Raw Material Characterization

Carbonate Rocks

The carbonate rocks make up 10 to 15% of sedimentary rocks. They largely consist

of two types of rocks.

Limestones which are composed mostly of calcite (CaCO3) or high Mg calcite

[(Ca,Mg)CO3], and Dolostones which are composed mostly of dolomite

[CaMg(CO3)2].

Because carbonate minerals in general are soluble in slightly acidic waters, they

often have high porosity and permeability, making them ideal reservoirs for

petroleum. For this reason they are well studied.

Limestone can be easily recognized in hand specimen or outcrop because of its high

solubility in HCl. A drop of such acid placed on the rock will cause it to fizz due to

the generation of CO2 gas.

A dolostone, on the other hand, will not fizz until a fine powder is made from the

rock or mineral. Also, dolostones tend to weather to a brownish color rock,

whereas limestones tend to weather to a white or gray colored rock. The brown

color of dolostones is due to the fact that Fe occurs in small amounts replacing some

of the Mg in dolomite.

Limestone uses and processing

Evaluation Scheme of Limestone

How to characterize the limestones?

1) Acid etching:

Acetic acid (1:5) dissolves only calcite not dolomite and AIR (clays and Qtz.). HCl (0.1

1. N) dissolves both calcite and dolomite (except large dol. Rhombs)

2) Petrography:

It is to study the microstructure of the limestone in terms of the texture and mineralogy.

This occurs by polarizing microscope (PL), scanning electron microscope (SEM),

cathodoluminescense (Cl),..and many other techniques.

Thin sections are prepared, where rock slabs less than 30 micron are stained and then

studied by (PL)

Staining

Feigel’s solution: Aragonite turns black, whereas calcite still colorless

Alizarine red S in 0.2 % HCl: Calcite and aragonite red, dolomite colorless

Potassium ferricyanide and Alizarine red S in 1.5 % HCl:

1. Calcite and Aragonite become red
2. Ferroan calcite becomes purple
3. Dolomite is colorless
4. Ferroan dolomite is turquoise

Petrography

1. Carbonate Components(%)(Comparison Charts)

2. Orthochem Type
3. Crystal Shape of Orthochem
4. Allochem Size
5. Allochem Roundness
6. Allochem Packing
7. Porosity Type

Carbonate Components

Skeletal grains

(variables)

Non-skeletal grains

(variables)

Orthochems

Micrite, sparite

Non-carbonate Components

Qtz, Argillaceous material

Fe-oxides, Gypsum

Orthochem Types

Mini-Micrite:

Is a crypto- and microcrystalline calcite or aragonite cements (<2 µm)

Micrite:

Is a crypto- and microcrystalline calcite or aragonite cements (<4 µm)

Sparite:

Is a carbonate cement (>4 µm). Varieties include orthosparite, neomorphic sparite, micro-,

pseudo and metasparite

Orthosparite:

1. occurs within pores.
2. has sharp boundaries between its crystals and pore limits.
3. has straight crystal boundaries and enfacial junctions.

Neomorphic sparite:

1. has irregular, curved and embayed crystals boundaries
2. has variable crystal size with remnants of micrite

Microspar (microsparite):

Neomorphic characterized by a crystal size between about 5 and 30 µm.

Pseudosparite:

Neomorphic characterized by a crystal size larger than 30 µm.

Metasparite:

Exhibits indications of metamorphic processes (e.g. granoblastic texture and complex twinning).

Crystal shape of orthochem

By examining the spatial relations between cements,  grains, substrate and pores, cement fabrics can be subdivided into five groups (Fig. 7.12):

1. Symmetrical cements around grains  represented by isopachous  and circumgranular cement rims, distinguished by the cement type and thickness of the cement rim.

2. Asymmetrical cements
   concentrated on the underside of grains or developed at grain contacts.
3. Cement crusts, fans and hemispheres forming millimeter to centimeter thick structures.

4. Calcite mosaics, differentiated by size, orientation and the type of crystal boundaries.
5. Syntaxial overgrowth cement, usually associated with echinoderm grains.

Allochem Size

Common size range of carbonate grains: The sizes of carbonate grains vary between <0.10 mm and several centimeters. Skeletal grains, oncoids, rhodoids, aggregate grains and extraclasts exhibit wide size ranges, peloids, ooids, pisoids and intraclasts rather narrow ranges:

1. Skeletal grains: Commonly within the range of ~0.250 mm to about 20 mm; coarse calcisiltite, calcarenite and calcirudite; medium silt, sand, granules and to coarse pebble size.
2. Peloids: Commonly about 0.10 to 0.20 mm (except for fecal pellets which can be larger); fine to medium-sized calcarenite; fine and medium sand size.
3. Ooids: <2 mm, commonly 0.5 to 1 mm; coarse and very coarse calcarenite.
4. Pisoids: >2 mm to several tens of millimeters, but sometimes <2 mm; calcirudite; granules to coarse pebble size.
5. Oncoids: < 2 mm to > 40 millimeter, commonly < 20 mrn; calcirudite; granules to very coarse pebble size.
6. Rhodoids: about 5 to 40 mm, commonly < 100 mm; calcirudite; fine to very coarse pebble size.
7. Aggregate grains: 5 to several millimeters, commonly < 1 mm; calcarenite and calcirudite; coarse sand to medium pebble size.
8. Intraclasts: < 0.10 to about 10 mm; calcarenite and calcilutite; fine sand to coarse pebble size.
9. Carbonate extraclasts: < 1 mm to several tens of millimeters; calcarenite and calcirudite.

Allochem Roundness

The roundness of fossils and bioclasts depends on:

1. the inherent roundness
2. the roundability of the skeletal grains (related to the microstructure and architecture of shells)
3. grain sizes (coarser particles round faster than finer ones)
4. the vigor and time-length of surf action, an area is exposed to.

The roundness is a function of grain composition, size, type of transport process, and transport distance, (Mills 1979).

Grain roundness in thin sections is commonly estimated by reference to a two-dimensional visual comparison scales or charts. These consist of sets of grain images of known roundness.

Allochem Packing

Mechanical compaction: is a process caused by sediment overburden and resulting in a general reduction of porosity and rock volume.

The thickness of the overburden necessary to produce compaction structures is controversial. Overburden results in the mechanical failure of grains. Compaction is commonly followed by pressure solution recorded by stylolites and solution seams (chemical compaction).

The degree of mechanical compaction is inhibited or reduced by early precompaction cementation and can be studied in grainstones by cement/grain ratios (Meyers 1980).

Chemical compaction: is exemplified by pressure solution resulting in stylolites and solution seams formed under burial conditions. Stylolites are irregular, suture-like contacts produced by differential vertical movement under pressure accompanied by solution.

Classification of pressure solution features by Logan and Semeniuk (1976). The classification concentrates on the morphology of stylolites.

Porosity

3) Limestone Classification

Folk Classification:

The Folk classification, which we will use in lab, is shown below. The classification divides carbonates into two groups:

Allochemical rocks are those that contain grains brought in from elsewhere (i.e. similar to detrital grains in clastic rocks).

Orthochemical rocks are those in which the carbonate crystallized in place. Allochemical rocks have grains that may consist of fossiliferous material, ooids, peloids, or intraclasts. These are embedded in a matrix consisting of microcrystalline carbonate (calcite or dolomite), called micrite, or larger visible crystals of carbonate, called sparite. Sparite is clear granular carbonate that has formed through recrystallization of micrite, or by crystallization within previously existing void spaces during diagenesis.

Dunham Classification:

The Dunham classification is based on the concept of grain support. The classification divides carbonate rocks into two broad groups, those whose original components were not bound together during deposition and those whose original components formed in place and consist of intergrowths of skeletal material. The latter group are called boundstones (similar to biolithite of the Folk classification). The former group is further subdivided as to whether or not the grains are mud-supported or grain supported. If the rock consists of less than 10% grains it is called a mudstone (potentially confusing if taken out of context). If it is mud supported with greater than 10% grains it is called a wackstone. If the rock is grain supported, it is called a packstone, if the grains have shapes that allow for small amounts of mud to occur in the interstices, and a grainstone if there is no mud between the grains.

Biomicrite (Wackestone). Small forams are embedded in a micritic matrix which contains also some glauconite and quartz grains

Biosparite (Grainstone). Nummulites are embedded in idiotopic and hypidiotopic crystals of orthochemical sparite (eosparite). Fabric selective pores (intraparticle “red arrows”), (intracrystal “blue arrows”) and not-fabric selective (Vuggy pores “green arrow”) are present. (Sample 2, Samalut Formation,, C.N, 32X)

Biosparite                  (Grainstone).

Nummulites of tangential “red arrows”contact are embedded in neomorohic sparite. (Sample 2,  P.P.L, 32X)

Biosparite                  (Grainstone).

Nummulites of tangential “red arrows” contact are embedded in neomorohic sparite. (Sample 2,  , C.N., 32X)

Pelmicrite (Grainstone). The rock consists mainly of small peloids (black arrow) embedded in a sparitic matrix. Glauconite (white arrow) and quartz (red arrow) grains are present.

Biopelsparite (Grainstone). Milliolids (Idalina sp.) are forming the main skeletal grains embedded in neomorphic sparite cement. They are rounded to very well rounded and of grain size ranges from coarse to medium sand. Moldic intraparticle (rare) and vug pores are dominant. Pellets are scattered in the whole rock. Mostly all allochems are of no contact.

Sparite (Crystalline limestone). Orthosparite (eosparite) and neomorphic sparite are totally of anhedral crystals composing the whole rock

Oosparite (Grainstone). Oolites and clasts are embedded in a sparitic matrix. Pores are vuggy and moldic

Microfacies Analysis

Fig. 14.29. Distribution of SMF Types in the Facies Zones (FZ) of the rimmed I carbonate platform model. Nearly all facie belts are characterized by assemblages consisting of several SMF Types. The distribution of SMF 14 (lag deposit) is not specially indicated,because  non deposition or strongly
reduced sedimentation occurs in many deep marine as well as shallow marine facies zones.

A: evaporitic, B: brackish.

References

1. Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture. In: Ham, W. E. (ed.), Classification of carbonate rocks: American Association of Petroleum Geologists Memoir, p. 108-121.
2. Embry, AF, and Klovan, JE, 1971, A Late Devonian reef tract on Northeastern Banks Island, NWT: Canadian Petroleum Geology Bulletin, v. 19, p. 730-781.
3. Folk, R.L., 1959, Practical petrographic classification of limestones: American Association of Petroleum Geologists Bulletin, v. 43, p. 1-38.
4. Folk, R.L., 1962, Spectral subdivision of limestone types, in Ham, W.E., ed., Classification of Carbonate Rocks-A Symposium: American Association of Petroleum Geologists Memoir 1, p. 62 84.
5. James, N.P., 1984, Shallowing-upward sequences in carbonates, in Walker, R.G., ed., Facies Models: Geological

Association       of         Canada,          Geoscience         Canada,          Reprint         Series        1,                              p.          213–228.

6. Scholle, P. A. and Ulmer-Scholle, D. S, 2003, A Color Guide to the Petrography of Carbonate Rocks: AAPG Memoir 77, 474 p

4) Limestone Mineralogy

1. Stained thin sections
2. XRD
3. DTA

The decomposition of Calcite (Fig. A) intoDTA lime “CaO” starts around 850 and ends at1020, but maximized at 1000 °C The decomposition of Aragonite (Fig. B) occurs at two steps:

1. The first is the irreversible conversion of aragonite to calcite. This begins around 460, ends at 520, but maximized at 500°C.

2. The second step is the decomposition of Calcite into lime “CaO”. This begins around 800, ends at 960 and maximized around 930 °C

occurs on two steps:The decomposition of Dolomite (Ca Mg(CO3)2) 

1. The first is to break apart the dolomite structure into MgCO3.CaCO3 then the decomposition of the Magnesite part (MgCO3) into periclase “MgO” and CO2. This begins around 690, ends at 850 and maximized at 830 °C.

2. The second step is the decomposition of the Calcite (CaCO3) part into lime “CaO” and CO2. This begins around 850, ends at 1020 and maximized around 960 °C.

black arrow

The MgCO3 (of the dolomite part) dissociation peak

Blue arrow

The CaCO3 (of the dolomite part) dissociation peak

Red Arrow

The CaCO3 dissociation peak

5) Limestone Chemistry

Classification based on total carbonate

The chemical restrictions for limestones used in cement production:

1. MgO % < 3
2. SO3 % < 1
3. P2O5 % < 1
4. Fe2O3 % < 0.01 (for white cement)

Total alkalies % < 0.6