Properties of rock described by mud loggers
It is recommended that colors of rock be described by mud loggers on wet samples under ten-powered magnification. The wet surface will decrease its value (lightness) of color, but does not change the chroma (color saturation). Dried cuttings may be viewed to allow better discrimination of subtle hues and color shades. The GSA Rock color chart that is supplied in every INTERNATIONAL LOGGING OVERSEAS unit could be used as a basis of color determination.
Rock color may be due to:
• Mass effect of the colors of its constituent grains
• Cement or matrix color
• Staining of cement or matrix
Below is a table of colors that are imparted by materials found in rocks.
When describing color, mud loggers should attempt to distinguish between:
1 Rock Particles
3 Matrix and Cement
Rock color may occur in combination or in patterns, e.g. mottled, banded, spotted or variegated. Suitable descriptions are:
Speckled, Spotted: marked with small colored spots
Banded – colors arranged in bands
Iridescent – it has a lustrous rainbow-like play of color caused by differential refraction of light waves (as from an oil slick) that tends to change as the angle of view changes.
Color can be a useful indicator of the depositional environment, especially in argillaceous rocks.
Colors of cuttings
The colors of cuttings could be altered after the samples are caught. Metal fragments in samples can rust and stain the samples. Diamond and sometimes PDC bits could produce very badly streaked and ground-up cuttings.
Color, in carbonates, may be of less importance than in clastics. Variations in color may be the result of the presence of detrital material (clay) or from the substitution of metallic ions into the mineral lattice.
When describing the color of carbonates, always stress the predominant color, though on many occasions the color will be mixed. When this occurs, use a suitable modifying adjective (e.g. variegated, mottled).
Texture is a function of the physical makeup of rock–namely, the size, shape, and arrangement (packing and orientation) of the discrete grains or particles of sedimentary rock.
Cuttings Shape (Argillaceous, Calcareous and Chemical Rocks)
Descriptive terms used for argillaceous, calcareous and noncarbonate chemical cuttings are:
* Blocky – chunky appearance, often used with subblocky
* Subblocky – often used with blocky
* Amorphous – having no definite form; it is quite common with claystones
found in the top hole section of the well
* Flat or Tabular
* Platy – flaky; often used to describe shale cuttings; often used with subplaty, fissile and subfissile
* Subplaty – often used with platy
* Fissile – capable of being split or divided in the direction of the grain or along natural planes of cleavage; a characteristic of shale; often used with subfissile and platy and subfissile
* Subfissile – often used with fissile; a property of shales
* Splintery – used to describe some carbonates, coals and shales
The mud loggers should always distinguish between shale, which exhibits fissility, and claystone, which yields fragments, which do not have parallel plane faces.
These terms are not used for the arenaceous / rudaceous cuttings.
Grain or Crystal Sizes (Arenaceous, Rudaceous lithologies and Clastic Carbonates)
size is a measure of the energy of the system where the rock was deposited.
Along with sorting, it has a direct bearing on porosity and permeability. For example, coarse-grained sandstones have often, but not always, larger pore throats, and thus have a better permeability, than finer-grained deposits. Size classifications are based on the Wentworth scale.
The mud loggers should always use a standard grain comparator get an accurate visual estimate when he is recording the grain or crystal size. INTERNATIONAL LOGGING OVERSEAS provides a mounted sieved sand grain standard comparator.
The comparator could be placed either on top or beside the cuttings in the sample tray.
The weighted average, taking into account the amount of each grain size, is reported.
Where the largest grains present are much larger than the weighted average, the maximum size should be reported (e.g. gen f-m, occ vf). When the range of grain sizes is so large and diverse the minimum and maximum of the range should be noted (e.g. vf –c).
Carbonate rocks contain both allochemical grains (grains produced by precipitation somewhere else and transported, usually short distances, to the depositional site and chemically precipitated minerals (either as pore-filling cement, primary ooze or as products of recrystallization and replacement). Therefore, the grain size described must be a double one, so that one can distinguish which constituent is being described.
Grain shape indicates the length of transport. It 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. Roundness refers to the extent to which originally angular edges and corners of the grain or sediment have been rounded off.
The five degrees of roundness 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 destroyed, but some comparatively flat faces may still be present. All original edges and corners have been smoothened 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.
The Grain Size Comparator like the figure above can be very useful in shape determination.
In addition, other descriptive terms used for carbonate grains and particles may be used to supplement the above descriptions. For example:
Sharp Elongate Bladed
Flat Rod-like Blocky
Platy Conchoidal Irregular
Disk Faceted Fibrous
Carbonate rocks contain five categories of grain/particle types (see table below). The roundness terminology used for clastic rocks may be used by mud loggers to describe such particles.
Sorting refers to the uniformity of grain size in a sediment or sedimentary rock.
Particles become sorted based on density because of the energy of the transporting medium. It involves shape, roundness, specific gravity, mineral composition and size.
It’s the most difficult and subjective assessment made by the mud loggers during sample descriptions.
Generally understood that if more than 50% of the cuttings are of the same modal size, and then the sample is well sorted. Most descriptions contain:
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 in carbonates (as in clastics) is a function of mean grain size. However, due to the various types of grains in carbonate rocks, very little is known about carbonate sorting.
Grain Size comparators may also be used by mud loggers to get an objective observation of grain sorting in a sample.
Hardness is the resistance to scratching. For cuttings examination, it is the physical parameter based on the amount of force required to break apart the cutting, using a sample probe.
Induration is the process by which sediment is converted into sedimentary rock, generally termed diagenesis. However, it is used also to refer to the hardness of the rock or how easily it breaks apart.
The Moh’s Hardness Scale, though not a quantitative scale, defines a mineral’s hardness by its ability to scratch another (one with a lower number).
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 by mud loggers 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.
Rudaceous and Arenaceous Rocks
Common descriptions for Rudaceous and Arenaceous Rocks are:
• Unconsolidated or Loose: Cuttings fall apart or occur as individual grains.
• Friable: Rock crumbles with light pressure. Grains detach easily with a
• Moderately Hard: Grains detach with a sample probe. Cuttings can be broken with some pressure.
• Hard: Grains difficult to detach. Extreme pressure causes cuttings to break between grains.
• Very Hard: Grains cannot be detached. Cuttings will break through the grains.
Common descriptions for Argillaceous Rocks, Carbonates and some Chemical Rocks are:
• Soluble: Readily dispersed by running water.
• Soft: No shape or strength. The material tends to flow.
• Plastic: Easily molded and holds shape. Difficult to wash through a sieve.
• Firm: Material has a definite shape and structure. Readily penetrated and broken by sample probe.
• Moderately Hard: Cuttings can be broken with some pressure.
• Hard: Sharp angular edges. Not readily broken by a probe.
• Very Hard: A lot of force is applied to break the cutting.
Other descriptive terms used for Argillaceous Rocks, Carbonates and some
Chemical Rocks include:
• Brittle – easily broken, cracked or snapped
• Crumbly – easily disintegrates into small pieces; friable
• Dense – compact
Luster is used to define the surface features of the cutting under reflected light. It is best to observe the surface texture with the naked eye and under the microscope, and with wet and dry samples. Rotating the sample tray under the light source also assists in describing the sample’s luster.
Common terms are:
• Coated – Precipitated or accretionary material on the surface of the cutting.
The cutting is not thick enough to develop a visible color.
• Vitreous, glassy, faceted – It appears as clear, shiny, fresh appearance.
• Silky, pearly (nacreous), polished – It appears as lightly etched or scoured.
• Frosted, dull, etched – It appears as deeply etched or scoured.
• Pitted – It appears as solution or impact pits, often pinpoint size.
• Striated – It appears as parallel abrasion lines or scratches.
Aside from the common textures discussed above, argillaceous rocks also exhibit observable surface texture under reflected light. Common terms are:
In carbonates, additional terms are used like:
• Rhombic – perfectly formed rhombs of nearly equal size, medium to coarse (usually pure dolomite)
• Sucrosic – Sugary, similar to rhombic, but finer, lacking the perfection of
crystal form (usually calcitic dolomite)
• Microsucrosic – very finely sugary, often quite friable (usually calcitic
• Grainy – Not vividly crystalline, but with definite grains, often chalky in part (limestone or dolomitic limestone)
• Oolitic – Spheroidal or smooth-surfaced grains with concentric internal
Slaking and Swelling
Marked slaking and swelling in water is characteristic of montmorillonite (a major constituent of bentonites) and distinguishes them from kaolins and illites. To test for swelling by mud loggers add water to the cutting and observe. Below are the terms used to describe slaking and swelling:
• Non-swelling – doesn’t break up in water even after adding 1% HCl
• Hygroturgid – swelling in a random manner
• Hygroclastic – swelling with irregular pieces
• Hygrofissile – swelling into flakes
• Cryptofissile – swelling into flakes only after adding 1% HCl
Cementation or Matrix
The difference between “cement” and “matrix” is one of degree, and may not be obvious in the cutting. Cement is deposited chemically and the matrix is deposited mechanically. Cement is a chemical precipitate deposited around the grains and in the interstices of rock as aggregates of crystals. The matrix consists of small individual grains that fill interstices between the larger grains. In general, where intergranular contact does not occur, the fill material between grains is a matrix.
Common cementing agents are:
• Sulfates (Gypsum, Anhydrite)
Minor cementing agents are:
• Iron Oxides (Hematite, Limonite)
• Phosphatic material
Chemical cement is uncommon in sandstone that has an argillaceous matrix. The most common cementing materials are silica and calcite.
Silica cement is common in nearly all quartz sandstones. Other siliceous cements are composed of opal, chalcedony and chert.
Dolomite and calcite also occur as a cement in sandstones (deposited as crystals in the interstices and/or as aggregates in voids) but it should be noted that both could occur as detrital or as coatings around the sand grains before these grains were lithified grains in the sandstone.
Anhydrite and gypsum cements are more commonly associated with dolomite and silica than with calcite.
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 a loss of porosity.
The term “cement” should not be used in recrystallized carbonates.
Fossils and Accessories
Microfossils or even fragments of fossils are used for correlation. Common fossils and microfossils encountered are: foraminifera (like Globigerina), ostracods, bryozoa, corals, algae, crinoids, brachiopods, pelecypods and gastropods. The mud loggers should record their presence and relative abundance in the samples being examined.
Fossil amount is estimated as:
10 to 25% Common
< 10% Trace
Accessory constituents or minerals may be significant indicators of the n depositional environment. They may also be used in correlation. The most common accessories are feldspars (both plagioclase and K-feldspar), micas (muscovite and biotite), chlorite, pyrite, glauconite, siderite, carbonaceous material, heavy minerals (magnetite, zircon etc), chert, and lithic fragments.
The minor accessories in carbonate rocks are commonly detrital or diagenetic products of terrigenous rock fragments with some carbonate terrigenous diagenetic minerals. The presence of sulfur and metallic sulfides is common and is an indication of anaerobic conditions of deposition and possible hydrogen sulfide hazards.
Most sedimentary structures are not discernible in sample cuttings but could be seen
in cores. There are however some structures that are evident in cuttings:
• Fractures / Microfractures – usually deduced because of the presence of
some type of filling
• Vugs / microvugs
• Micro-veinlets / veins
• Slickensided surfaces – should be carefully scrutinized; bits will scrape the
sides of the borehole, giving a striated/glassy appearance on the cuttings
Visible porosity is easier to determine on a dry sample by mud loggers than on a wet one. A magnification of 10x is frequently adequate to establish the amount of relatively visible porosity in a dry sample. Higher magnification is occasionally needed to determine relatively visible porosity. Theoretical maximum porosity for a clastic rock is about 26%. This is normally much reduced by other factors. When estimating porosities use:
The mud loggers should never use the numerical values in estimating porosity, just the descriptive terms.