SAMPLE DESCRIPTION TECHNIQUES General Hints

Dont spend ages deciding on exact percentages. It is not an exact science and the amounts of each lithotype are only relative.

Do a not use As Above (A/A) more than three times or so before doing another full description. It is easy to miss subtle changes by doing this. Ideally every sample should be described, but practically this is impossible.

Never let too much of a backlog of samples accumulate. Use Samplex trays to keep samples for review purposes if available. It is possible to cut the bottom off of polystyrene or cardboard cups to use as temporary storage after samples have been described. Always describe samples on the metal trays.

Cuttings samples are collected wet from the shale shaker. Samples should be described whilst still wet and should not be left to dry out or re-hydrated with water. Put samples in plastic bags, or at least cover them up, if it is to be sometime before they are described. However, the texture of certain formation types can be seen more clearly when dry, so occasionally keep a portion of sample for later, dry examination.

Mudlogger to have representative samples of all potential mud additives that may be used in the well. If you think there is contamination from drilling fluid additives then check with your reference set. Remember, these may change somewhat after being subjected to the pressures and temperatures of circulation.

Systematic Sample Description Introduction This section describes how samples should be described systematically. It is recommended that you establish a methodical routine to describe samples so that each is described consistently and to the same standard each time. As a minimum please try to follow the checklists presented here. They are meant to ensure that all relevant aspects of the samples are described and help to assure quality.

Sample Description Overview Samples include cuttings samples created from the action of the bit, core chip samples, sidewall core samples and, occasionally, samples taken from junk baskets, stuck to drill collars and hole openers etc. All samples should be examined for :-

Lithology and accessory minerals, microfaunal content Texture and fabric Porosity and permeability Hydrocarbon type and content

Essentially, all of these samples can be described in the same way although rock fabric and texture is more difficult in small cuttings. Whilst we advocate a systematic examination technique you should never limit your descriptions to those aspects described here if you feel that there are other relevant aspects. Always note down any unusual features which may be useful for identification purposes later. If there are micropalaeo or geochemical personnel making examinations of samples at the wellsite then make use of their results in your descriptions. Never ignore any source of information or data about the samples. The first part of this chapter deals with the description and testing of samples. The second part of the section is devoted to show evaluation and description. The description of samples is split into four main categories:-

General descriptive sequence Clastics Carbonates Evaporites

General Sequence of Description The sequence of description of cuttings samples should follow the same standard routine:-

1. Look at the sample tray without the microscope. Are the various lithotypes easily distinguishable? Ensure there is sufficient sample on the tray and that it is a maximum of one layer thick.

2. Put the sample tray in the UV box, is there any fluorescence? Is this natural

mineral fluorescence or is there a possible show? 3. Put the sample tray under the microscope at low magnification and look round

the tray. As the sample tray is processed the various lithotypes are aggregated by the washing action.

4. A visual determination of the relative percentages of the various lithologic

components should be made and entered on the Wellsite Description Sheet. Dont be afraid to go back and change these percentages when you have described the lithotypes as it is sometimes better to gauge percentages after the descriptions and the sample has dried a bit.

5. Methodically describe the samples as per the guidelines laid out below. 6. Perform lithotype tests as required. 7. Check and describe shows as per guidelines later in this section. 8. Fill in as much information as possible on the Wellsite Description Sheet. 9. Review percentages again. 10. Do samples fit in with changes in ROP or LWD curves? 11. See how sample fits in to overall sequence of samples.

Estimating Percentages This becomes easier with experience. The main things to remember are: - Do not spend hours agonising about each percentage -

you havent got time. It is not an exact science and you will not be penalised for being 10% out. Use a chart (e.g. Figure 4.23)

Estimate percentages to nearest 5 - 10%. Anything less than 5% is usually designated as rare trace to good trace.

If the cuttings are gradational between a number of end members, e.g. sandstone, siltstone and claystone then estimate the relative percentages as best you can but make a note of the gradational nature. Pick criteria to decide what is what and stick by it. Discuss these criteria with your relief and with the Mudloggers too. You dont all want to be reporting totally different lithologies for the same depth interval on your reports.

Always reassess percentages after you have described the lithotypes. It is often easier to see when you have got your eye in on the lithotypes. Figure 4.23

Description of Clastic Rocks Overview Clastic rocks are those built up of pre-existing rock types produced by the processes of weathering and erosion. They are normally transported to their place of deposition and may be subsequently subject to cementation and slight chemical changes. Typical sedimentological analysis of clastic rocks based on mineralogy and thin section work is not possible at the wellsite due to constraints of time and equipment. A classification system based on size and texture, however, has evolved over the years and is pretty much standard for all clients. The following methodical scheme can be followed when describing samples. The operator could have their own scheme then use theirs as a preference. Descriptions are generally made on the Wellsite Description sheets using abbreviations. This saves time and space. However, it is not compulsory to use abbreviations. If you feel it is easier to use full words then please do. However, do not use your own abbreviations as other people may not be able to understand them. Either use standard abbreviations or full words. Abbreviations should generally be used on the Wellsite Lithology Log but not on reports and the Final Composite Log. Again this will depend on client preference. Descriptions should be made for clastic rocks using categories in the following order: -

CATEGORY DESCRIPTION Rock Type Sandstone, siltstone etc. Colour Use standard colour charts Grain Colour Mainly applies to sandstones Hardness How resistant are the cuttings to applied force Cuttings Shape General shape of cuttings Grain Size Use standard grain size charts, mainly sandstones Grain Shape Use grain shape chart, mainly sandstones Sorting Use sorting chart if available Cementation/Matrix Types of cement and matrix Porosity/Permeability Visual determinations only Accessory Minerals Glauconites, micas etc, with some qualifier as to abundance Unusual Features Microfossils, fissures, etc. Hydrocarbon Shows See separate section

Try to describe each of categories in turn for each sample description. Obviously grain shape will not be applicable to claystones, for example, so ignore categories which do not apply. Each of the categories is now described in more detail:-

Rock Type The type of rock or lithotype you are looking at is usually fairly readily identifiable at a glance to any reasonably trained geologist. A quick look at the grain size and a dab of acid is usually sufficient to make a fairly confident identification of most rock types. Sometimes it may not be quite so easy: -

Gradational lithotypes. Silty claystones, argillaceous sandstones, sandy siltstones are always difficult to describe and to put relative percentages on. As noted before, try not to spend hours agonising over percentages and make a note of the gradations on worksheets and descriptions. Qualifying names such as those above and tuffaceous claystone etc. must only be used if there is a significant amount of the qualifying rock type present.

Consolidation. Distinguish the rock types based on the degree of consolidation. When soft, claystone should be referred to as clay etc.

Contaminants. Mud additives, metal shavings and cement can all dilute the sample to varying degrees. You will need to inform the Drilling Supervisor and Mud Engineer about the contaminants and if they become too pervasive and start to mask the sample then inform the Drilling Supervisor and Operation Geologist.

Loose Sand Grains. Loose sand grains are a common feature on sample trays and need to be given more than a cursory examination.

Firstly, are they sand grains? Crystalline barite looks very similar to fine loose sand grains and you must examine the grains carefully to ensure they are sand. Barite dries with a dusty white sheen and crushes easily to white powder.

On top hole, the sandstone units are fairly unconsolidated and poorly cemented. The action of the bit and the jetting action of the mud are enough to completely disaggregate the sand grains but you should check for any residual cement or matrix around the grains and also for mineral growths on the grains.

Loose sand grains from deeper in the well may be a sign of overbalance. If the sandstone cuttings are not cleared quickly from the bit then they will be subjected to much grinding action by the bit itself which will dissagregate the grains. If there is little overbalance then the sandstone cuttings should be liberated fairly quickly and should be seen as proper sandstone cuttings. Again check for residual cement and matrix in loose grains. Where, grains are loose it is impossible to estimate permeability and porosity.

Metamorphic/Igneous lithotypes. It can sometimes be difficult to identify some igneous and metamorphic rock types based on physical appearance in cuttings. It is enough to roughly classify them and make a full description. As long as they are described in reports then if there is much interest then samples can be analysed in a laboratory away from the wellsite to get an exact identification. Dont spend a great deal of time describing them as they are unlikely to be of particular interest in oil industry terms other than for provenance studies.

If you cannot identify a particular rock type then dont worry. As long as you note its presence, do a full description and make a stab at identification this will be good enough.

Colour. Colour is very subjective. Each individual has their own idea of colour and it is a pointless exercise to argue over relative shades. To make this category more objective it is strongly recommended that all members of the wellsite crew use a standard Rock Colour Chart such as that produced by the Geological Society of America (Figure 4.24). It consists of a small booklet where each page consists of small oblongs of different hues of the same colour. Each colour has a name and a number Grip a small representative piece of wet cutting with tweezers. You will need at least a

moderately sized piece. Put the colour chart, with the page of the approximate colour visible, under the microscope. Compare the cutting with each hue in turn until you get the closest match Most operators prefer the colour to be written down whilst others may require the colour code. It is recommended that the colour is described when the cutting is wet as it will be much lighter when dry. Also the power of the microscope lamp and the amount of magnification you use can affect the hue of the colour. Try to make colour comparison using the same lamp power and magnification if possible. If a lithotype is vari-coloured then describe it as being so, give a dominant colour and try to estimate the percentages of each colour. Does one colour remain dominant or does it change with depth? Can you tell if the colours are mottled, streaked etc. in nature?

Figure 4.24

Colour can also be used as simple diagnostic tool to aid in mineral and environmental determination. Colour Mineral Indications Environmental indications

Red - Orange Iron Ferric-oxydised state indicative of oxygenated environments, e.g. deserts and river systems. The Tertiary Lark Formation (Robertsons) above the Balder Formation, parts of the Upper Cretaceous Rdby Formation and Permian Kupferscheifer

Light green Iron Ferous reduced state indicative of reducing environment. Greens and purple reduction spots are found in parts of Tertiary, Triassic etc.

Bright Green Glauconite, Chlorite and Chamosite

Glauconite is common in many horizons and is thought, in many cases to be a product of fish faeces. It is common on lower marine shelf environments. Chlorite and chamosite (an oxy-chlorite) may be found in sediments derived from nearby metamorphic sources or in deeper wells as products of diagenesis.

Blue Tuffaceous Blue colouration is common in Balder Formation which is of volcanic origin Dark grey -brown black - olive black

Carbonaceous material

Anoxic environments, usually marine in nature. This allows preservation of organic material and disseminated iron sulphide is commonly associated. The Kimmeridge Clay/Draupne Formation of the North Sea is characteristically a brown black-olive black colour and is one of the best hydrocarbon source rocks in the world.

Yellow - ochre Limonite Limonite covers a range of hydrated iron oxides and iron hydroxides. It is a weathering product of all iron containing minerals.

Brown Oil Check for shows! Sandstones and coarser When describing sandstones and pebbles you will need to describe not only the colour of the constituent grains and clasts, but also their transparent, translucent or opaque nature. This will help in mineral identification. Also note any surface discoloration of the grains or if there are any coloured inclusions. See also later in Subsection on grain shape and surface features.

Hardness and fracture This category is intended to reflect the degree of induration, cementation and compaction of the lithology and how the sample fractures. The common adjectives used to describe hardness include:

Loose lse Grains disaggregate when the sample dry. Not used on clay/shale rocks.

Friable fri Loose grains can be separated by pressure from the fingers.

Firm frm Grains can be separated with a probe. Hard hd Grains difficult to detach, pressure results in cuttings

breaking grains. Very hard v hd Individual grains cannot be detached and cuttings

break through grains. For clay based lithologies, the following terms can be used:

Very soft v sft Can be dispersed by water/drilling mud. Soft sft No shape or strength, very easily deformed. Sticky stky Sticks to fingers and sample probe. Plastic plas Easily moulded and retains shape, difficult to wash through

sieve. Firm frm Definite shape and structure, penetrated and broken by probe. Hard hd Sharp angular edges, not easily broken by probe. Variously

subdivided as moderately to very hard. To determine hardness, then, you will need to crush a number of grains using a probe or tweezers to get a representative hardness for each lithotype. If there is a range of values then make a note of this. Is there some reason for hardness ranges? Does the hardness vary with colour or calcareous content? Further terms are often applied to clay based lithologies which describe the solubility to water of the claystone. This is especially so when dealing with tophole clays. Various terms are used, soluble hydrateable, hygroturgid etc. and they all mean pretty much the same - the lithology disintegrates when exposed to water. It is recommended that when dealing with soft, soluble lithologies that:-

You occasionally check how the sample is being washed by the Mudlogger. Is much of the claystone fraction being washed away?

Get the Mudlogger to put a small pile of unwashed sample on the sample tray. This allows a direct comparison between the washed and unwashed sample.

Try to make estimates of percentages based on unwashed sample rather than washed sample in these circumstances. The non-soluble fraction may be considerably enhanced by the washing process.

When testing for hardness the fracture or break of the cutting can described. The break may be described using the following terms: -

Crumbly crmly Easily crushed into constituent parts Brittle brit Breaks into small pieces when fractures Conchoidal conch Curved fracture planes such as those seen in flint Hackly hkly Irregular break with no preferred fracture orientation Splintery splty Very hard and splinters into sharp pieces when broken

or may be described using terms defined in the next section on cuttings shape.

WENTWORTH SIZE CLASS SIZE mm. ADJECTIVE SIZE TERMINOLOGY

BOULDER

COBBLE

PEBBLE

GRANULE

SAND

SILT

CLAY

GR

AVEL

MU

D

MU

DD

Y

CONGLOMERATE

SANDY

SILTY

CLAYEY

COARSE GRAINED

MEDIUM GRAINED

FINE GRAINED

VERY FINE GRAINED

MICRO GRAINED

VERY COARSE TO PEBBLY

256.0

64.0

4.0

2.0

1.0

0.50

0.25

0.125

0.0625

0.0039

Cuttings Shape This category is largely used to describe the cuttings shape of clay based lithologies but can be applied to other lithologies and the fracture or break of all lithologies (see above). It does not refer to constituent grains of cuttings, which are described later. Cuttings shape and size are strongly influenced by rock type, bit type and the degree of overbalance. If the well is close to balance, hold down force is low and cuttings are freely liberated. They tend to be bigger and fresher looking when seen at the surface. If the cuttings get too big then the circulation system will not be able to carry them quickly up the hole. They will tend to become abraded by constant impacts with other cuttings and the borehole wall. If there is a high overbalance then cuttings are not freely liberated and they are much smaller. They may be rolled round on bottom and will have a more rounded and less fresh appearance. Influences on bit shape are discussed below The common, fairly self explanatory, descriptive adjectives used for cuttings shape include:

Amorphous amor No shape - generally due to hydration of sample, preferred fracture orientation masked.

Blocky blky Square, angular appearance with no preferred fracture orientation Platy plty Flat appearance with rounded edges, preferred fracture plane Subfissile sbfiss Flatter and more elongate than platy, not as sharp edged as

fissile, preferred fracture plane Fissile fiss Generally flat and elongate with sharp edges, marked fracture

orientation. May be curved if pressure caving.

Various qualifying parameters are commonly used to grade these such as slightly, very etc. and terms such as sub-blocky are also used for intermediate stages. However, as it is difficult to define what sub-blocky actually means it is recommended that terms such as this are not used.

Grain Size. This refers to constituent grains of cuttings (sandstones, siltstones etc.) and not to the cuttings themselves. Evaluation should include both the dominant size grade and the range of grades in the sample. A sandstone, for example, may be predominantly coarse grained yet still exhibit a range of grain size from medium to very coarse. Comparison with the Wentworth scale is in Figure 4.26.

Figure 4.25

Figure 4.26

Grain size can be easily measured using one of the grain size comparison charts that are available in many of the Mudlogging units such as the old Exlog one in Figure 4.27. It also gives comparisons for sorting and roundness which are also most useful. Figure 4.27 It must also be noted that the grain size will also be affected by the mesh size of the shale shakers that are being used. Always be aware of the shaker screen size that is currently in use and how it will affect the samples. On tophole it is common for large mesh sizes to be used so that cuttings and even fairly coarse sand grains will go straight through and thus will not be seen in samples. The common set up at all times is to have coarser screens above finer ones on the shale shakers. This is why it is important that representative samples must be taken from both screens and mud cleaner/centrifuge output to ensure that, for instance, fine sand grains are not missed. As it is difficult to gauge comparative grain sizes in whole rock samples of cuttings and core chips it is best to disaggregate the sample to evaluate grain sizes. Simply crush a few grains of sandstone in a spot tray or on a spare metal tray and spread the grains out so they are one layer thick. Use the grain comparison chart to gauge the grain size and also you will get a much better idea of sorting. It is easy to note other characteristics of the rock at this stage.

Grain Shape and surface features This refers to constituent grains of cuttings (sandstones, siltstones etc.) and not to the cuttings themselves - see cuttings shape. Grain shape is usually applied to arenaceous rocks and refers to the angularity and smoothness of the grains. It can be subdivided into the following classes of sphericity and angularity:-

Angular Ang Flat, plane surfaces, terminating in acute or right angles, thin, sharp edges.

Sub-angular Sbang Flat surfaces terminating in corners. Sub-rounded SbRndd Rounded corners and increasingly surfaces. Rounded Rndd Rounded surfaces, edges and corners. Well rounded W Rnd Becoming spheroidal.

SPH

ERIC

ITY HIGH

LOW

Very Angular

.15

Angular

.20

Sub- Angular

.30

Sub- Rounded

.40

Rounded

.60

Well Rounded

.85

ANGULARITY

From Powers

Sphericity is a measure of how spherical a grain is and this is illustrated in this chart from Powers (Figure 4.28). Grain shape and sphericity give an indication of the maturity of the sandstone. However, these can be affected by later mineralisation so that the original grains are eroded, coated or overgrown. Any evidence for this needs to be described, as it will effect grain shape and porosity and permeability. A number of terms are commonly used: -

Figure 4.28

Pitting Surfaces of grains have small holes caused by chemical solution or physical impacts.

Staining Thin veneer of mineralisation with a coloured, barely noticeable, powdery appearance. Iron staining is common but may also be oil staining!

Coating Thicker veneer of mineralisation on grain surfaces. Frosting As its name implies a white powdery coating to grains caused abrasion of

surfaces. Glassy Mineral overgrowths into void with flat crystal surfaces apparent.

Check all translucent and transparent grains for internal features such as inclusions and fractures.

Sorting Sorting applies to the coarser clastic rocks. The degree of sorting should be classified according to the following terms:

Well sorted well srtd Range of particle size confined to two adjoining grain sizes. Siltstones must be well sorted by this criteria.

Moderately sorted mod srtd Range of particle size confined to four adjoining grain sizes.

Poorly sorted prly srtd Range of grain sizes over more than four grain sizes. Sorting gives an indication of the textural maturity of the sediment and is one of the major influences on porosity and permeability. As it is difficult to gauge comparative grain sizes in whole rock samples of cuttings and core chips it is best to disaggregate the sample to evaluate sorting. Simply crush a few grains of sandstone in a spot tray or on a spare metal tray and spread the grains out so they are one layer thick. Ensure you are not looking at well sorted laminations of different grain sizes.

Porosity and Permeability Porosity and permeability in drilled cuttings are difficult to evaluate and are determined, at best, only very subjectively. Take a good look at a number of samples and cuttings and see how well they are cemented or if there is a common matrix. Sorting also affects porosity and should be evaluated first as it may be a porosity indicator. These preliminary indications are useful to reinforce evidence from the technique described below. Evaluation is best carried out by examining dry sample. Representative cuttings may be selected from the dried portion of the sample, or more simply select pieces from the wet sample to describe lithology and then let them dry out. Porosity may then be estimated by placing a drop of water on a dried cutting while viewing through the microscope. The speed at which the water is absorbed by the cutting will help in subjectively evaluating porosity and permeability. Where distinguishable, porosity should be described using the following terms:

Trace Tr 0 - 5% Poor Pr 5 - 10% Fair Fr 10 - 20% Good Gd 20 - 30%

The evaluation of porosity should also include an evaluation of the type of porosity present (e.g. intergranular, vuggy, pin-point, etc.). Permeability will be related to the porosity to a certain extent. If you can see porosity but water absorption rates are low then you must assume that the permeability is fairly poor.

Cementation and Matrix Cement is caused by the precipitation of minerals within the porosity of sedimentary rocks. Matrix is defined as fine material that accumulates between the larger grains of clastic rocks such as sandstones, conglomerates and breccias. The presence of cementation and matrix will obviously have a direct influence on porosity and permeability. The type and degree of cementation or matrix should be noted. Observe the relationship between the cement, matrix and the constituent grains. Cements The most common cements are:-

Silica (SiO2) This is the most common cement but may grow in optical continuity with quartz grains and can be difficult to spot. Look for residual surfaces, flat crystal surfaces. Assume silica cement if no reaction with acid.

Calcite (CaCO3) Can be recognised as will react strongly with dilute hydrochloric acid. The cutting should disintegrate into constituent grains as cement is dissolved.

Dolomite (CaMg(CO3)2) Slower reaction with dilute hydrochloric acid, cement may bubble for sometime and cutting may start to disaggregate after some time.

Siderite (FeCO3) Dull yellow brown and white, will react slowly with application of acid. Relatively rare as cement.

Pyrite (FeS2) Bright yellow gold and metallic, very recognisable but relatively rare. Work safely; use spot plates for acid tests and keep acid fumes away from microscope lenses. Matrix The most common matrix materials in sandstones are silt and argillaceous material. Kaolinite is also very common and generally appears as a soft to firm and white clayey material. It is often a breakdown product of feldspars. Note also that kaolinite matrix can become separated from the sand grains, especially when drilling with PDCs or in an overbalanced situation and can produce rock flour.

Unusual Features Note any unusual feature about the clastic rocks you are describing. The relevance of these may not be immediately apparent but may be later. These features will, obviously, be more apparent in whole rock samples such as cores and sidewall cores (see later in this section).

Accessories/Modifiers Note and identify, where possible, any minerals (e.g. glauconite) or inclusions in the sample. Their relative abundance should also be noted. The following table can be used as a rough guide:-

Rare 0 - 2% Trace 2 - 4% Common 4 - 6% Abundant 6 - 8% Very abundant 8 - 10%

Common accessory minerals are:- Glauconite Glauc Dark green - green black, generally rounded grains especially if recycled. In situ it

indicates a shelfall marine environment Pyrite Pyr Gold colour, may be disseminated, small nodules or crystalline fragments Mica (Micromica) mic (mmic) Mica flakes are common in sandstone whilst small mica fragments (micromica) are

common in finer clastic rocks. Chlorite is also common and is a distinctive green colour.

Carbonaceous Mat.

Carb mat Dark black specks of carbonaceous material are very common. In cores this may include layers of carbonaceous material, rootlets and plant debris.

Kaolinite Kao White clayey material, found as matrix and as decomposed feldspar grains Coal / Lignite Coal / lig Brittle, black coal layers may be observed in cores, lignite is softer and browner Siderite sid Yellow brown or dark brown pellets.

Identifying argillaceous material in the matrix is sometimes difficult. Log responses can help. Kaolinite should have a negligible effect on the Gamma Ray, whilst illitic and smectites clays will have more of an effect. Muscovite mica affects gamma ray more than biotite. If identification is difficult then make as full a description as possible and, if there is anything significant, further tests can be carried out later if required. Again, do not spend hours trying to identify obscure or rare minerals unless you feel they are significant. Your time is much better spent identifying the main characteristics of the sample. Fossil material can be included as accessories. Macrofossil fragments are fairly common but it is generally impossible to identify the origin. Microfossils can be very common in some samples and, with a good microscope, many can be identified. This need only be a general name. Again dont spend hours on microfossils, the samples will be subject to analysis after (or in some cases during) the well.

4.6.8 Description of Carbonate Rocks Introduction The classification and description of carbonate rocks at the wellsite is constrained by the samples and the equipment available. There have been many attempts over the years to classify carbonate rocks and it is difficult to do even with thin sections and microscopes. Of necessity then the wellsite classification of carbonate rocks is based on a fairly simple scheme proposed by Dunham (1962). His classification is based on depositional textures produced by varying degrees of environmental energy.

DEPOSITIONAL TEXTURE RECOGNIZABLE DEPOSITIONAL TEXTURE NOT RECOGNIZABLE

Original Components not bound together during Deposition

Contains Mud(particles of clay and fine silt size)

Mud-supported Grain-supported

Lacks mud and is

grain-supported

Original componentswere bound togetherduring deposition...

as shown be intergrownskeletal matter,

lamination contrary to gravity,or sediment-floored cavities that

are roofed over by organic orquestionably organic matter andare too large to be interstices.

Less than10 percent grains

More than10 percent grains

Mudstones Wackestone Packstone Grainstone Boundstone

Crystalline Carbonate

(Subdivide according to classification designed to bear on

physical texture or diagenesis)

From Dunham Figure 4.29

Carbonate rocks are generally deposited in shallow, warm marine environments. Clastic input is minimal allowing the waters to be clear and light. If the original textures are present then the Dunham classification can be used and some basic inferences as to energy of deposition made. If the carbonate rock has been subject to recrystallisation then a description giving the basic rock features and crystal size is generally all that is possible at the wellsite.

CATEGORY DESCRIPTION Rock Type Limestone, Dolomite, gradations in between. Define in terms of

Dunham if possible. Some definitive tests are possible. Colour Use standard colour charts Hardness How resistant are the cuttings to applied force Cuttings Shape General shape of cuttings Crystal Size If crystalline Crystal Shape If appropriate Sorting If appropriate, in packstones and grainstones etc. Cementation/Matrix Types of cement and matrix Porosity/Permeability Visual determinations only Accessory Minerals Glauconites, etc, with some qualifier as to abundance Unusual Features Microfossils, fissures, etc. Hydrocarbon Shows See separate section

Follow this systematic sequence of description as illustrated in the above table and discussed in more detail below.

Rock Type The main carbonate rock forming minerals are calcite, aragonite and dolomite. Calcite, iron and magnesium are commonly substituted to produce a whole range of rock types of which limestone (predominantly CaCo3) and Dolomite (CaMg(CO3)2) are by far the most common. The following table shows a guideline for basic nomenclature based on calcium percentages (as opposed to magnesium):-

Percentage Calcium Rock Name 0 - 50% Magnesite 50 - 60% Dolomite 60 - 75% Calcareous Dolomite 75 - 90% Dolomitic Limestone 90 - 100% Limestone

Determination of carbonate rock type at the wellsite usually consists of working out via simple tests what the rock is and then fitting it in to the Dunham classification. If crystalline, the rock should be described in terms of its crystal size. The Mudloggers usually have a calcimeter - a specialised apparatus to measure total carbonate content of samples. This can give relative abundances of limestone and dolomite and may help to make a basic classification of the carbonate. Whilst it does not provide a definitive evaluation it is certainly more objective than dropping cuttings in acid. There are a number of tests described below that allow more specific determinations of rock character to be made. Whilst limestones and dolomites are the commonest rock types there are several other subtypes that are commonly used. Chalk Very fine grained, predominantly white limestone made up of the skeletons of a planktonic algae called coccolithospheres. Chalk is very widespread in the Cretaceous but many limestones can be said to have a chalky texture and it is a readily identifiable rock type. Marl This is a fine grained calcareous mudstone. There are various definitions of marl and it is recommended that the term is not used with either calcareous claystone or argillaceous limestone being used instead. Even here it is often difficult to pick between these two terms as they are just gradational members of the same series. Ideally when put in dilute hydrochloric acid a calcareous claystone cutting will retain its integrity whilst an argillaceous limestone cutting will not, leaving a clayey residue in the spot tray. Arenaceous Limestones Differentiate sandy limestone and calcareous sandstone in the same way as argillaceous limestones. Drop cuttings in dilute hydrochloric acid and see if they disaggregate.

Determinative Testing for Carbonates SAFETY NOTE

The simplest test to show if a cutting is a carbonate is to drop some dilute acid on it. You need to be careful on two counts when you do this:-

d

mple. Use s

Putting acid directly onto the cuttings below the microscope can cause damage to the microscope lens if effervescence is strong and bubbles up quickly sending acidic fumes anmoisture into the air. Make sure the sample trays are cleaned thoroughly. Drill water is sometimes impure and a deposit of carbonate material can build up on the trays that will fizz readily on the application of acid. This can be mistaken for calcareous sa

Use the following procedure to give provisional indications of carbonate content. Requirements:- Clean sample trays and spot trays Distilled water Dilute Hydrochloric acid Appliance of heat 1. Wash cuttings in distilled water. Mud additives may

be calcareous or oil based fluids or glycol can coat grains (as they are designed to do) and inhibit reactions.

2. Dip the cutting in diluted HCl (10%) for a few seconds in a spot tray. If the sample is a carbonate it will effervesce and become etched. Etching emphasises the texture and structure and gives you more information on the cuttings.

pot trays for analysis involving acid.

3. If the sample effervesces allow the reaction to go to completion. The insoluble residue may consist of clay, kerogen, anhydrite, pyrite, etc., and helps determine the presence of some otherwise difficult to detect minerals as accessories.

4. If the rate of effervescence is slow then suspect that the cuttings are partially dolomitic.

5. Place suitable washed cuttings in a spot tray, cover with dilute HCL and heat. If the sample is dolomitic its effervescence will increase considerably until a strong reaction occurs. Again check for residues and accessory minerals. Be careful whilst handling hot acid.

6. If the sample fails to effervesce in diluted and heated HCl, it is probably not a carbonate.

Use the Mudloggers calcimeter to get a more reliable measurement of both the calcium and magnesium carbonate in a particular sample. These can be determined by this instrument in a few minutes. This data should complement and not replace your own observations however.

Distinguishing Calcite from Dolomite by Stainin The following procedures outline the technique of calcite-staininMaterials required:

Requirements:- Clean spot tray and 3-5 cm diameter dish Distilled water Dilute Hydrochloric acid Paper Towel to dry Alizarin Red solution

The following procedure is recommended for calcite staining wit

1. The larger dish should be filled about half-full with the2. Wash cuttings or core chips with fresh or distilled wat3. Place the dry fragments into the staining solution. 4. The staining of the fragments becomes visible after o5. Examine the samples in the solution. 6. Calcite will be stained red and dolomite is unstained.

g Techniques

g of ditch samples at the wellsite.

h Alizarin Red;

staining solution. er dry.

ne minute.

The staining solution can be re-used, but must not be poured back into the container of fresh solution.

4.6.9 Description of Evaporite Rocks Overview Evaporites are sediments resulting from the evaporation of saline water. As such they are not sources of hydrocarbons or reservoir rocks, but are important as trap producing mechanisms and seals. They are found in strata of every age from the Cambrian and in many basins throughout the world. There are two methods of evaporite formation:-

Sabkha - From the Arabic term referring to wide, salt encrusted supra-tidal areas such as those in Abu Dhabi. Occasional flooding of the surface and subsequent evaporation of the seawater leads to deposition of gypsum and salts. Anhydrite is formed by the re-solution and replacement of gypsum. Whilst thin evaporite units are formed using this mechanism, thicker units are harder to account for.

Basinal - This method requires a basin with restricted inflow of saline fluids. The fluids evaporate and evaporites start to form when saline water concentration is above 50% of the original volume. This method of formation is more likely to give thicker evaporite intervals than that of sabkha although it does depend on the frequency of influx waters which may tend to dilute the basinal waters.

Minerals tend to be formed in the reverse order of their solubilities but this also is temperature dependant. Gypsum, for example, will be deposited below 42C, and anhydrite above. Assuming constant conditions, a typical evaporite sequence or cycle is shown below. However, this ideal sequence may be disrupted by influxes of water into the basin, subsequent recrystallisation and temperature changes.

5. Potash and Magnesium Salts KCl (Sylvite) & KMgCl3.6H2O (Carnallite) 4. Halite (Rock Salt) NaCl 3. Gypsum or Anhydrite CaSO4.2H2O , CaSO4 2. Dolomite CaMg(CO3)2 1. Limestone CaCO3

The description of evaporite rocks follows similar categories to those of clastics and carbonates and, apart from rock type recognition will not be expanded upon further than that presented in the table below:-

CATEGORY DESCRIPTION Rock Type A number of identification methods can be used to correctly identify

rock type. Colour Use standard colour charts Hardness and fracture How resistant are the cuttings to applied force Cuttings Shape General shape of cuttings Crystal Size If crystalline Crystal Shape If appropriate Solubility In salts etc. Recrystallisation Evidence for re-crystallisation Porosity/Permeability Visual determinations only. Likely to be almost always zero. Accessory Minerals Where appropriate - likely only to be other evaporites or, rarely

argillaceous material Unusual Features Microfossils, fissures, etc. Hydrocarbon Shows Not normally associated with evaporites

Identification of Evaporites - Rock Name Identification of evaporite rocks at the wellsite is sometimes straightforward, at other times it is anything but. The mudlogger must use a number of different lines of evidence to deduce the presence of evaporites and to identify them correctly. Correct identification is useful as, because the deposition of evaporite rocks is cyclical in nature, then it may give evidence as to positioning within a particular section The main identification tools at your disposal are:-

Drill rate and torque LWD/MWD - Wireline Mud chloride levels Cuttings quality and quantity Gas levels

Colour of cuttings Solubility of cuttings Taste of cuttings Chemical tests

These are summarised in the table below: -

Identification Tool Description Drill rate and torque Drill rate and torque may give early indications of evaporites. Anhydrite will drill very slowly, may

cause bit bouncing, and torque will be fairly consistent. Salts, on the other hand, tend to drill fairly quickly with a uniform drill rate and low, uniform torque.

LWD/MWD & Wireline The LWD/MWD tool even with a basic gamma ray configuration is an important indicator of evaporites. Most evaporites will have a very low, if not zero, reading. However, the potassium salts will give readings in excess of 200 API and even 500+ in the case of sylvite. More advanced LWD tools may give densities which are useful in rock-type identification. Wireline tools may help with further identification at the end of a hole section or if intermediate logs are run.

Mud chloride levels If a salt saturated water based mud or oil based mud with water phase salt saturated is used then salt will be seen at the surface. If a salt is drilled and the mud is not salt saturated then the chlorides content of the drilling fluid will increase dramatically. The salt bed will gradually dissolve and cause hole enlargement (is lag increasing?) and the mud will gradually salt saturate if the bed is thick enough. If you suspect that salt beds have been drilled then get the Mud Engineer to do a chloride check on the mud to confirm it. Alternatively, if the Mud Engineer gets an unexpected increase in chlorides he may ask you if salt beds have been drilled. If this is the case then check some of the other indicators. Note, however, a complication brought about by bottom hole temperatures. As temperature increases with depth, the saturation point of any particular drilling fluid will also increase. Mud systems that are salt saturated at surface temperature may be undersaturated at depth (providing the bottom hole temperature in a sequence of salt is high enough) and take salt into solution. When circulated up the annulus the mud cools and the saturation point falls, resulting in precipitation of salt. This can result in the confusing situation where salt continues to be seen in the cuttings some time after the salt bed has been penetrated.

Cuttings quality and quantity

There will be no change in cuttings when drilling the harder evaporites in the sequence such as anhydrite, dolomite etc. However, with a mud that is not salt saturated there will be a marked decrease in cuttings quantity when drilling salts. This is because the salt is dissolving into the mud and thus all the cuttings will dissolve before they reach the surface. This will result in the shakers appearing clean and the Mudloggers complain that they have no sample to collect! Ensure that bags are suitably marked up to explain the lack of sample. Do not just throw the bags away as confusion may later ensue when the samples are processed onshore - where are the missing samples?

Gas levels

Because evaporites are essentially lacking in porosity, gas levels are generally very low when drilling them. Of course if there is any secondary porosity some gas may be found and the gas levels may be masked if gas is seeping into the hole at other levels.

Colour of cuttings Most of the evaporites are colourless to white or very light grey with some translucence. Polyhalite may have a red tinge, whilst anhydrite is white, hard and dense. Use colour comparison charts as appropriate.

Solubility of cuttings Cuttings of salt can look very similar to sand grains in some circumstances. However, the salt will crush quite easily and is readily soluble in fresh water.

Taste of cuttings

Salt obviously tastes salty! Some of the salts, such as carnallite, can taste particularly bitter whilst other evaporites, such as polyhalite and anhydrite have no taste.

Chemical tests

There are basic chemical tests that can be carried out to identify common evaporites. Mudlogging units should have the necessary chemicals and you should check at the beginning of the well that they have these if evaporites are prognosed (see next page).

Mineral Chemistry Colour Taste Solubility S.G. Sonic DT GR CommentsAnhydrite CaSO4 White, light grey, rrly lt blue None None 2.96 50 0 Very hard, bit may bounce

Barytes Ba(SO4) colourless, white, yellow None None 4.40 - - Rare, commonly associated with anhydrite

Bischofite MgCl2.6H2O Colourless, white Very bitter

V Soluble 1.60 - 0 Hole enlargement if mud not salt saturated

Carnallite KCl.MgCl2.6H2O White, slightly red & yellow Very bitter

V Soluble 1.61 78 200 Hole enlargement if mud not salt saturated

Epsomite MgSO4.7H2O Colourless, white Very bitter

V Soluble 2.10 Similar to Carnallite

Glauberite Na2SO4.CaSO4 Colourless, white, grey, yellow white

- None 2.70 - 2.80

- - Rarely developed, found with mirabilite

Gypsum CaSO4.2H2O Colourless, white, rr pink yellow and grey

None None 2.32 52.5 0 Above 500-700 m only

Halite NaCl Colourless, tinted when impure Salty Soluble 2.17 67 0 Hole enlargement if mud not salt saturated, uniform drill rate

Hexahydrite MgSO4.6H2O - - - - - - RareKainite MgSO4.KCl.3H2O Colourless, white & light grey Very

salty & sl bitter

Soluble 2.13 - 225 Rare, commonly associated withhalite

Kieserite MgSO4.H2O Colourless, white & light grey None V Soluble 2.57 - 0 Often associated with Sylvite

Langbeinite K2SO4.2MgSO4 Colourless, white, occasionally sl pink

None V Soluble 2.83 52 275 Rarely developed

Mirabilite Na2SO4.4H2O Colourless, white Salty Soluble 1.4 - 1.5

- - Fairly rare. Associated with halite

Polyhalite K2SO4.2MgSO4.2CaSO4.2H2O

Grey white, sl yellow & red None V Soluble 2.78 57.5 180 Often associated with Anhydrite

Sylvite KCl.MgCl2.6H2O Colourless, white Bitter and salty

Soluble 1.98 74 >500 Similar to rocksalt

The table below summarises evaporite rock properties: -

Barium Chloride test for Anhydrite and Gypsum

Requirements:- Clean test tube Distilled water Dilute Hydrochloric acid Filter Paper Barium Chloride

The presence of either anhydrite and gypsum can be confirmed by the following test:

1. Pick out several good cuttings from the sample and place them in a test tube.

2. Fill the test tube with distilled water and wash the grains thoroughly by

shaking the test tube vigorously. 3. Pour off the water and then repeat the washing sequence again. 4. Add dilute hydrochloric acid. 5. Shake the test tube vigorously. 6. Filter the solution 7. Add two drops of barium chloride. 8. The presence of calcium sulphate (indicative of anhydrite or gypsum) will

be confirmed by the appearance of a pearly white discoloration of the fluid.

Evaporation test for Anhydrite and Gypsum

Requirements:- Clean evaporating dish Distilled water Dilute Hydrochloric acid Small heater

The presence of either anhydrite and gypsum can be confirmed by the following test:-

1. Wash a small quantity of cuttings in distilled water. 2. Dissolve the cuttings in dilute hydrochloric acid in an evaporating

dish. 3. Put the dish on a small heater or other warm surface. 4. If calcium sulphate is present a small rim of acicular crystals will

develop.

4.6.10 The Description of Oil and Gas Shows Overview One of the most important duties of the mudlogger is to systematically examine all samples for hydrocarbon shows. This following subsections:-

Define a show Give an overview of factors which influence shows Describe gas shows and how to evaluate them Describe oil shows and how to evaluate them

What is a Show? A show may be defined as the presence of hydrocarbons, as liquid or gas, in the cuttings or drilling fluid returned to the surface. A proper show means significant amounts of hydrocarbon and not just a few ppm of C1. Defining significant is not easy as it will vary from well to well. Experience will teach you what is significant and these subsections try to give some background information on this.

Factors which Influence a Show: A number of factors are known to influence a show:-

Factor Description Rock Properties The type and magnitude of the porosity and permeability greatly influence the magnitude or amount of

any show. Highly porous and permeable rocks are rapidly and often completely flushed by the mud filtrate so there may be a large show in the mud stream, detected by oil in the pits, while the cuttings will have very little residual oil and only a poor show. Impermeable rocks tend to retain their formation fluids throughout the drilling process so there will be a small show in the mud stream but a comparatively better show in the ditch samples. Vuggy or fracture porosity has a very high permeability coupled with a very simple pore geometry. This type of rock may be flushed almost entirely of its contained fluids the instant it is penetrated so that the shows will be quite small, even for good producing intervals.

Type of Hydrocarbon

In general, heavy oil will be flushed less easily than light oils or gas. The in-situ reservoir conditions will govern the gas-in-mud and oil-in-mud concentration. Solution of large amounts of gas in oil will result in shrinkage of the oil, diminishing the amount that will enter the mud. Dry gas and distillate reservoirs are difficult to evaluate because of the lack of oil in either the mud or the cuttings. For these, a chromatographic analysis of the mud gas is very useful. An oil reservoir is easier to detect because the cuttings provide direct evidence.

Drilling Rate The magnitude of the show in the drilling mud will be directly proportional to the rate of penetration because this rate governs the rate at which hydrocarbons are added to the mud stream. A rapid penetration rate reduces the time the formation is subjected to the differential pressure that may exist between the mud and formation fluids, thereby diminishing the flushing effect.

Density and Viscosity of the Drilling Fluid

The greater the density of the drilling fluid, the greater will be the pressure differential existing between the mud and the formation fluid pressure, resulting in increased flushing action. Jet drilling bits also increase the flushing action. If the mud is too light, there will be a tendency for the formation up the hole to bleed gas into the mud and provide an undesirable background of gas. When the mud has a high viscosity, the release of the gas from the mud at surface is inhibited. If the mud is not de-gassed efficiently, the gas detector will show a large and persistent background reading that may obscure a genuine show. If the gas in the mud continues to build up, the gas detector may eventually become saturated and thus not be able to log any new shows.

Depth of the Well A deep well is usually associated with high pressure differentials and slow drilling rates, both of which will reduce the magnitude of any shows. In deep wells the hole size is usually smaller so that less rock is pulverised per meter of penetration. Circulation times are longer in deep wells, resulting in greater chances for mixing and dilution of hydrocarbons in the cuttings.

Miscellaneous Drilling Conditions

Occasionally the geologist will encounter an anomalous show, i.e. one that just does not seem valid. For gas shows, suspect the addition of some chemicals or diesel oil to the mud. Some additives used in the mud may cause it to foam.

4.6.11 Evaluation and Description of Gas Shows Gas Basics The evaluation of gas shows can be more subjective than that of oil shows. You are dependent on the Mudloggers gas system working properly. If the gas system does not work then you will have no gas to record and analyse. This is why it is most important that you ensure that the Mudloggers gas system is regularly maintained and calibrated. This is one of the more important aspects of your QA of the Mudlogging Unit. The main hydrocarbon gases recorded by the Mudlogging unit devices are:-

Methane C1 CH4 Ethane C2 CH3, CH3 Propane C3 CH3, CH2, CH3 Butane iC4, nC4 CH3, (CH2)2, CH3 Pentane iC5, nC5 CH3, (CH2)3, CH3

C1 through C5 are the common names used to denote these gases at the wellsite. The relative proportion of these gases can give an indication of the type and composition of reservoir fluids (See Gas Ratios below). Gas is recorded by the Mudloggers in two ways:-

Total Gas: All hydrocarbon gases totalised and constantly measured and recorded in percentage ( 1% = 10,000 ppm)

Chromatograph: Components, usually just C1 to C5 recorded cyclically every few minutes (ppm).

The Total Gas trace is the one that is usually reviewed and which provides a continuous monitor of gas levels produced from the well. It is measured as a percentage gas in air. It must be realised that these gas readings are all relative. The values recorded depend on so many factors that the levels are only a qualitative and not a exact quantitative value. With this in mind it must be realised that it is impossible to compare values between wells and, sometimes, within the same well. Gas levels are affected by: - Mud Properties The higher the overbalance, the lower the gas levels will be in the mud. Also gel strength has

an effect, a thick mud will retain the gas more. Gas Trap Efficiency The positioning of the gas trap in the ditch/header tank is crucial. If it is not placed in the

correct position it may miss the main flow and gas readings will be lower. The trap itself may be too high or low in the mud, partially blocked with cuttings etc. These factors will affect the mud flow through it and the amount of entrained gas that can be released. The Mudloggers should always check the trap regularly. Some gas traps have an air slot to dilute the gas. If this becomes partially blocked the gas values will be artificially enhanced.

Configuration of surface system

Gas may be lost at the bell nipple, flow line (if partially uncovered) and from the ditch/header tank. More gas will be liberated if the ditch has a larger surface area.

Gas Detectors Gas levels will be affected by the efficiency and calibration of the Mudloggers system. Hole Size The bigger the hole drilled the more gas will be released per depth unit penetrated.

All these factors affect the gas levels and the amount of a gas show. Whenever a gas show is reported it must also be in conjunction with the reported background levels. For example if a connection gas was reported as 5% then this may seem significant, but if the true value was actually 1% over a background of 4% then this will not be as significant.

Gas Definitions The following definitions are provided for a better understanding of the various origins of gas shows, and to establish a standard terminology for reporting them.

Type Description Background gas

This is the sustained level of gas continuously carried in the drilling mud over a period of time. 'Continuously carried' does not mean that entrained gas in the annular mud is not knocked out when it reaches the shaker, sandtrap and de-gasser. Background gas enters the annular mud at one or several points of the open hole and is dissipated at the surface. The gas may bleed from a specific formation up-hole, or it may be the gas released from cuttings. The best way to estimate background gas is to eye-ball it on a chart or plot. You can quickly eliminate any bad readings.

Liberated gas

Gas that is liberated by the drilling process as the bit cuts the formation. Some gas will be liberated from the cylinder of rock cut and will be entrained in the mud, other gas will be remain in the cuttings.

Produced gas

Gas that seeps into the wellbore from formations already penetrated. This condition will exist when a state of underbalance exists.

Gas peak When the gas level rises above average readings and peaks out over a short period of time. Maximum gas levels should be recorded by Mudloggers on a gas peak summary sheet. Total gas and chromatographic breakdown should be measured and recorded above background.

Recycled gas

After a large gas peak much of the gas will be extracted as it passes through the shaker and down through the surface systems. However, if the mud is thick it is possible for some of the gas to remain entrained in the mud and be re-circulated round the system. A muted and much broader gas peak will be recorded one full circulation time later. If this starts to become a problem then the rig de-gassers should be used.

Gas show Any deviation in the amount or composition of gas from the established background. A gas show may or may not indicate a significant gas accumulation.

Trip gas The mud circulated out of the hole following a trip often contains formation fluids and gases. It may come from one or several zones of the open hole, but usually comes from the bottom where the mudcake has not had time to seal effectively. Trip gas can be a show just encountered prior to making the trip. Trip gas will usually peak and decrease rapidly. If high gas readings persist for some time after bottoms up, it might be a legitimate show. If sufficiently large, not all of the trip gas will be removed from the mud system by the degasser. In this case it is usual to see an anomalous, but much muted, gas peak one full circulation after the initial appearance of the trip gas, as the gas entrained in the mud is re-circulated. Trip gas should be measured at its peak. It should be recorded as the levels above background.

Connection gas

This develops when drilling is interrupted to add a joint of pipe to the drilling string. Bottom connection gas results from gas introduced at the bottom of the hole through swabbing action of the bit and collars passing freshly drilled hole, and the difference in hydrostatic pressure between static mud weight and effective circulating density when pumps are stopped. Connection and trip gas are often good indicators of the pressure differential across the bottom of the hole, and, used in conjunction with other formation pressure sensitive parameters, (e.g. cuttings size and shape, drill rate etc.), provide useful data on the degree of mud hydrostatic overbalance or underbalance. Connection gas need not come from the bottom of the hole, however, careful observation of lag times is required to check for this about right is not good enough! It only takes a few strokes to clear hundreds of feet of small diameter, collar filled hole. Connection gases should be measured at the peak and recorded as values above background.

Kelly Air or 'Top connection gas'

Is the result of air being introduced into the drill string at the surface during a connection. This causes, after the connection is made, a lightened section of mud to be pumped through the system and can become contaminated with entrained gases. It can give a small gas peak a full circulation after a connection.

Swab gas: Very similar in mode of occurrence to connection gas but caused when the pipe is pulled up but not at a connection. Swab gases should be measured at the peak and recorded as values above background.

Micro- or Blender gas:

(Very rarely used offshore nowadays). During the course of drilling it is important to compare the continuously measured gas from the ditch trap with a single, independent sampling technique. Specific quantities of mud or cuttings are placed in a blender with a measured quantity of water. This is capped and agitated and the released gas is drawn off by vacuum across to a gas detector. This system, called microgas-analysis (the results being called blender gas), shows the amount of gas trapped in the mud or cuttings. The units of gas measured from diluted mud should be 1/2 to 2/3 less than the ditch gas readings from the same ditch mud. When it is equal to or greater than a corresponding ditch gas measurement, the funnel viscosity may be dangerously high and the PV/YP properties of the drilling mud (yield point in particular) are not correctly proportioned. Blender gas values cannot be compared on a parallel basis with the ditch gas readings but are indicative of the porosity-permeability characteristics important to the geological evaluation of the formation.

Gas Kick This has two meanings. The usual meaning is a sudden influx of gas into the wellbore i.e. the well kicks. However, some geologists refer to a sudden increase in gas levels, incorrectly, as a gas kick. This leads to misunderstandings and unnecessary panic so the term should always be qualified with an explanation and only use it in its well control context.

General Hints, Tips and Guidelines The following detail some general hints, tips and guidelines to ensure that you produce the best descriptions under the prevailing conditions.

Sample Catching Quality There is not much point making elaborate descriptions if the samples are not being collected properly and the sample is not representative. It is a wise precaution to explain to each Mudlogger exactly how you want the samples caught. An overview of sample collection can be found in the Miscellaneous Section at the back of this manual. Things to check regularly:-

Occasionally check how the Mudloggers or sample catchers are catching the samples to ensure consistency. Is the sample representative?

Are there differences between individuals in how they carry out sample catching? It can have noticeable effects on sample appearance. Do the samples noticeably change at Mudlogger shift changes?

Is the lag correct? Do samples tie in with drilling breaks or LWD responses? A lag check should be performed at least once a day.

Sample Processing Quality It is also important that samples are processed correctly. A representative sample should be taken from the fine sieve and, ideally, the sample on the cuttings tray should be uniformly one cutting deep across the tray for optimum descriptive conditions. Again, it is useful to show the Mudloggers exactly how you want the samples presented. Things to check regularly:-

Samples are being processed in the correct manner. A representative sample is being taken from the sieve. Make sure sample processing such as shale densities are carried out

consistently on each shift.

Other useful tips

If the formation is unconsolidated, soft and sticky then it is highly likely that soft clays will be easily washed away. Over zealous sample washing can completely wash away soft clays. Get the Mudloggers to put a small pile of unwashed sample on the tray in these circumstances.

Ensure samples are washed to same amount and that drilling mud is washed away. This especially applies to oil and glycol based mud. Insufficient washing can cause grains to have a coating around, or stain on, the grain which makes analysis difficult. The colour is masked and it prevents the cutting reacting with acid properly.

Always check the coarse sieve from time to time to check for cavings. Large cuttings found here are also useful as they give better clues to rock fabric and allow you to make better descriptions. Provenance of large cuttings/cavings is, however, often questionable, so be careful with these.

Ensure amounts, type and size of cavings are monitored closely. When cavings are present or suspected, this fact should be noted on the worksheet. For example, drilling in a limestone succession whilst experiencing cavings from overlying shales, the percentages may be 90% shale, 10% limestone. These would be recorded with the added note: 'shale all cavings'

Macroscopic as well as Microscopic As well as describing individual samples the mudlogger should also look at the broader view. Lay out sample trays - either metal trays or plastic samplex trays - on a bench in the unit in depth order. By doing this subtle color or texture changes can be seen which may be missed in individual samples. If changes do occur then look again at individual samples to see if any changes have been missed. Do these changes tie in with tops? These changes can be used for descriptive intervals on logs and reports.

Equipment for Sample Description Equipment List The following list shows the all equipment required for proper sample description and analysis. The list, below, outlines who should provide such equipment in normal circumstances.

Equipment Who provides it? Notes Microscope Mudloggers Should be kept clean and periodically maintained.

Keep acid fumes away from lenses. Microscope Lamp Mudloggers Should give adequate light even on highest

magnification. Sample trays Mudloggers Should be metal trays. Ensure they are kept clean

and rust-free. Check there is no build up of limescale on tray surface.

Prod, tweezers Mudloggers These should be good quality and not bent. Good tweezers are essential for manipulating individual cuttings samples. If they are bent ask the Mudloggers to order new pairs.

Spot Trays Mudloggers These enamel trays can be used for examining individual cuttings for shows and chemical tests. Tests using acid should always be done in the spot trays. They should be washed thoroughly after use.

UV box Mudloggers Essential for evaluation of shows. Should be either close to fume cupboard or should have means of evacuating fumes.

Colour Comparison chart

Mudloggers The AAPG color comparison charts should be used by all geologists to ensure consistency.

Percentage/Grain size charts

Mudloggers All the mudlogging companies have their own handy visual comparison & grain size/roundness.

Chemicals Mudloggers Acids, chemical thinner, alizarin red. Ideally should be kept in fume cupboard.

Sample Description Sheet

Client Pads of description sheets are provided by client. These should be filled in as fully as possible.

Plastic Samplex Trays

Mudloggers The loggers have only a limited supply of metal trays. These trays are good for comparing samples over significant depth intervals.

Abbreviations list Client Most clients have a standard list of descriptive abbreviations both in summary and in full. Use standard abbreviations if required.

SAMPLE DESCRIPTION TECHNIQUESGeneral HintsSystematic Sample DescriptionIntroductionSample Description Overview

General Sequence of DescriptionEstimating Percentages

Description of Clastic RocksOverviewRock TypeColour.Sandstones and coarser

Hardness and fractureLooseVery softCrumbly

Cuttings ShapeGrain Size.Grain Shape and surface featuresAngularPitting

SortingPorosity and PermeabilityTrace

Cementation and MatrixCementsMatrix

Unusual FeaturesAccessories/ModifiersRare

4.6.8Description of Carbonate RocksIntroductionRock TypeChalkMarlArenaceous Limestones

Determinative Testing for CarbonatesDistinguishing Calcite from Dolomite by Staining Techniques

4.6.9Description of Evaporite RocksOverviewRock Type

Identification of Evaporites - Rock NameDrill rate and torqueLWD/MWD & WirelineMud chloride levelsGas levelsColour of cuttingsSolubility of cuttingsTaste of cuttingsChemical tests

Barium Chloride test for Anhydrite and GypsumEvaporation test for Anhydrite and Gypsum

4.6.10The Description of Oil and Gas ShowsOverviewWhat is a Show?Factors which Influence a Show:

4.6.11Evaluation and Description of Gas ShowsGas BasicsMethane

Mud Properties

Gas Definitions

General Hints, Tips and GuidelinesSample Catching QualitySample Processing QualityOther useful tipsMacroscopic as well as Microscopic

Equipment for Sample DescriptionEquipment List