FE 02-16G Sample Description Format

Nội dung text: FE 02-16G Sample Description Format

1. Sample Description • Major functions • Porosity and Permeability • Description format • Describing clastic rocks • Describing carbonate rocks • Describing other chemical rocks • Describing igneous and metamorphic rocks
2. Porosity and Permeability • Porosity is a measure of the volume of void space in the rock. It determines the amount of fluid that is present in a rock. • Permeability is a measure of the capacity of a rock for transmitting fluid and it is dependent on effective porosity and the mean size of the individual pore spaces. It has a direct bearing on the amount of fluid that can be recovered.
3. Sample Description Examples • Sst: lithic, lt gy-off wh, vf-f gr, occ med gr, sbang- sbrd, mod w srtd, fri, sl arg mtx, v wk calc cmt, mica, glau, p-fr vis por, tr-5% blu wh fluor, slow strmg bl wh cut, no cut color, no res, p oil show. • CLYST: lt gy-med gy, occ dk gy, sbblky-blky, mod hd, mic mica, sl calc. • Ls: oolitic grainstone, buff-brn, med gr, mod hd, arg, Brach, glau, gd vis por, no oil show
4. Color • Rock color may be due: – Mass effect of the colors of its constituent grains – Cement or matrix color – Staining of cement or matrix • Use a GSA rock color chart for standardization of color • Observe samples when they are wet • Dried cuttings may be viewed to allow a better discrimination of subtle hues and color shades • When describing color, distinguish between rock particles, staining, matrix/cement and accessories
5. Color: Depositional Environment Color Depositional Environment Red and brown Oxidizing environment Green & grey Reducing environment Dark brown Possible source rock Black Anaerobic environment
6. Cuttings Shape • Blocky Subblocky • Amorphous Elongate • Flat or Tabular Platy or flaky • Subplaty Subfissile • Fissile Irregular • Splintery
7. Grain Size • Grain size and sorting have a direct bearing on porosity and permeability • Size classifications are based on the Wentworth scale • Report weighted average • If largest grains present are much larger than the average, the maximum size should be reported • If the grain size range is large and diverse, report the minimum to maximum size (e.g. vf – vc) • Use Grain Size comparator chart
8. Sphericity • 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.
9. Particle Shape: Roundness vs. Sphericity
10. Sorting • Sorting is the measure of dispersion of the size frequency distribution of grains in a sediment or rock. It involves shape, roundness, specific gravity, mineral composition and size. • Along with Grain Size both have a direct bearing on porosity and permeability • Most difficult and subjective assessment • A function of mean grain size • If more than 50% of the cuttings are of the same modal size, the sample is well sorted
11. Hardness and Induration • Hardness is a physical parameter based on the amount of force required to break apart the cutting using a simple probe • Induration is the process by which a sediment is converted into a sedimentary rock. It is function of the type and quantity of the cement
12. Luster • Describes the surface features of a cutting under reflected light • Observe features with naked eye and under microscope and when wet and dry • Rotating the sample tray under the light source also helps in describing luster
13. 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 • Add water to dried cuttings
14. Relationship Between Texture, Porosity and Permeability • Porosity (and possibly permeability) may decrease with increased sphericity and rounded grains. • Permeability decreases with decreasing grain size because pore throats are smaller and the capillary pressure goes up.
15. Matrix Materials • Silt acts as a matrix, hastening cementation by filling interstices • Clay is a common matrix material
16. Cementing Agents • Common cementing agents: – Calcite (most common) – Silica (most common) – Sulfates (Gypsum, Anhydrite) – Clays – Dolomite • Minor cementing agents: – Siderite – Fe oxides – Pyrite – Zeolites – Phosphatic minerals
17. Cementing Agents: Sulfate Cements • Anhydrite and gypsum cements are more commonly associated with dolomite and silica than with calcite
18. Fossils and Accessories • Minerals or fossils in trace quantities • Have great diagnostic and descriptive value • If the accessory mineral could not be identified it should be carefully described
19. Fossils: Estimation of Abundance > 25% Abundant 10% – 25% Common < 10% Trace
20. Crystal Structure Terminology • Anhedral - no visible crystal form • Subhedral - partly developed crystal form • Euhedral - well developed crystal form
21. Visual Porosity • Easier to determine with a dry sample than on a wet one • A magnification of 10x is frequently adequate to establish the amount of relative visible porosity in a dry sample. • Samples with good porosity should always be examined for hydrocarbon shows • The porosity in rudaceous and arenaceous rocks is primarily interparticle