Some Criteria & Procedures For Rock

2.0. Some Criteria & Procedures For Rock
&
Mineral Identification
2.1. Testing Methods:
2.1.1. Tests with Dilute HCL (10%)
There are different types of observations to be made on the results of treatment with 10 % dilute acid:
a) Degree of effervescence:
Limestone and Dolomite determination are characterised by effervescence values arrived at by the use of 10 % hydrochloric acid, Pure limestone effervesces violently while pure dolomite will not effervesce at all in cold 10 % acid. Dolomite will effervesce slowly if heated in the acid. These components are seldom encountered in a pure state, as usually they are mixtures of one or other predominating or either one mixed with shale. For example, it might be debatable whether to call a material shaley limestone or limy shale. One procedure by which to arrive at a standard for making the distinction is as follows:- Place several pieces of material in a depression of the spot plate and cover with 10 % HCL. If it begins to effervesce immediately the material is probably limy than dolomitic. Warm the spot plate add acid and let at effervesce until additional acid will create no more effervescence. If the cuttings still regain their original general shape, they were limy shale : if they no longer have their original shape but only appear as a residue left behind the limestone was predominant and the cutting were Shelly lime. Impurities slow the reaction, but these can be detected in residues. Oilstained limestones are often mistaken for dolomites, because the oil coating the rock surface prevents acid from reacting immediately with CaCo3, and a delayed reaction occurs. The shape, porosity and permeability will affect the degree of reaction, because the greater the exposed surface, the more quickly will the reaction be completed.
b) Nature of residue:
Carbonate rocks may contain significant percentages of chert, anhydrite, sand, silt or argillaceous materials that are not readily detected in the untreated rock fragments.
Not all argillaceous material is dark colored, and, unless an insoluble residue is obtained, light colored argillaceous material is generally missed. During the course of normal sample examination in carbonate sequences, determine the composition of the noncalcareous fraction by digesting one or more rock fragments in acid and estimate the percentage of insoluble residue. These residues may reveal the presence of significant accessory minerals that might otherwise be masked.
c) Oil reaction:
If oil is present in a cutting, large bubbles will form on a fragment when it is immersed in dilute acid.
2.1.2. Hardness
Scratching the rock fragment surface is often an adequate way of distinguishing different lithic types. Silicates and silicified materials, for example, can not 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 chart 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 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.
2.1.3. Parting (Fissility)
Shaly parting, although not a test, is an important rock character. The sample logger should always distinguish between shale, which exhibits parting or fissility and mudstone, which yields fragments which do not have parallel plane faces.
2.1.4. Slaking and Swelling
Marked slaking and swelling in water is characteristic of montmorillonites (Shale rich in bentonite) and distinguishes them from kaolinite and illites.
2.2.Thin Sections
Certain features of rocks may not be distinguishable, even under the most favorable conditions without the aid of thin sections. Thin sections adequate for routine examination can be prepared without the use of the refined techniques necessary to produce slides suitable for petrography study. Some of the questions of interpretation which might be clarified by the use of thin sections, include the following:- mineral identification, grain-matrix relationships, grain-cement relationships, pore space relationships and distribution, grain sizes, source rock quality. Although wetting the surface of a carbonate rock with water, or mineral oil, permits “in depth” observation of the rock, some particles, or particle-matrix relationships still remain obscure until the rock is examined by transmitted light, plane and/or polarized. Once these features have been recognized in thin sections, they are frequently detectable in whole fragments, and only a few thin sections may be needed in the course of logging a particular interval. It is important to have polarizing equipment available for use in thin section examination – many features of the rock texture, and some minerals, are most readily recognized by the use of polarized light.
2.3. Staining Technique for Carbonate Rocks
The distinction between calcite and dolomite is often quite important in studies of carbonate rocks. For many years several organic and inorganic stains have been used for this purpose, but with varying degrees of success. Friedman (1959) investigated a great variety of stains for use in identifying carbonate minerals. He developed a system of stains and flow charts for this purpose.
These vary in ease of application, but most are not practical for routine sample examination. The reader is referred to Friedman’s paper for an extensive discussion of carbonate mineral stains. One stain that is applicable to routine sample examination and is both simple and rapid is “Alizarin Red S.” This stain can be used on any type of rock specimen, and it has proved especially useful in the examination of cuttings. The reactions to acid of chips of dolomitic limestone or calcareous dolomite are often misleading, and the rapid examination of etched chips does not always clearly show the calcite and dolomite relationships. Alizarin Red S. shows clearly the mineral distribution. Calcite takes on a deep red color; other minerals are uncolored.
2.6. Heavy Mineral Studies
Heavy mineral studies are used today, primarily when a geologist is seeking information concerning the source areas and distribution patterns of siliciclastic sediments. Their use as a correlation tool is limited. Excellent descriptions of techniques are available in the literature.


2.7. Tests for Specific Rocks and Minerals
Many of the more perplexing problems of rock mineral identification can be solved by the use of thin sections. However, certain simple and rapid tests are discussed as follows:
2.7.1. Clay
Shales and clays occur in a broad spectrum of colors, mineral composition and textures. Generally, their identification is done with ease; however light colored clay or kaolinite is commonly mistaken for finely divided anhydrite.
The two may be distinguished by simple tests:
i) Anhydrite will dissolve in hot dilute hydrochloric acid, and when cooled, will recrystallize out of solution as acicular needles. Clay remains insoluble in the hot dilute acid.
ii) Barium Chloride BaCl2 test: anhydrite dissolved in hot dilute acid will get turbid with drops of Barium Chloride solution and a white preciptate will deposite on the bottom of the test tube.
2.7.2.
Chert
Recognition of the more common varieties of chert and siliceous carbonates, generally is not a problem. Weathered chert, however, is often found to be soft enough to be readily scratched and mistaken for clay or carbonate.
Lack of reaction with acid, generally distinguishes this type of chert from carbonates. Clay and tripolitic chert may require petrography techniques for differentiation. In thin sections under polarized light, chert commonly has a characteristic honey-brown color.
2.7.3. Evaporites
a) Anhydrite and Gypsum
They are usually readily detected in cuttings. Anhydrite is more commonly associated with dolomites, than with limestones, and is much more abundant in the subsurface than gypsum. At present, there appears to be little reason to distinguish anhydrite from gypsum in samples. Anhydrite is generally harder and has a pseudo-cubic cleavage; the cleavage flakes of gypsum have “swallow-tail” twins. Anhydrite can be readily recognized in thin sections by its pseudo-cubic cleavage, and, under polarized light, by its bright interference colors. The dilute hydrochloric acid test is a valid and simple test for anhydrite or gypsum in cuttings. Place the cutting(s) in a watch glass and cover with acid.
Heat on a hot plate to 250° F + (120° C +) and wait for the sample to start dissolving. If anhydrite or gypsum is present, acicular gypsum crystals will form around the edge of the acid solution as it evaporates. If the sample contains much carbonate, a calcium chloride paste may form and obscure the acicular gypsum crystals. Dilute the residue with water, extract and discard the solution and repeat the test. *A simple method of distinguishing finely divided anhydrite from silt is a scratch test. This can be done by two methods:-
A. Rub glass rod on residue in bottom of glass test plate and listen for gritty sound.
B. Place
 a drop of liquid containing the residue on a glass cover slip and cover with another slip. Rub them together between thumb and forefinger. Examine slips under microscope for scratch marks, or listen for gritty sound.
b)_Salts
Are rarely found at the surface and generally do not occur in well samples. Unless salt-saturated or oil-base mud is used, salt fragments or crystals dissolve before reaching the surface. The best criteria for detecting a salt section are:-
· While clastics (sands/shale’s) and carbonates (limestone’s/dolomites) tend to drill at irregular ROP’s evaporites generally drill at a fairly uniform rate.
· Chloride content of the drilling mud will increase and the fluid will tend to flocculate (get very viscous).
· Cuttings returns at the shale shaker will be minimal but may indicate small amounts of carbonate material as it is commonly found in association withevaporates.
· A sudden influx of abundant caved material in the samples.
· A sharp increase in drilling penetration rateSalts are commonly associated with cyclical carbonate sections and massive red bed sequences. In the former, they are usually thin bedded and often occur above anhydrite beds. Potassium-rich salts, the last phase of an evaporation cycle, are characterized by their response on gamma ray log curves.
2.7.4. Phosphate
Place on the suspected mineral (either on the hand specimen or on an uncovered thin section) a small crystal of pure white ammonium molybdate. Allow one or two drops of dilute nitric acid to fall on the crystal. If the rock contains phosphate, the crystal rapidly takes on a bright yellow color.
2.7.5. Feldspar
The presence, quantity and type of feldspar constituents can be important in the study of reservoir parameters in some sandstones. Feldspars are usually characterized by variable colours and angular edged particles.
2.7.6. Bituminous Rocks
Dark shales and carbonates may contain organic matter in the form of Kerogen or Bitumen. Carbonates and shales in which the presence of bituminous matter is suspected, should be examined by thin section and pyrolysis-fluorometer methods for possible source rock qualities. Dark, bituminous shales have a characteristic chocolate brown streak which is very distinctive.
GENERAL REMARKS ON SAMPLE DESCRIPTION
1. The major changes in the formations should be noted and the appearance of new formation martial should be described as carefully as possible.
2. The logger upon coming on duty should familiarise himself with the samples from formations drilled while he was off-duty. This will aid him in soothing cavings and changes of formation characters.
3. In some cases the sample retrieved will not represent the formation at all. For example, evaporites sections drill with a fresh water base mud will dissolve the formation salts as they rise in the annulus. In this case the previous mentioned criteriae should be borne in mind.
4. In arriving at the geological descriptions of the formation drilled, such things as drilling rate and present depth of the bit should be taken into consideration. At fairly deep depths there is a tendency for cutting from a formation to become dispersed along the mud column and straggling out sometime after the formation has been drilled through. For instance after a sand has been drilled through, cuttings from this sand may continue to appear in samples for time.
5. Always bear in mind that contamination of an intervals cutting can occur for several reasons and must be excluded from the description.

 

posted by Geology on 07:39

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