Lithology Descriptions

CATAGORY

DESCRIPTION

ROCK TYPE

Sandstone, siltstone, etc.

COLOUR

Use standard colour charts

GRAIN COLOUR

Mainly applies to sst

Adjective to describe hardness

HARDNESS

For clay based lithologies

Fracture or break of cutting

CUTTING SHAPE

General shape of cuttings

GRAIN SIZE

Use standard grain size chart

Use grain shape chart GRAIN SHAPE & SURFACE FEATURES

Secondary mineralization which cause erotion, coatation or overgrowth in original grains

SORTING

Use sorting chart

CEMENTATI ON/ MATRIX

Types of cement/matrix

POROSITY & PERMEABILI TY ACCESSORI ES/ MODIFIERS

Visual determinations only Quoted with some qualifier as to abundance

REMARKS 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. 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!). 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. 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 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 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 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 Sand / Sandy (Coarse, medium, fine, very fine grained), Mud (Silt/silty & Clay/clayey – Micro grained) 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 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 Glassy – Mineral overgrowth into void with flat crystal surface apparent Well sorted – Range of particle size confined to two adjoining grain sizes Moderately sorted – Range of particle size confined to four adjoining grain sizes Poorly sorted – m Range of grain sizes over more than four grain sizes Silica – Most common but difficult to spot. Look for residual surface, flat crystal surface Calcite – React strongly with dilute HCl. Cutting should disintegrate into constituent grains Dolomite – Slower reaction, cement may bubble for sometime and cutting start to disintegrate after some time Siderite – Dull yellow brown & white, will react slowly with application of acid Pyrite – Bright yellow gold and metallic. Very recognizable 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. 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.