What is Mudlogging?

BASIC MUDLOGGING
Formation Evaluation Procedures
In the early days of drilling, when cable tools were the only means to make hole, the
notion of sampling well bailings to find out what the bit was penetrating seemed
unimportant, and even a waste of time. Drilling proceeded until oil and gas flowed,
and it continued as soon as production dropped off. If a well was going to produce, it
would produce - sampling well bailings would never change that.
Today, it is still true that formation evaluation cannot change a well's production
potential, but completion techniques suggested by formation data can. Formation
evaluation is done routinely. Methods for evaluation of subsurface formations now
include the sophisticated use of seismic surveys, records from nearby wells, the
driller's log, mud logs, core samples, and a multitude of wireline well logs.
Mud logging is a useful evaluation technique that has developed since the advent of
rotary drilling in the 1920s. Since mud circulates constantly during drilling, mud
logging can provide information on a continuous formation sample.
A mud logger checks mud for oil and gas and collects bit cuttings for analysis. Bit
cutting analysis is very useful, since it can tell much about rock types and formation
characteristics, which must be known for mapping formation beds. Information
gathered by mud logging is recorded on a mud log.
Basic mud logging involves:
· Depth and ROP determination
· Lag time determination
· Cuttings sampling and lithological description
· Gas sampling and analysis
ILO Mud Logging Unit
The basic ILO mud logging unit is xx x xx x xx and weighs x tons. It contains the
following equipment:
· 2 THD
· 2 Chromatographs
· 1 Integrator
· CO2 Detector
· H2S Detector
· 4 computers
· 2 continuous recording charts
· 1 Laser jet
· 1 Epson printer to plot out logs
· 1 Panasonic dot matrix
· 2 Microscopes
· 1 Microgas blender
· Pit Sensors
· Rig floor sensors
· Depth sensors
· Mud weight sensors
· Mud Temperature sensors
· Mud resistivity sensors
· Mud press, laboratory glassware and chemicals to allow chemical tests on
cuttings and mud
· Equipment for catching samples
On offshore floating rigs additional equipment is installed:
· Rig-motion compensated depth system
The unit could be equipped handle coring jobs, shale density, shale factor, mineral
staining and other specialized tests.
The ILO mud logging unit should be able to provide:
· Personnel trained in the interpretation of all derived data, capable of providing
interpretation and observations to the wellsite geologist and to the company man.
· Monitor drilling parameters
· Monitor mud properties and the pit volumes
· Monitor gas and its components using FID THA, Chromatograph-Integrator, CO2
and H2S.
· Data processing, storage, display and transmission to off wellsite locations
· Print out, store and display mud logs, pressure logs and other logs needed at the
wellsite.
· Do pore pressure monitoring and estimation
· Record on charts drilling parameters
· Perform specialized tests and analyses
· Handle core samples
Depth and ROP Determination
Lag Time Determination
To the mud logger, the most important drill string information involves the numbers,
lengths, ODs and IDs of its different components. Before the bit is run back in the hole,
the mud logger has to get a list of the BHA along with the bit information from the rig
floor.
The BHA figures are used, in conjunction with mud pump output, to calculate the
capacity of the drill string and the annulus. The time necessary
Xxxxxx
Gas Sampling and Determination
Introduction
Equipment / Hardware
The Gas Trap
The Vacuum and Pneumatic System
Total Hydrocarbon Detector
Chromatograph
HP Integrator
CO2 Detector
H2S Detector
Blender Gas Analysis
Mud Still
Gas Show Determination and Analysis
Sources of Gas
Effects of Drilling on Gas Shows
Recording Gas Information
Gas Data Analysis and Presentation
Gas detection along with cuttings sampling is the heart of basic mud logging.
Trip and Connection Gases
Samples and Sample Preparation
Introduction
Cuttings are an ideal source of samples to establish lithological information to fill in
boundaries and characteristics defined by more precise but less informative
measurements.
The recovery of cuttings from the hole is a more complicated situation than gas
recovery. With good mud conditions gas peaks can correspond to breaks in ROP and
can confirm a bed or lithological boundary. With drill cuttings, bed boundary definition
is not a clear-cut matter. You can not determine formation boundaries with any degree
of precision by using cuttings.
Its density, viscosity and velocity govern the carrying capacity of the mud. The particle
load will not always be constant and uniform. In long annular sections, we can expect
some sorting of cuttings by mass and possibly by shape. Because of this, some of the
denser or larger cuttings will arrive at the surface later. Tabular-shaped cuttings like
shale will present more resistance to settling than spherical cuttings like sand grains.
There are three flow regimes:
· Plug flow
· Laminar flow
· Turbulent flow
Plug flow is a rare condition. It is usually seen in slow moving, extremely viscous fluids.
Laminar flow is the ideal flow condition for drilling fluids. It results in minimal pressure
losses and borehole erosion. Turbulent flow is a less ideal condition because of
increased pressure losses.
For cutting recovery, the significance of the flow regime is in the velocity distribution
across the path of flow. In laminar and turbulent flows, the maximum velocity is
attained at the center of the annulus. The solid particles will tend to slip across the
annulus from higher to lower velocity levels in the flow, inducing a further degree of
vertical mixing of cuttings.
Drill string rotation enhances the randomness of the mixing by sweeping cuttings back
towards the high velocity region.
The lag time determined by the use of carbide represents the minimum time
necessary for cuttings to reach the surface travelling in the high velocity region of
either the laminar or the turbulent flow regime.
On arrival at the surface drill cuttings will be physically and chemically weathered,
flushed of fluids and mixed with material drilled earlier or debris from the borehole
walls.
Introduction
Despite the presence and use of sophisticated remote and downhole measuring
devices, rock samples derived from drilling and coring remain the most important
source of data to the geologist. It is important to understand the means by which the
samples are taken and the environment through which they are transported back to
the surface.
During and after drilling several types of rock samples may be taken from the wellbore:
· Drill cuttings
· Core chips
· Sidewall core samples
· Unusual samples, such as samples taken from the bit, junk basket and stabilizers
In addition to rock samples, fluid samples can also be obtained. These include:
· Drilling fluid samples
· Formation fluid samples
Collection and Preparation
Introduction
The client selects what type of samples and at what interval these samples will be
collected at the start of the well. A common sample interval is 10 feet or 10 meters.
Ideally, a sample interval should be selected so those samples will be caught at 15-
minute intervals. If the client demands a shorter time interval then the sample quality
declines and other mud logging tasks are neglected. Generally, the sample intervals
are shortened as the hole is deepened and the ROP is slower.
The principal location for cuttings sampling is the shale shaker. A sample catching
board must be located below the edge of the double deck screens. When the sample
is thought to be up the sample catcher or mud logger must go to the shale shaker and
collect the composite sample from the sample catching board. The sample catching
board is then cleaned of any excess sample so that fresh sample could be allowed to
accumulate.
If air drilling is used to drill the well, samples are collected from the blooie line.
Periodically the valve in the bypass line of the blooie line is opened and the
accumulated samples can be retrieved.
The desander, desilter and the mud centrifuge are employed to remove the finest
material. These equipment, esp. the desander, should be sampled on a regular basis,
time permitting, to establish the quantity and the appearance of sand and fine solids
commonly contaminating the mud system.
There are generally three types of samples collected at the wellsite:
· Large unwashed samples bagged into cloth bags
· Sieved, rinsed and dried samples bagged into brown Kraft paper clasp envelopes
· Sieved, rinsed samples for lithological and hydrocarbon evaluation
· Paleo samples bagged in cloth bags
· Unwashed and preserved with bactericide geochemical samples bagged into tins
· Mud samples in tins
· Metal shavings bagged in sample cloth bags (if required)
· Wellbore fluids in tins or appropriate container
The materials used in gathering a sample are:
· coarse seive (No. 8)
· fine sieve (No. 80 or No. 120)
· sample trowel or spade
· Rubber gloves (esp. when oil-based mud is in use or when handling metal
shavings)
· bucket, if the quantity of samples to be collected is large
The materials used in bagging and storing the samples are:
· tin can (for geochemical and mud samples)
· bactericide (for geochemical samples)
· cloth bags for unwashed samples (usually Hubco 5” x 7”)
· brown Kraft paper clasp envelopes for sieved, rinsed and dried samples
Figure 2 Hubco cloth bags and brown Kraft paper clasp sample envelopes.
Preparing to Get Samples
Before collecting samples, the cloth sample bags, brown Kraft clasp envelopes and
tins should be properly marked. These should be labeled with the following information:
· Oil company name
· Well name and number
· Well locat ion (optional)
· Sample depth interval or Sample depth (for mud samples; not needed for metal
shaving samples)
· Date and Time of collection (for mud, wellbore fluid, metal shaving samples)
· Set number
Make up some white cardboard boxes. This is where the brown Kraft clasp envelopes
will be stored. After sometime, properly marked boxes for storing the unwashed
samples need to be assembled. These boxes should also be properly marked with the
following information:
· Oil company name
· Well name and number
· Well location (optional)
· Sample depth interval (From x to y). x represents the start of the interval of the first
sample in the box. It is also the bottom interval of the last sample in the previous
box of the same set number. y represents the bottom interval of the last sample in
the box.
· Set number
· Box number (Box x of y) where x represents the present cumulative number of the
box and y is the total number of boxes. The number for y will be written when the
sample boxes are sent to town during casing periods or at the end of the well.
Large Unwashed Samples
This is the first sample that is collected and bagged at every sample interval. To collect
and bag an unwashed sample:
1. When the sample is up as indicated by the DLS computer or by the worksheet
calculations, pick up the coarse sieve (No. 8), a sample trowel or spade and the
required number of properly marked Hubco cloth sample bags.
2. At the shale shakers, collect the sample from the accumulated pile on the sample
board. Take the sample from different places in the pile to get a representative
sample of the interval. Put the samples into Hubco cloth bags and onto the coarse
sieve.
3. Clean the sample board to allow fresh samples to accumulate.
4. Hang the closed cloth bags to dry. It is preferable that the site where the bags
would be hanged is sheltered.
5. When the samples have been sufficiently dried, place them into a cardboard box.
6. When the box is full, write down the interval of the samples in the box. You can
also write down the number of sacks in the box.
Sieved, Rinsed and Dried Samples
To collect and bag the sieved, rinsed and dried samples:
1. At the sink, place the coarse sieve with the unwashed sample on top of the fine
sieve (No. 80 or No. 120).
2. Rinse the sample. Take note of the percentage of cavings (coarse cuttings ) left on
the coarse sieve. Inform the pressure engineer if there is an increase in the
percentage of cavings. This is useful information for pore pressure estimation.
When drilling using water-based muds through very soft clays, care must be taken
to wash away as little of the clay as possible.
If oil-based mud is being used the samples have to rinsed first in a detergent
solution to remove the mud.
3. Remove the No. 8 sieve, slightly rinse the accumulated sample in the fine sieve of
excess mud. Rinsing with a detergent might be needed to remove excess mud if
an oil-based system was used.
4. Use a metal tray to scoop the accumulated sample. Fill the tray with sample. You
may need to use more than one metal tray if the sampling program requires more
than one set of sieved, rinsed and dried samples. Leave some sample for
lithological and hydrocarbon analyses.
5. Place a sample tag on the tray. Sample tags are printed using Excel. Once the
sample tags are printed out, the sheet is cut into vertical strips. These strips are
stuck to the wall near the sink.
6. Store the sample tray in the oven.
7. After sometime, when the sample is dried use a pair of tongs or alligator pliers to
get the sample trays from the oven. Do not handle the trays with your bare hands,
as they are hot.
8. Place the tray into the plastic mud cup or similar container. Scoop out the samples
into cup.
9. Scoop out the samples and place them into the properly marked brown Kraft
clasp envelopes.
10. Place the envelopes into the corresponding white cardboard boxes.
Samples for Lithological and Hydrocarbon Evaluation
To collect samples for lithological and hydrocarbon evaluation:
1. After the samples in the fine sieve have been slightly rinsed (see Step 3 of the
above section), use a metal tray to collect a sample. Rinse it again slightly and
shake the tray to get an even distribution of the sample. The sample must be one
layer thick. Thick-layered samples more often than naught give erroneous cuttings
percentages. The sand grains tend to settle at the bottom, xxxxxx.
2. Lean the tray against the wall or splashboard of the sink to let the excess water
drain off.
3. Place a tag on the sample tray.
4. Observe and describe the sample under the microscope and the UV box. In
intervals where very soft and soluble clays have been drilled you can include into
your claystone percentage an estimate of the amount of clay that you think has
been washed away.
Occasionally the client would want the samples temporarily stored in plastic sample
trays or a cutting lithology log would be required. A cut ting lithology log xxxxxx.
Paleo Samples
Geochemical Samples
Mud Samples
Metal Shaving Samples
In some jobs, you will be required to collect metal shavings from ditch magnets located
in either the possum belly of the shale shakers or the flowline. Ditch magnets usually
have a rope tied around it so that it can be pulled out and cleaned. Metal shavings are
collected at regular time intervals, say once or twice a day. To collect metal shavings:
1. Pull out the ditch magnets.
2. Use a non magnetic scraper (wood or plastic) to initially scrape the majority of the
metal shavings from the magnet/s. Scrape the magnets clean by using one hand
with extra protective equipment, rubber gloves and safety glasses. This should be
done slowly so as not to cause hand injuries. Place the metal shavings in a bucket.
3. In the unit, place Sieve no. 8 over Sieve no. 80. Put the sample into the coarse
sieve.
4. Wash the wash the samples carefully so as not to cause cuts. All formation
cuttings and mud are to be removed.
5. Collect all the metal shavings using the metal sample trays.
6. Dry the metal shavings in the oven.
7. When they are dried, weigh the metal shavings using the electronic balance.
8. You may be required to store the samples in cloth bags. If so the bags must be
labeled correctly (s ee Preparing to Get Samples section).
9. Report the weight to the company man.
Examination and Interpretation of Cuttings
Materials Needed for Examining Cuttings
The materials needed to examine a sample are:
· Binocular microscope
· Sample probes and tweezers
· Sample trays and dishes
· UV box
· Acids and solvents
· Water
· Size and color comparators
Binocular microscope lenses should be kept clean to reduce eyestrain. Lenses should
be cleaned with either lens paper, facial tissue or very soft cloth.
Several types of trays and dishes may be used for examining the sample under the
microscope. ILO units are provided with metal sample trays and trays subdivided into
five compartments.
Methods of Logging
There are two general methods of sample description and logging, the interpretive
system and the percentage system.
The interpretive log is preferable but its accuracy depends in some measure on the
quality of the samples, and the geologist's familiarity with the local stratigraphic section.
Obvious caved material is to be disregarded, and only the lithology believed to be
representative of the drilled section is logged. If several different rock types are
present in the sample, all assumed to be derived from the drilled interval, they are
logged as discrete beds, interbeds, intercalations, lenses, or nodules, rather than as
percentages. The interpretation in this case is based on the geologist's knowledge of
the section. On interpretive logs, lithologic contacts are drawn sharply, and the entire
width of the log column is filled with appropriate symbols. One hazard in this form of
logging is that of overlooking unexpected repetitions of lithologic types.
Experience and good training are essential for making a good interpretive log.
Generally the person examining the samples is best qualified to recognize lithologic
and formational contacts. Although formation contacts should be picked on the basis
of sample evidence rather than on mechanical logs, the latter, as well as drilling time
logs, can be useful in defining boundaries of specific lithologic units, and zones of
porosity.
In percentage logging, the geologist, after eliminating the obvious foreign matter and
unquestionable caved material, plots each rock type with a width of symbols
proportional to its percentage in the sample. This system of logging may be used to
advantage in areas where:
1. the details of the stratigraphy are unknown
2. samples are of very poor quality
3. no mechanical logs are available
4. the sampled interval is very large compared to the thickness of the rock units
5. the stratigraphic sequence is interrupted by structural complications
6. the person studying the samples is inexperienced or is not a professional
geologist
The principal disadvantages of this system are that lithologic breaks do not show up
sharply on the log, and the intricate logging of each rock percentage often gives a
confused and meaningless picture of the rock present.
A modification of interpretive logging that has been employed is a combination
interpretive/percentage log.
Problems In Interpreting Drill Cuttings
Contamination
There are many potential sources of contamination to consider when undertaking
estimates of lithology percentages or determining hydrocarbon shows. These are:
· Cavings
· Recycled cuttings
· Mud chemicals
· Cement
· Metal
· Evaporite sections
· Drilling mud
· Oil contamination
· Miscellaneous contaminants
Cavings
Cavings is often material identical to what has already been seen from much higher in
the hole. This spalling of previously penetrated rocks into the ascending mud stream is
particularly pronounced after trips of the drill stem for bit changes, drill stem tests,
coring operations or other rig activities. Most samples will contain caved material. Soft
or thinly bedded brittle shales and bentonites cave readily and may be found in
samples representing depths hundreds of feet below the normal stratigraphic position
of those rocks.
Caved fragments tend to be larger than fragments of rock from the bottom, and they
are typically rounded by abrasion. It may also sometimes consist of splintery rock
fragments. They may be concavo-convex or blocky to subblocky in shape. If cavings
occur in appreciable quantities, it may suggest a serious underbalance regime.
Recycled Cuttings
Recycled cuttings are cuttings that are not removed from the drilling fluid at the shale
shakers, desanders and desilters. They are recycled through the system. They may
be recognizable as small, abraded, rounded rock fragments.
Mud Chemicals
Some mud chemicals may be confused with rock types or components of rocks.
Some of these are:
· Lignosulphonate
· Bentonite gel
· Barite
· Mica flakes and other lost circulation material
Lost Circulation Material
A large variety of substances may be introduced into the hole to combat lost
circulation difficulties. These include such obviously foreign materials as feathers,
leather, burlap sacking, or cotton seed hulls, as well as cellophane (which might be
mistaken for selenite or muscovite), perlite, coarse mica flakes and calcium carbonate.
Mica flakes and calcium carbonate might be erroneously interpreted as formation
cuttings.
Most of these extraneous materials will float to the top of the sample tray when it is
immersed in water, and so can be separated and discarded .at once. Others may
need more careful observation. Generally, the sudden appearance of a flood of freshlooking
material, which occupies the greater part of a sample, is enough to put the
mud logger on his guard. As a check, he can collect samples of lost circulation
material from the mud engineer.
NOTE: LCM material like mica flakes, nut kernels, fibers must be removed from the sample by
panning.
Other Mud Chemicals
Lignosulphonates resemble lignites. Bentonite gel may be erroneously identified as
montmorillonite clay in a poorly mixed system. Barite might be mistaken for sand
grains.
Cement
Cement contamination is usually encountered when drilling after casing has been set
or while sidetracking the well. Cement fragments in cuttings are easily mistaken for
sandy, silty or chalky carbonate, very fine argillaceous sandstone or sandy siltstone.
However, most cements are of an unusual texture or color, frequently have a glazed
surface, tend to turn yellow or brown when immersed in dilute HCI, and are usually full
of fine black specks. The latter are sometimes magnetic, in which case the fragments
of cement can be removed from the cuttings with the aid oil a small magnet. If the
identification of cement is questionable, the well record should be examined to
determine where casing was set or cement poured.
Using phenolpthalein solution can readily identify cement. Cement samples will turn
red when they react with phenolpthalein solution.
Metal
Metal is occasionally found in samples and frequently comes from the wear and tear
of the inside walls of the casing by the drill string. The use of rubber drill pipe
protectors usually minimizes this wear and tear. If an appreciable amount of metal
shavings is observed in the sample, the company man must be informed.
Pipe scale may also contaminate the samples. They are usually rusty and rarely
present a logging problem.
Evaporite Sections
Evaporite sections drilled using water-based muds presents a slight problem. Salts
dissolve and there is no lithological indication of their presence in lagged samples.
However, they can still be recognized by:
· Evaporites usually drill at very consistent rates regardless of depth or lithological
variations.
· Gas values through evaporite sections are nil or very low.
· There will be either no returns or very poor returns at the shakers.
· Anhydrite, limestones and dolomites are frequently found in association with other
evaporite deposits.
· The chloride content of the mud will increase significantly.
Drilling Mud
In examining unwashed or poorly washed cuttings, it is often important to be able to
recognize the drilling muds that are used. An inexperienced mud logger may confuse
drilling mud with soft clay, bentonite, or sometimes gypsum or a carbonate.
Though washing of the unwashed sample in water will generally remove most mud
contamination. If necessary, lithic fragments can be broken open to see if the interior
(fresh) differs from the surface (coated).
Oil-base and oil-emulsion muds coat the cuttings with oil. Care must be taken to
distinguish such occurrences from formation oil. They are generally recognized
because they coat all cuttings regardless of lithology, rather than being confined to one
rock type. Such contamination can sometimes be removed by washing the samples
with a detergent or with dilute HCl (see section on sample handling when oil based
mud is used).
Lignosulfonate muds may present problems in samples used in palynological studies.
Oil Contamination, Pipe Dope etc.
Foreign substances, such as pipe dope, grease, etc., from the rig operations
sometimes enter the mud stream. Oil may be used to free stuck drill pipe. When
foreign oil contamination is suspected, cuttings should be broken and their fresh
surface examined. Naturally-occurring oil will tend to stain the chips throughout;
contamination will remain on or near the surface of the chip.
Miscellaneous Contaminants
Other lithic materials which may be present in cutting samples and obscure their real
nature, or might be logged as being in place, include rock fragments used as
aggregate in casing shoes.
Miscellaneous Interpretation Problems
Rock Dust
If samples are not washed sufficiently, a fine dust composed of powdered rock or dried
drilling mud may cover the cutting chips with a tightly adhering coat. In such cases,
care should be taken that a fresh surface of the rock or cutting is described. Wetting
the samples will tend to remove this coating, but if the chips are saturated with oil, the
powder may still adhere to the surface even after immersion in water, unless a wetting
agent or ordinary household detergent is used. These comments are particularly
applicable to limestone and dolomite where the powdered rock film tends to be in the
form of crystals which may mask the true texture of the rock. In this case, the best
procedure is to break a few chips and obtain fresh surfaces for description.
Powdering
Powdering is the pulverization of the cuttings by regrinding (failure of the mud to
remove cuttings from the bit), or by crushing between the drill pipe and the wall of the
borehole. It can result in the disappearance of cuttings from some intervals and the
erroneous logging of chalky limestone where none exists.
Fusing
Shales drilled by a diamond bit may be burned and fused, resulting in the formation of
dark gray or black hard fragments that resemble igneous rock.
Air-Gas Drilling Samples
Cuttings from wells drilled with air or gas instead of mud are usually made up of small
chips and powder, which makes sample examination difficult. Often a simple
screening of the cuttings to eliminate the powder will facilitate the examination. When
the cuttings are entirely of powder, little can be done beyond describing basic rock
types and colors. When the cuttings are carbonates, the basic rock type will be
difficult to determine because dolomite powder effervesces as readily as limestone
powder.
Where well indurated shale sections are air drilled, the samples can be cleaned
conveniently by washing them with care on an 80 mesh screen. This cleaning
procedure should be required, where feasible, as the dust coating on particles will
mask the true color, texture and even the basic lithology of the drilled section. When
"mist" drilling is done, particles can become plastered with fine mud that is removable
only by a washing process; simple screening does not suffice.
Sample Lag Correction Error
Lag time is the time required for cuttings to travel from the bottom of the hole to the
place at which they are collected. If new hole is drilled during this time interval, the
depth assigned to the samples will be greater than the depth from which the cuttings
originated.
Despite the many methods available for determination of lag time and for the correct
labeling of depths on the sample bags, the person catching the samples sometimes
does the sample collection incorrectly, or not at all. Subsequent sample studies are
thus affected by significant discrepancies between the indicated sample depth and
true sample depth. As a result of these discrepancies:
1. lithologies are plotted at incorrect depths
2. interpolation of true ded.,hs becomes time consuming and requires unnecessary
log manipulation
3. uncertainties as to the character of the formation penetrated may be introduced
If erroneous lag correction is suspected or known, the mud logger examining the
samples should try to plot the lithologic information at their true depth. If the
discrepancy from the true sample depth is not determinable, or is questionable, the
samples must be plotted as labeled, with an appropriate note in the remarks column.
Lag correction is best controlled at the well site.
Spread
Spread is the separation of large from small cuttings by relative slippage (also called
differential settling) in the mud stream. The fine cuttings may overtake the coarse
chips during their journey up the borehole. This results in the wrong sequence of rock
types or very mixed samples.
"Boiler-housing" or "Dog-housing" of Samples
Unfortunately, because of inclement weather, lack of interest or supervision,
breakdowns, or fast drilling, the sample catcher will occasionally sack up a number of
samples only once during his tour. However, he then labels the samples as if they
were properly caught at specific intervals. This collection procedure is known as
"boiler-housing" or "dog-housing". Any mud logger can readily see the errors inherent
in this practice.
Sample Examination and Description
Once the percentages of the various constituents of the sample have been estimated,
a sample description should be made on the work sheet.
Order Of Written Description
Descriptions are written using abbreviations. The AAPG standard set of abbreviations
are normally used, but, some clients would require that their own set of abbreviations
be used. Descriptions are written following a standard order. A standardized order of
describing cuttings has the following advantages:
· Reduces the chance of not recording all important properties
· Increases the uniformity of description among geologists and mudloggers
· Saves time in obtaining specific information from descriptions
The following order is used:
1. Rock type – either underlined or capitalized and followed by classification
2. Color
3. Texture – including cuttings shape (argillaceous and calcareous lithologies), grain
size, roundness, sorting, hardness or induration
4. Cement and/or matrix materials
5. Fossils and accessories
6. Porosity and oil shows
The sample description for carbonate rocks is a bit varied depending on what type of
nomenclature the mud logger and/or the geologist wants to use.
Examples:
Sst: lithic, lt gy, off wh, vf-f gr, occ med gr, sbang-sbrd, mod w srtd, fri, arg, mica, glau,
p-fr vis por, tr-5% blu wh fluor, slow strmg bl wh cut, no cut color, no res, p oil show.
CLYST: lt gy-med gy, occ dk gy, sbblky-blky, mod hd, mic mica, sl calc.
Ls: oolitic grainstone, buff-brn, med gr, mod hd, arg, Brach, glau, gd vis por, no oil
show.
Parts Of A Sample Description
Rock Types
Rock type descriptions consists of two fundamental parts:
· Basic rock name – either underlined or capitalized; some clients would require
that the full basic rock name be written. Example: Sst or Ss (for sandstone),
Limestone, etc.
· Proper compositional or textural classification term – lithic, quartzose, oolitic
grainstone, packstone, etc.
Color
Rock color may be due:
· Mass effect of the colors of its constituent grains
· Cement or matrix color
· Staining of cement or matrix
Rock color may occur in combination or in patterns, e.g. mottled, banded, spotted or
variegated. It is recommended that colors be described on wet samples under tenpowered
magnification. The GSA Rock color chart that is supplied in every ILO unit
could be used as a basis of color determination.
Below is a table of colors that are imparted by materials found in rocks.
Color Material
Yellow, red or brown shades Limonite or hematite
Gray to black Carbonaceous or phosphatic material, iron
sulfide, manganese
Green shades Glauconite, ferrous iron, serpentine, chlorite,
epidote
Red or orange mottlings Surface weathering or subsurface oxidation by
action of circulating waters
The colors of cuttings could be altered, after the samples are caught, by oxidation or
overheating. Metal fragments in samples can rust and stain the samples. Drilling
additives may also cause staining. Diamond and sometimes PDC bits could produce
very bad streaked, ground up cuttings.
Texture
Texture is a function of the size, shape and arrangement of the component elements
of the rock.
Grain or Crystal Sizes
Grain size and sorting have a direct bearing on porosity and permeability. Size
classifications are based on the Wentworth scale.
The mudlogger should always use a standard grain comparator when he is recording
the grain or crystal size. ILO provides a plastic transparent film positive grain
comparator or a mounted sieved sand grain standard comparator. The comparator
could either be placed on top or beside the cuttings in the sample tray.
Shape
Shape involves both sphericity and roundness.
Sphericity refers to the comparison of the surface area of a sphere of the same
volume as the grain, with the surface area of the grain itself (Shell). ????
Roundness, refers to the sharpness of the edges and corners of a fragment or grain.
It is an important characteristic that deserves careful attention in detailed logging. Five
degrees of roundness are distinguished. These are:
· Angular – edges and corners are sharp and there is little or no evidence of wear.
· Subangular – faces are untouched but edges and corners are rounded.
· Subrounded – edges and corners are rounded to smooth curves. The areas of
the original faces have been reduced.
· Rounded – original faces have been almost completely destroyed, but some
comparatively flat faces may still be present. All original edges and corners have
been smoothen to rather broad curves.
· Well rounded – no original faces, edges or corners remain. The entire surface
consists of broad curves. There are no flat areas.
Sorting
Sorting is the measure of dispersion of the size frequency distribution of grains in a
sediment or rock. It involves shape, roundness, specific gravity, mineral composition
and size.
Payne (1942) suggested a sorting classification:
· Good – 90% in 1 or 2 class sizes
· Fair – 90% in 3 or 4 class sizes
· Poor – 90% in 5 or more class sizes
Sorting comparators may also be used to get an objective observation of grain sorting
in a sample.
Hardness (may be move)
Scratching the rock fragment surface is often an adequate way of distinguishing
different: lithic types. Silicates and silicified materials, for example, cannot be
scratched, but instead will take a streak of metal from the point of a probe. Limestone
and dolomite can be scratched readily, gypsum and anhydrite will be grooved, as will
shale or bentonite. Weathered chert is often soft enough to be readily scratched. and
its lack of reaction with acid will distinguish it from carbonates. Caution must be used
with this test in determining whether the scratched material is actually the framework
constituent or the cementing or matrix constituent. For example, silts will often scratch
or groove, but examination under high magnification will usually show that the quartz
grains have been pushed aside and are unscratched, and the groove was made in the
softer matrix material.
Parting (maybe move)
Shaly parting, although not a test, is an important rock character. The mud logger
should always distinguish between shale, which exhibits parting or fissility, and
mudstone, which yields fragments, which do not have parallel plane faces.
Slaking and Swelling (maybe move)
Marked slaking and swelling in water is characteristic of montmorillonite (a major
constituent of bentonites) and distinguishes them from kaolins and illites.
Cementation or matrix
Cement is a chemical precipitate deposited around the grains and in the interstices of
a sediment as aggregates of crystals or as growths on grains of the same composition.
Matrix consists of small individual grains that fill interstices between the larger grains.
Cement is deposited chemically and matrix is deposited mechanically.
Chemical cement is uncommon in sandstone which has an argillaceous matrix. The
most common cementing materials are silica and calcite.
Silica cement is common in nearly all quartz sandstones. This cement generally
occurs as secondary overgrowths deposited with detrital quartz grains. Opal,
chalcedony and chert are other forms of siliceous cement. Dolomite and calcite are
deposited as crystals in the interstices and as aggregates in voids. It should be noted
that dolomite and calcite could be indigenous to the sandstone. They may be found as
detrital grains or as coatings around the sand grains before these grains were lithified.
Anhydrite and gypsum cements are more commonly associated with dolomite and
silica than with calcite.
Additional cementing materials of minor importance are pyrite, siderite, hematite,
limonite, zeolites and phosphatic material.
Silt acts as a matrix, hastening cementation by filling interstices, thus decreasing the
size of interstitial spaces. Clay is a common matrix material, which may cause loss of
porosity.
Compaction and the presence of varying amounts of secondary quartz, secondary
carbonate and interstitial clay are the main factors affecting pore space in siliciclastic
rocks. While there is a general reduction of porosity with depth due to secondary
cementation and compaction, ranges of porosity vary considerably due primarily to
extreme variations in secondary cement.
Fossils
Microfossils, or even fragments of fossils, are used for correlation. Common fossils
and microfossils encountered are: foraminifera, ostracods, bryozoa, corals, algae,
crinoids, brachiopods, pelecypods and gastropods. The mud logger should record
their presence and relative abundance in the samples being examined.
Accessories
Accessory constituents or minerals may be significant indicators of the depositional
environment. They may also be used in correlation. The most common accessories
are glauconite, pyrite, feldspar, mica, siderite, carbonaceous material, heavy minerals,
chert, and lithic fragments.
Sedimentary Structures
Most sedimentary structures are not discernible in sample cuttings. They are present
in cores. Common structures are stratification
Porosity and Permeability
Porosity is a measure of the volume of void space in the rock. Permeability is a
measure of the capacity of a rock for transmitting fluid and it is dependent on effective
porosity and the mean size of the individual pore spaces. Porosity determines the
amount of fluid that is present in a rock; permeability has a direct bearing on the
amount of fluid that can be recovered. Generally, the smaller the grain or crystal size
the lower the permeability.
Visible porosity is easier to determine on a dry sample than on a wet one. A
magnification of 10x is frequently adequate to establish the amount of relative visible
porosity in a dry sample. Higher magnification is occasionally needed to determine
relative visible porosity.
Samples with good porosity should always be examined for hydrocarbon shows.
In siliciclastic rocks three types of porosity are common:
· Intergranular – pore space between grains or particles of a rock
· Moldic – due to the leaching of soluble grains
· Fracture – due to fractures
Intergranular porosity is the most common type and it is easily observed in cuttings.
Generally, it is difficult to detect the other two types of porosity. The presence of
coarsely crystalline vein calcite is often the only indication of the occurrence of
fractures. Moldic porosity is very difficult to recognize.
Porosity in carbonate rocks is generally classified into:
· Intergranular – pore space between grains or particles of a rock
· Intercrystal – pore space between crystals of a rock
· Vuggy – pore space between grains or crystals of a rock wherein the space is
equal or larger than the size of the individual grains or crystals. It usually has the
form of irregular voids.
· Moldic – due to the leaching of soluble grains
· Fracture – due to fractures
It is very hard to distinguish between vuggy and fracture porosity in carbonate rocks.
There are two classifications of porosity in carbonate rocks:
Choquette and Pray
Archie
Choquette and Pray Carbonate Carbonate Porosity Classification
This carbonate porosity classification emphasizes geologic or genetic interpretation. It
is one of the best and most widely used porosity classifications. The authors
recognized 15 basic types of porosity. Each type is distinct and can be defined by
such attributes as pore size, pore shape, genesis and association relative to either
particular constituents or overall fabric. The table below lists the different types of
carbonate porosity.
Table 1 Choquette and Pray's carbonate porosity classification.
Fabric Selective Not Fabric Selective Fabric Selective Or Not
Interparticle Fracture Breccia
Intraparticle Channel Boring
Intercrystal Vug Burrow
Moldic Cavern Shrinkage
Fenestral
Shelter
Growth-framework
Seven of the 15 types are extremely common and probably form the bulk of the pore
space in carbonate rocks. These are: interparticle, intraparticle, intercrystal, moldic,
fenestral, fracture and vug porosity.
The single element in determining the difference between interparticle, intraparticle
and intercrystal porosity is the position of the pore with respect to the fabric elements.
Cavern porosity is determined solely by size. The pore size is at least man-size or
larger.
Archie’s Carbonate Porosity Classification
This classification deals primarily with the physical properties used for evaluating or
exploiting the fluid contents of the rock. It consists of two parts: one refers to the
texture of the matrix, including grain size; and the other to the character plus frequency
of the visible pore structure.
The classification of the matrix gives lithological information on the minute pore
structure (not visible under 10x magnification) between the crystals or carbonate
grains. Below is a tabulation of the three matrix classifications.
Table 2 Matrix classification of Archie's classification of carbonate porosity.
Class Appearance of Hand
Sample
Appearance under 10x
magnification
I
Compact, Crystalline
Crystalline, hard, dense,
sharp edges and smooth
faces on breaking.
Resinous.
Matrix made up of cruystals
tightly interlocking, allowing
no visible pore space
between the crystals, often
producing “feather edge”
appearance on breaking.
II
Chalky, Earthy
Dull, earthy or chalky
appearing, hard to soft.
Crystalline appearance
absent because the small
Crystals, less effectively
interlocking than above,
joining at different angles.
Extremely fine texture may
Class Appearance of Hand
Sample
Appearance under 10x
magnification
amount of crystals or
carbonate particles are less
tightly interlocked, thus
reflecting light in different
directions.
still appear “chalky” under
this power, but others may
start appearing crystalline.
Grain size for this type is
less than 0.2 mm.
III
Sucrosic or Granular
Sandy or sugary appearing. Crystals less effectively
interlocked, fracture
generally along individual
crystal faces giving a rough
or sandy appearance.
Generally more space
between the crystals.
Oolitic, pisolitic and other
granular textures also fall in
this class.
When examining carbonate rocks it is common to find rock types that are gradational
between the three types. For example, Archie type I is often found in association with
type III. Rocks in this category can be denoted as I/III with the dominant type given as
the numerator. This indicates a rock grading between type I and III, and/or close
association of the two types, but closer to or dominated by type I.
The crystals or grains comprosing the matrix are further described within the
classification according to size; e.g. C, M, F, etc.
The character of the visible pore size is classified according to size as listed below.
Class Visible pore size character
Class A No visible porosity, at 10x magnification, or where pore size
is less than about 0.2 mm. in diameter.
Class B Visible porosity, greater than 0.02 mm but less than 0.125
mm. Cannot ber seen without magnification.
Class C Visible porosity greater than 0.125 mm, but less than the
size of cuttings (2 mm). Can be seen by the naked eye.
Class D Visible porosity as evidenced by secondary crystal growth
on faces of cuttings or “weathered-appearing” faces
showing evidence of fracturing or solution channels; where
pore size is greater than the size of the cuttings.
Hydrocarbon Shows
The recognition and evaluation of hydrocarbon shows is one of the more important
responsibilities of the mud logger. He should be familiar with the various methods of
testing for and detecting hydrocarbons. Cuttings with good porosity should always be
tested for hydrocarbon shows.
The order of hydrocarbon shows description is:
· Odor (type and strength)
· Oil stain or bleeding (distribution, intensity, color)
· Sample fluorescence (percentage, intensity, color)
· Cut color fluorescence (speed, character, intensity and color)
· Cut color and intensity
· Cut residue (intensity and color)
See the section on Hydrocarbon shows and tests.
Recording of Data
Data should be first written on a worksheet. A summary of lithological descriptions
should be made f
xxxxxxxx
Lithological Classification and Description
Noncarbonate Clastics
Carbonates
Distinguishing between dolomite and Limestone
Tests with Dilute HCI (10%)
There are at least four types of observations to be made on the results of treatment
with acid:
1. Degree of effervescence: limestone (calcite) reacts immediately and rapidly,
dolomite slowly, unless in finely divided form (e.g. along a newly made scratch).
While the effervescence test cannot yield the precision of chemical analysis or Xray,
it is generally adequate for routine examination. Unless the sample is clean,
however, carbonate dust may give an immediate reaction that will stop quickly if
the particle is dolomite. Impurities slow the reaction, but these can be detected in
residues. Oil-stained limestones are often mistaken for dolomites because the oil
coating the rock surface prevents acid from reacting immediately with CaCO3, and
a delayed reaction occurs. The shape, porosity and permeability will affect the
degree of reaction because the greater the exposed surface, the more quickly will
the reaction be completed.
2. Nature of residue: carbonate rocks may contain significant percentages of chert,
anhydrite, sand, silt, or argillaceous materials that are not readily detected in the
untreated rock fragments. Not all argillaceous material is dark colored, and,
unless an insoluble residue is obt ained, light-colored argillaceous material is
generally missed. During the course of normal sample examination in carbonate
sequences, determine the composition of the non-calcareous fraction by digesting
one or more rock fragments in acid and estimate the percentage of insoluble
residue. These residues may reveal the presence of significant accessory
minerals that might otherwise be masked.
3. Oil reaction: if oil is present in a cutting, large bubbles will form on a fragment
when it is immersed in dilute ac id. See Section under Reaction in Acid of Oil-
Bearing Rock Fragments for more details of this method.
4. Etching: etching is usually done on core chips or whole rock fragments. Etching a
carbonate rock surface with acid yields valuable information concerning the
texture, grain size, distribution and nature of non-carbonate minerals, and other
lithologic features of the rock.
Etching is accomplished by sawing or grinding a flat surface on a specimen, which
is then submerged for a short time (10 to 30 seconds) in dilute acid with the flat
surface parallel to the surface of the acid. After etching the surface is carefully
washed by gentle immersion in water, taking care not to disturb the insoluble
material adhering to the surface of the specimen. Limestone specimens etched in
HCI usually develop an "acid polish”. Insoluble materials such as clay, silt, sand,
chert, or anhydrite will stand out in relief against the soluble matrix. Dolomite
crystals usually stand out also, inasmuch as the acid attacks them more slowly
than calcite. The internal structures of fossils, oolites, and detrital fragments are
commonly revealed on an etched surface. If the appearance of the etched
surface is so diagnostic that a permanent record is desired, an acetate peel can
be made or the surface can be photographed.
Staining Technique for Carbonate Rocks
The distinction between calcite and dolomite is often quite important in studies of
carbonate rocks. For many years several organic and inorganic stains have been
used for this purpose, but with varying degrees of success.
Friedman (1959) investigated a great variety of stains for use in identifying carbonate
minerals. He developed a system of stains and flow charts for this purpose. These
vary in ease of application, but most are not practical for routine sample examination.
See appendix for Friedman’s staining system.
One stain that is applicable to routine sample examination is Alizarin Red S. This stain
can be used on any type of rock specimen, and it has proved especially useful in the
examination of cuttings. The reactions to acid of chips of dolomitic limestone or
calcareous dolomite are often misleading, and the rapid examination of etched chips
does not always clearly show the calcite and dolomite relationships. Alizarin Red S
shows clearly the mineral distribution. Calcite takes on a deep red color; other
minerals are uncolored. See Appendix I, Section 10.3 for a discussion of the
preparation and application of this stain.
Insoluble Residues
Carbonate rocks may contain significant percentages of chert, anhydrite sand, silt, or
argillaceous materials that are not readily detected in the untreated rock fragments.
The study of cherts and associated residues has been a common practice for many
years in certain areas. A description of the methods for the preparation, examination,
and description of the residue are provided in Appendix I, Section 10.4.
Versenate Analysis
Versenate analysis is a relatively fast and inexpensive method for determining
quantitatively the calcite/dolomite ratios of given carbonate rocks. The method has
shown merit in the mapping of intimately associated limestone and dolomite. It is
based on the color reaction of a reagent on crushed and sieved carbonate samples.
(Preparation and Technique in Appendix I, Section 10.5).
Evaporites
Igneous Rocks
Metamorphic Rocks
Test on Minerals
Salts (possibly transfer to Evaporites)
Salts are rarely found at the surface and generally do not occur in well samples.
Unless salt-saturation or oil-base mud is used, salt fragments or crystals dissolve
before reaching the surface. The best criteria for detecting a salt section are:
· occurrence of "salt hoppers" (molds of dissolved salt crystals in other rock
fragments)
· marked increase in salinity of the drilling mud
· sudden influx of abundant caved material in the samples
· sharp increase in drilling penetration rate
· electrical log character, particularly the sonic, density and caliper logs
Cores are the most direct method of determining whether salt is present, but they are
not usually cut in salt sections.
Salts are commonly associated with cyclical carbonate sections and massive red bed
sequences. In the former, they usually are thin bedded and often occur above
anhydrite beds. Potassium-rich salts, the last phase of an evaporation cycle, are
characterized by their high response on gamma ray log curves.
Siderite
Its characteristic brown color and slow rate of effervescence with dilute HCl usually
readily distinguishes siderite. It often occurs as buckshot-sized pellets. The presence
of siderite or iron dolomite in the same rock with calcite may be difficult to recognize
and the following stain procedure is recommended for use when such cases are
suspected.
1. The polished face of chip is immersed for 5-10 minutes in a hot concentrated
solution of caustic potash to which a little hydrogen peroxide is added at intervals
during treatment.
2. The surface is finally washed and dried in the air.
3. Siderite is stained brown while ferrous dolomite (ankerite) takes a weaker stain
and ordinary dolomite remains colorless; calcite is roughened but is not destroyed
and chamosite retains its green color unless carbonate of iron is present.
This method is equally applicable to powders.
Bituminous Rocks
Dark shales and carbonates may contain organic matter in the form of kerogen or
bitumen. Thin section and pyrolysis-fluorometer methods should be used to examine
carbonates and shales in which the presence of bituminous matter is suspected for
possible source rock qualities. Dark bituminous shales have a characteristic chocolate
brown streak that is very distinctive.
Hydrocarbon Evaluation
Introduction
Although petrophysical analyses, coupled with well testing, may give a conclusive
determination of the presence of commercial quantities of oil, it is still the mud logger’s
responsibility to report and log all hydrocarbon shows.
Unfortunately, there are no specific criteria to establish if a show represents a
potentially productive interval. The color and intensity of the stain, fluorescence, cut,
cut fluorescence and residual cut fluorescence will vary with the specific chemical,
physical, and biological properties of each hydrocarbon accumulation. The aging of
the shows (highly volatile fractions dissipate quickly) and flushing by drilling fluids or in
the course of sample washing tend to mask or eliminate evidence of hydrocarbons.
The presence or absence of obvious shows cannot always be taken as conclusive. In
many cases, the only suggestion of the presence of hydrocarbon may be a positive
cut fluorescence. In other cases, only one or two of the other tests may be positive.
Sample Examination Procedure For Hydrocarbon Shows
It is a good practice, to examine both unwashed and the sample for lithological
evaluation under the UV box. This will allow you to take note of the background
fluorescence. In practice, this is rarely done due to time constraints.
One of the first signs of an entry into a possible hydrocarbon-bearing zone would be a
drilling break. This is an increase in ROP due to the porous nature of the sediment
drilled. On most wells, the drilling program would call for a flow check and a CBU
when two to five meters into the drilling break. CBU is circulating until all the samples
are recovered and evaluated.
If there is a significant gas peak, a spot sample should be taken and examined under
the UV light. The spot sample should include an unwashed sample, lithological sample
and a mud sample.
The mud logger must get all the relevant chemicals and tools ready before drilling a
potential hydrocarbon zone.
Below is a typical flow of sampling procedures that are done when hydrocarbon shows
are encountered.
1. Whenever there are significant gas shows, the mud logger must get a mud
sample from either the flowline or the possum belly, aside from the regular sample
or bottoms up sample. If the significant gas peak arrives in between sampling
intervals, a spot sample is caught along with a mud sample.
2. The mud sample is poured into a shallow dish and placed under UV light to
observe the signs of fluorescence in the mud or droplets of immiscible oil popping
to the surface. If nothing is seen, water is added to the mud and the mixture is
stirred. Again the sample is observed under UV light.
3. The unwashed sample is also observed under UV light. The distribution, color and
intensity of the fluorescence, if any, is noted.
4. For the lithological samples, smell the sample first before observing it under the
microscope. If an oil odor is detected record it (see section on Odor). Under the
microscope, an estimate is made of the percentage of oil-stained cuttings. The
appearance (oily, waxy, dry residue), distribution (even, patchy, spotty, streaks)
and color of the oil stains will be described. If oil-stained cuttings are observed
these can be separated and placed into the depressions of the spot plate.
5. The sample tray is then observed under UV light. The proportion of the cuttings
fluorescing and its distribution (even, spotty, pinpoint, patchy, streaky), color and
intensity of the fluorescence, if any, is noted. Some samples, if none were
selected in Step x, are placed into the depressions of the spot plate.
6. The samples in the spot plate are observed under the microscope for any oil
stains. This is also to verify the lithology of the sample that will be subjected to the
solvent cut test.
7. If no stains were observed, the spot plate is put inside the UV box and the
samples are subjected to a solvent cut test.
Odor
Odor may range from heavy, characteristic of low gravity oil, to light and penetrating,
for condensate. Some dry gases have no odor. Describe the odor, as oil odor or
condensate odor. Report its strength as good, fair or faint. Faint odors may be
detected more easily on a freshly broken of odor (in cores) or after confining the
sample in bottle for 10-15 minutes.
Staining and Bleeding
The amount by which cuttings and cores will be flushed on their way to the surface is
largely a function of their permeability. In very permeable rocks only very small
amounts of oil are retained in the cuttings. Often bleeding oil and gas may be
observed in cores, and sometimes in drill cuttings, from relatively tight formations.
The amount of oil staining on cuttings and cores is primarily a function of the
distribution of the porosity and the oil distribut ion within the pores. The color of the
stain is related to oil gravity. Heavy oil stains tend to be a dark brown, while light oil
stains tend to be colorless.
The color of the stain or bleeding oil should be reported. Ferruginous or other mineral
stains may be recognized by their lack of odor, fluorescence or cut.
Fluorescence
There are numerous fluorescent compounds in crude petroleum. A number of them
will fluoresce in the visible spectrum. Generally, dense, low gravity oils will fluoresce at
the longer, infrared, red and orange visible wavelengths, while the lighter, high gravity
oils will fluoresce at shorter, yellow or blue, visible wavelengths.
Unfortunately, crude oil is not the only material found in cuttings that will fluoresce
under ultraviolet light. Some minerals and sample lithologies will also fluoresce. The
fluorescence colors emitted are dark and the intensity is low. Below is a table of
fluorescence of some common minerals and lithologies.
Table 3 Table of common minerals and lithologies and their fluorescence color.
Mineral Fluorescence color
Dolomite and magnesian
limestone
Yellow, yellowish-brown to dark brown
Aragonite and calcareous
mudstones
Yellow-white to pale brown
Chalky limestones Purple
Foliated paper shales Tan to grayish brown
Anhydrite Blue to mid gray
Pyrite Mustard yellow to greenish brown
Some mud additives may also exhibit traces of fluorescence. Pipe dope, the heavy
metalized grease used to lubricate and seal threaded tool joints of drill pipe and drill
collars, is a source of fluorescence contamination. Pipe dope has a very bright gold,
white or bluish-white fluorescence normally indicative of a light, high gravity oil or
condensate. In natural light, it has a heavy, viscous appearance and a blue-black or
brown metallic color. While high gravity oil or condensate is transparent and gold in
natural light.
A reservoir containing heavy, low gravity oil might be passed over because bright
white fluorescence of pipe dope might mask the darker color of the oil. In addition, low
gravity crude has is dense and dark brown in natural light.
It is important to check all samples under ultraviolet light, regardless of whether oil is suspected.
Examination of mud, drill cuttings and cores for hydrocarbon fluorescence under
ultraviolet light often indicates oil in small amounts, or oil of light color which might not
be detected by other means. All samples should be examined. Color of fluorescence
of crude ranges from brown through green, gold, blue, yellow, to white; in most
instances, the heavier oils have darker fluorescence. Distribution may be even,
spotted, or mottled, as for stain. The intensity range is bright, dull, pale, and faint.
Pinpoint fluorescence is associated with individual sand grains and may indicate
condensate or gas. Mineral fluorescence, especially from shell fragments, may be
mistaken for oil fluorescence, and is distinguished by adding a few drops of a solvent.
Hydrocarbon fluorescence will appear to flow and diffuse in the solvent as the oil
dissolves, whereas mineral fluorescence will remain undisturbed.
Solvent Cut Test
Introduction
Oil-stained samples which are old may not fluoresce; thus failure to fluoresce should
not be taken as decisive evidence of lack of hydrocarbons. All samples suspected of
containing hydrocarbons should be treated with a reagent. The most common
reagents used by the geologist are chlorothene, petroleum ether, and acetone. The
next topic provides a list of reagents that can be used for the solvent cut test.
Chemicals Used For The Solvent Cut Test
The solvent cut test is useful in determining the quality of the show. You can use
different solvents for this test:
· chloroform (trichloromethane)
· carbon tetrachloride (tetrachloromethane, perchloromethane)
· ethylene dichloride (sym-dichloroethane, 1,2-dichloroethane, ethylene chloride,
dutch oil)
· methylene chloride (methylene dichloride, dichloromethane)
· 1,1,1-trichloroethane (methyl chloroform, chlorothene)
· 1-1-2-trichloroethane (vinyl trichloride, beta-trichloroethane)
· trichloroethylene (ethylene trichloride, triclene, tri, trike or TCE)
· acetone
· petroleum ether
The most common reagents used by the geologist are chlorothene, petroleum ether,
and acetone. The next topic provides a list of reagents that can be used for the
solvent cut test. The use of ether gives a more delicate test for soluble hydrocarbons
than chlorothene or acetone. However, the ether being used should be tested
constantly, for the least presence of: any hydrocarbon product will contaminate the
solvent and render it useless. Chlorothene is recommended for general use although
it too may become contaminated after a long period. Acetone is a good solvent for
heavy hydrocarbons but is not recommended for routine oil detection.
CAUTION: CARBON TETRACHLORIDE is a cumulative poison and SHOULD NOT BE USED for
any type of hydrocarbon detection.
CAUTION: Proper ventilation is important when using petroleum ether as it may have a toxic effect
in a confined space. In addition, PETROLEUM ETHER AND ACETONE are very INFLAMMABLE
and must be KEPT AWAY FROM OPEN FLAMES.
Do’s and Don’t’s
Do not store the solvents in plastic containers or bottles. It has been observed that
after sometime the solvent evaporates or emits a pale yellow-white fluorescence. Try
to replace the contents of the small glass bottle that you use for tests.
Before performing a solvent cut test, a test the solvent should be checked under UV
light for fluorescence.
When working with solvents always work with small quantities in a well-ventilated area.
Remember to wash your hands after using them. Do not eat without washing your
hands after handling them.
Solvent Cut Test
The most reliable test for hydrocarbons is the solvent cut test. Another name for it is
the cut fluorescence or “wet cut" test.
If hydrocarbons are present, fluorescent "streamers" will be emitted from the sample
and the intensity and color of these streamers are used to evaluate the test. Some
shows will not give a noticeable streaming effect but will leave a fluorescent ring or
residue in the dish after the reagent has evaporated. This is termed a "residual cut."
It is recommended that the "cut fluorescence" test be made on all intervals in which
there is even the slightest suspicion of the presence of hydrocarbons. Samples that
may not give a positive cut or will not fluoresce may give a positive cut fluorescence.
This is commonly true of the high gravity hydrocarbons that may give a bright yellow
cut fluorescence. Distillates show little or no fluorescence or cut but commonly give
positive cut fluorescence, although numerous extractions may be required before it is
apparent.
Generally low gravity oils will not fluoresce but will cut a very dark brown and their cut
fluorescence may range from milky white to dark orange. An alternate method
involves picking out a number of fragments and dropping them into a clear one-ortwo-
ounce bottle. Chlorothene or acetone is poured in until the bottle is half full. It is
then stoppered and shaken. Any oil present in the sample is thus extracted and will
color the solvent. When the color of the cut is very light, it may be necessary to hold
the bottle against a white background to detect it. If there is only a slight cut, it may
come to rest as a colored cap or meniscus on the top surface of the solvent.
Before a solvent cut test is to be performed, ensure the reliability of your solvent by
placing a few drops of the solvent on a depression in the spot plate and observing it
under UV light. If the solvent is “good” no fluorescence should be observed.
To do a solvent cut test:
1. Place a few drops of solvent, enough to immerse the sample, on the sample in
the depression in the spot plate. Be careful not to get the cutting agent into the
rubber of the dropper as it might “contaminate” the solvent by giving it a pale
yellowish fluorescence.
2. Observe the following:
· Cut speed: This is an indication of both the solubility of the oil and the
permeability of the sample. The speed can vary from instantaneous to very slow.
· Cut nature: coloration of the solvent with dissolved oil may occur in a uniform
manner, in streaming manner or in a blooming manner. A streaming cut also
indicates low oil mobility.
· Cut color fluorescence and intensity: Observe the color and the intensity of the
oil in the solvent under both UV and natural lights. The cut color observed under
UV light could be called a cut color fluorescence (example: bright blue white cut
fluorescence).
· Cut color and intensity: After observing the sample under UV light observe the
sample under natural light. The cut color observed in natural light is just called cut
color (example: very light brown cut color or no cut color).
The shade of the cut depends upon the gravity of the crude, the lightest crudes
giving the palest cuts, therefore, the relative darkness should not be taken as an
indication of the amount of hydrocarbon present. A complete range of cut colors
varies from colorless, pale straw, straw, dark straw, light amber, amber, very dark
brown to dark brown opaque.
· Residue color and intensity: The solvent dissolves rapidly under the heat of the
UV light, sometimes leaving a residue of oil around the cutting on the spot plate.
The true color of the oil can then be observed. The intensity and opacity of color,
especially of the residue, is an indicator of the oil density and the quantity of oil
originally in the cutting.
A faint "residual cut" is sometimes. discernible only as an amber-colored ring left
on the dish after complete evaporation of the reagent
The hydrocarbon extracted by the reagent is called a "cut."
3. If the sample shows all the possible signs of being oil-bearing but has no solvent
cut, the sample is crushed using the metal probe and it is observed for a solvent
cut. The cut is called a crushed cut. Sometimes, adding a little dilute acid will.

Mudlogger : Eltayeb Hajmedani

Shaa.eltayeb@yahoo.co.uk