
staining/fluorescence is distributed within the lithology…i.e. is the free
oil/staining/fluorescence:
1. Coating on grains.
2. Free globular in rock matrix (intergranular)
3. Intercrystalline, vuggy (carbonates).
4. In fractures (very important state the depth of oil invasion within the fractures).
5. OR evenly (uniformly) dispersed (source rock Claystones).
Loss of volatiles
Ü
Ü
Ü
For best results and consistency it is best to test the samples for shows as soon as
they are collected.
The reason behind this is that some light grade oils and condensates will be lost over
time by evaporation.
This shouldn’t be and issue for the WSG as it is advanced prior to entering or during
drilling of a target reservoir that they spend the majority of your time in the
mudlogging unit.
Oil Show Description (SOBM Systems)
Ü For obvious reasons, it is very difficult to ascertain any Oil Shows in cuttings drilled with
OBM.
Ü When you look at a sample tray of cuttings drilled with OBM under the fluoroscope the
whole sample tray will fluoresce.
Ü Great care must be taken reporting ANY shows to your SOG (town) and on your
Complog/Litholog.
Ü The ability to see ‘REAL’ shows will largely depend on the nature (API gravity) of the real
oil.
Ü Masking is the term we use to describe what the OBM does to the real oil shows – it
MASKS them!
Ü The OBM (even after washing with detergent) will tend to coat the cuttings with a film of oil.
Ü As stated previously if there is ANY porosity or permeability in the cuttings (e.g. drilled
Sandstones and Silstones), then during the drilling process and the cuttings transit from
TD to surface in the annulus, permeable/porous cuttings with be FLUSHED to some
extent by the hydrostatic pressure and flow of the mud etc.
Ü This process can TOTALLY MASK the real oil shows in the cuttings.
Ü In general, you will only be able to distinguish real oil shows if the fluorescence/visible light
oil staining is significantly different from the OBM.
Ü The rule being that identification of real oil shows vs. OBM is easier when the real oil is a
lower API gravity that the base oil in the OBM. i.e heavier lower API grade, darker (API
gravity of 15-25° API).
Ü Most modern OBM give quite a distinct moderately bright yellowish green fluorescence.
TEPI OBM seems to give a dull orange DF.
Dry gas
no fluorescence
Gas/condensate white to blue-white, frequently
"spotty"
35-45º API
blue-white to light yellow
25-35º API
light yellow - dark straw yellow
15-25º API
dark straw yellow - orange brown
less than 15º
orange brown - no fluorescence
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Examination Process & Tips
Ü
Ü
Ü
Ü
Ü
Ü
Ü
For WBM cuttings follow EXACTLY the same procedure and description technique as
you would do with cuttings drilled with water based mud (WBM).
BUT… Keep samples of BASE OIL and MUD in the fluoroscope for comparison.
Change the reference mud sample at each shift change – there may have been
additions/changes to the mud since your last sample examination.
Perform various cut fluorescence tests of the sample of mud and base oil – keep these
as reference, note visible light colour, UV fluorescence, colour of cut and residue under
UV and visible light.
It can be useful to report these show for comparison on your DGR.
In OBM, quite often the visible light oil stain is easier to see than the UV fluorescence.
Thoroughly check samples for visible free oil in pore spaces.
If running LWD logs use them as reference and pay particular close attention to samples
that LWD resistivity is high. Sandstones AND Claystones. Increasing resistivity (over the
normal compaction trend) can be an indication of entering a source rock formation.
Examining Core Chips/SWC (OBM)
Ü
Ü
Ü
Real oil shows can easily be determined from core samples and SWC.
There will be a ‘flush zone’ on the outer surface of the cores which will be a function of
the formations porosity and permeability. It will be clearly visible under the fluoroscope.
When examining chips or whole SWC try to liberate some of the fresh formation for
examination – be careful to avoid any contamination as this will affect the overall cut.
WBM contaminants
Ü
Ü
Ü
Ü
Be aware that in some modern WBM systems it is quite common for synthetic oil
products to be added. These generally act as drilling torque reducers/lubricators.
GlydrillTM being one of these products and is run at 5% in the mud system.
This can give a significant contamination mud therefore to the drilled cuttings as well.
When drilling with these products, keep reference samples of the Glydrill and mud as if
you were drilling in a OBM regime.
Other Contaminants – PIPE DOPE
Ü
Ü
What’s that?
Pipe dope (essentially thick grease) is applied to the drill pipe during connections,
occasionally the dope may end up in the sample. It will generally give a golden brown dull
fluorescence and will occur as greasy blobs.
Recording Oil Shows on the Litholog
Ü
Ü
As well as a description of the Oil Show being included after the lithology description it is
also required to annotate the litholog/mudlog with black bars over the intervals exhibiting
shows.
These are normally on a grading scale of 1-3 or 1-6 depending on the oil company.
Discuss.
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Other tests for Oil Shows – Emulsion ‘Pop’ Test WBM
Ü
Ü
Ü
Ü
Ü
You can also see shows in the mud system, commonly smell hydrocarbons in the
shaker, flowline area and even see clear hydrocarbons floating on top of the mud
(pits, flowline).
Samples of fresh mud from the flowline can be collected and poured into a tray,
inspect the mud samples under the fluoroscope for shows and on some occasions oil
may be seen ‘popping’ at the surface of the mud.
Then add some water to the mud which lowers the viscosity of the mud and
separates the mud from the oil. By this method small samples of oil can be skimmed
of the top of the mud.
Finally the mixture can be placed in a bottle and shaken. The results should be
monitor and the results described. Light grade oils are liable to evaporate so the
sample should be closely monitored.
You can repeat the process with bulk wet cuttings.
Emulsion ‘Pop’ Test – Classification table
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Example #1
Ü
SHOWS in 100% SST: no noticeable odour on core, 30% light creamy yellowish
staining, 80% even moderately bright yellowish white direct fluorescence, dull very
slow diffuse milky white solvent cut from aggregate, instant diffuse white crush cut,
very pale yellow tea, moderate spotted milky white residual ring in UV light, nil to rare
trace pale brown residue.
Example #2
Ü
SHOWS in 100% medium SST: strong petroliferous odour, 70% spotted dark black
visible oil staining and local greasy coating on grains, free solidified waxy oil seen in
pore spaces, 100% even very dull direct fluorescence, dull very slow diffuse yellow
solvent cut, dark brown black tea, thick moderately bright yellow residual ring in UV
light becoming dull brown black with time, thick brown black oil residue.
Example #3
Ü SHOWS in 100% SST: moderate petroliferous odour, 70% light creamy yellow to
locally dark patchy brown visible staining, 90% moderately bright to bright milky
yellow direct fluorescence, instant flashing milky white solvent cut from loose grains
and aggregate, very pale yellow tea, moderate spotted milky white residual ring in UV
light, nil to very pale yellow residual veneer.
Example #4
Ü
SHOWS in 100% Fine grained SST: no noticeable odour(looks wet/waterized?) no
visible staining, possible trace spotted traces of black bituminous flecks? 80% dull
with trace <10% pinpoint spotted bright white direct fluorescence, moderate slow
diffuse milky white solvent cut, no tea, faint weakly spotted milky white residue in UV
light, nil in visible light, possibly some mineral Feldspar fluorescence?
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