Lithology Descriptions
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CATAGORY DESCRIPTION REMARKS ROCK TYPE Sandstone, siltstone, etc. Gradational lithotypes – only be used if there is a significant amount of qualifying rock type present. Consolidation – when soft claystone should be referred to as clay, etc. Contaminants – mud additives, metal shavings and cement, Loose sand grains – are they sand grain? Loose sand grain from deeper section may be sign of overbalance. Metamorphic/Igneous lithotypes – could be difficult to identify. COLOUR Use standard colour charts Red – Orange (Iron-Ferric-oxidized stated); Light green (Iron-ferrous reduced state); Bright green (Glauconite, chlorite & chamosite); Blue (Tuffaceous); Dark grey – brown black – olive black (Carbonaceous materials – anoxic environ, usually marine); Yellow – ochre (Limonite – hydrated iron oxides and iron hydroxides); Brown (Oil – check for shows!). GRAIN COLOUR Mainly applies to sst Describe the colour of constituent grains and clasts, but also their transparent, translucent & opaque nature. Also note surface discolouration of grains or any coloured inclutions. HARDNESS Adjective to describe hardness Loose (lse) – Grains disaggregate when dry (not for clay/shale rocks) Friable (fri) – Loose grains can be separated by pressure from the fingers Firm (frm) – Grains can be separated with probe Hard (hrd) – Grains difficult to detach, pressure results in cuttings breaking grains Very hard (v. hrd) – Individual grains cannot be detached & cuttings break through grains For clay based lithologies Very soft (v. sft) – Can be dispersed by water/drilling mud Soft (sft) – No shape or strength, easily deformed Sticky (stky) – Sticks to fingers and sample probe Plastic (plss) – Easily moulded and retains shape, difficult to wash through sieve Firm (frm) – Definite shape & struc can be penetrated by probe Hard (hrd) – Sharp angular edges, not easily broken. Varies to mod hrd to v hrd Fracture or break of cutting 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 Splintery (splty) – Very hard and splinters into sharp pieces when broken CUTTING SHAPE General shape of cuttings Amorphous (amor) – no shape Blocky (blky) – Square, angular appearance Platy (plty) – Flat appearance with rounded edges Subfissle (sbfiss) – Flatter & more elongate than platy, but not as sharp edged as fissile Fissile (fiss) – Generally flat and elongate with sharp edges Pressure caving – concave, flat and thin. Sometimes very large in size GRAIN SIZE Use standard grain size chart Sand / Sandy (Coarse, medium, fine, very fine grained), Mud (Silt/silty & Clay/clayey – Micro grained) GRAIN SHAPE & SURFACE FEATURES Use grain shape chart Angular (Ang) – Flat, plane surface, terminating in acute or right angles, thin, sharp edges Subangular (Subang) – Flat surface terminating in corners Subrounded (Subrnd) – Rounded corners and increasingly surfaces Rounded (Rnd) – Rounded surface, edges and corners Well Rounded (Wl.rnd) – Becoming spheroidal Secondary mineralization which cause erotion, coatation or overgrowth in original grains Pitting – Surface of grains have small holes caused by chemical solution or physical impacts Staining – Thin veneer of mineralisation with coloured, barely noticeable powdery appearance e.g. iron / oil staining Coating – Thicker veneer of mineralisation on grain surface Frosting – As a white powdery coating to grains caused abrasion of surface
Transcript of Lithology Descriptions
CATAGORY DESCRIPTION REMARKS
ROCK TYPE Sandstone, siltstone, etc.
Gradational lithotypes – only be used if there is a significant amount of qualifying rock type present.Consolidation – when soft claystone should be referred to as clay, etc. Contaminants – mud additives, metal shavings and cement,Loose sand grains – are they sand grain? Loose sand grain from deeper section may be sign of overbalance.Metamorphic/Igneous lithotypes – could be difficult to identify.
COLOUR Use standard colour charts
Red – Orange (Iron-Ferric-oxidized stated); Light green (Iron-ferrous reduced state); Bright green (Glauconite, chlorite & chamosite); Blue (Tuffaceous); Dark grey – brown black – olive black (Carbonaceous materials – anoxic environ, usually marine); Yellow – ochre (Limonite – hydrated iron oxides and iron hydroxides); Brown (Oil – check for shows!).
GRAIN COLOUR Mainly applies to sst Describe the colour of constituent grains and clasts, but also their transparent, translucent & opaque nature. Also
note surface discolouration of grains or any coloured inclutions.
HARDNESS
Adjective to describe hardness
Loose (lse) – Grains disaggregate when dry (not for clay/shale rocks)Friable (fri) – Loose grains can be separated by pressure from the fingersFirm (frm) – Grains can be separated with probeHard (hrd) – Grains difficult to detach, pressure results in cuttings breaking grainsVery hard (v. hrd) – Individual grains cannot be detached & cuttings break through grains
For clay based lithologies
Very soft (v. sft) – Can be dispersed by water/drilling mudSoft (sft) – No shape or strength, easily deformedSticky (stky) – Sticks to fingers and sample probePlastic (plss) – Easily moulded and retains shape, difficult to wash through sieve Firm (frm) – Definite shape & struc can be penetrated by probe Hard (hrd) – Sharp angular edges, not easily broken. Varies to mod hrd to v hrd
Fracture or break of cutting
Crumbly (crmly) – Easily crushed into constituent partsBrittle (brit) – Breaks into small pieces when fracturesConchoidal (conch) – Curved fracture planes such as those seen in flintSplintery (splty) – Very hard and splinters into sharp pieces when broken
CUTTING SHAPE General shape of cuttings
Amorphous (amor) – no shapeBlocky (blky) – Square, angular appearancePlaty (plty) – Flat appearance with rounded edgesSubfissle (sbfiss) – Flatter & more elongate than platy, but not as sharp edged as fissileFissile (fiss) – Generally flat and elongate with sharp edgesPressure caving – concave, flat and thin. Sometimes very large in size
GRAIN SIZE Use standard grain size chart Sand / Sandy (Coarse, medium, fine, very fine grained), Mud (Silt/silty & Clay/clayey – Micro grained)
GRAIN SHAPE & SURFACE FEATURES
Use grain shape chart
Angular (Ang) – Flat, plane surface, terminating in acute or right angles, thin, sharp edgesSubangular (Subang) – Flat surface terminating in cornersSubrounded (Subrnd) – Rounded corners and increasingly surfacesRounded (Rnd) – Rounded surface, edges and cornersWell Rounded (Wl.rnd) – Becoming spheroidal
Secondary mineralization which cause erotion,
coatation or overgrowth in original grains
Pitting – Surface of grains have small holes caused by chemical solution or physical impactsStaining – Thin veneer of mineralisation with coloured, barely noticeable powdery appearance e.g. iron / oil stainingCoating – Thicker veneer of mineralisation on grain surfaceFrosting – As a white powdery coating to grains caused abrasion of surfaceGlassy – Mineral overgrowth into void with flat crystal surface apparent
SORTING Use sorting chartWell sorted – Range of particle size confined to two adjoining grain sizesModerately sorted – Range of particle size confined to four adjoining grain sizesPoorly sorted – m Range of grain sizes over more than four grain sizes
CEMENTATION/ MATRIX Types of cement/matrix
Silica – Most common but difficult to spot. Look for residual surface, flat crystal surfaceCalcite – React strongly with dilute HCl. Cutting should disintegrate into constituent grainsDolomite – Slower reaction, cement may bubble for sometime and cutting start to disintegrate after some timeSiderite – Dull yellow brown & white, will react slowly with application of acidPyrite – Bright yellow gold and metallic. Very recognizable
POROSITY & PERMEABILITY Visual determinations only Take a good look for cementing or is there a common matrix. Sorting also affects porosity. 0-5% = Trace, 5-10% =
Poor, 10-20% = Fair, 20-30% = Good. Type of porosity e.g. intergranular, vuggy, pin-point etc.
ACCESSORIES/ MODIFIERS
Quoted with some qualifier as to abundance
Note any minerals (e.g. glauconite) or inclusions in the sample with relative abundance: 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. If insitu, indicated shelf all marine environ.Pyrite (pyr): Good colour, may be disseminated, small nodules or crystalline fragments.Mica (mic) : Mica flakes in sst whilst small mica fragments (micromica) are common in finer clastic. Chlorite = distinctive green colour.Carbonaceous Mat (carb.mat) : Dark black specks of carbon material are very common.Kaolinite (kao) : White clay material, found as matrix and as decomposed feldspar grains.Coal / Lignite (coal/lig) : Brittle, black coal layers. Lignite is softer and browner.Siderite (sid): Yellow brown or dark brown pellets.
