Sedimentary Rocks (petrography): Oky Sugarbo, St., M.eng

SEDIMENTARY ROCKS (PETROGRAPHY)

Oky Sugarbo, ST., M.Eng.

Referensi

Petrography Petrology is the study of the:

• Origin (how did the rock form) of the rock. • Genesis (after forming how did the rock change through time) of the rock.

Petrology is done through the:

• Description of the rock. • Analysis of the rock. • Classification of the rock. • Where they occur. • Theories of their origin.

Petrography is done through the:

• Description of the rock. • Analysis of the rock. • Classification of the rock. • Where they occur. • Theories of their origin; by thin-section examination method (microscopy polarisation tool) 4

Batuan Sedimen

Tipe Proses

Klasifikasi Aplikasi

Zhangye Danxia landform, China

Sedimentary rocks are rocks which are formed by the “turning to stone” of sediments and that sediments, in turn, are formed by the breakdown of yet-older rocks (Huton 1875).

Menurut Tucker (1991), 70 % batuan di permukaan bumi berupa batuan sedimen. Tetapi batuan itu hanya 2 % dari volume seluruh kerak bumi. Ini berarti batuan sedimen tersebar sangat luas di permukaan bumi, tetapi ketebalannya relatif tipis.

1. Grains 2. Matrix mud-sized sediment between the grains. May be primary (deposited at same time or soon after grains), or secondary (formed by diagenetic alteration of grains.

3. cement chemical precipitates in pore spaces

4. pore space primary or secondary

framework grains

Framework Grains major minerals quartz feldspar clay minerals

MATRIX matrix

accessory minerals mafic, heavy minerals stable, non opaque, Z, R, T, rare element rock fragments igneous (IRF & VRF) sedimentary (SRF) metamorphic (MRF) chemical cements silicate, carbonate, iron oxide, sulfate

FRAMEWORK GRAINS

cement CEMENT

Major minerals quartz Si O2

Quartz structure varies with origin: Monocrystalline quartz is igneous Polycrystalline quartz is metamorphic

framework silicate

Single-crystal grains = monocrystalline ( in igneous rocks)

Major minerals quartz Si O2

Quartz structure varies with origin: Monocrystalline quartz is igneous Polycrystalline quartz is metamorphic

framework silicate

Multiple interlocking quartz grains = polycrystalline (in metamorphic rocks)

One crystal in the grain

Two or more crystals in the grain

Major minerals feldspar

framework silicate

plagioclase

Orthoclase KAlSi3O8 Plagioclase NaAlSi3O8 - CaAl2Si2O8

Plagioclase

Orthoclase

Petrography – thin sections • • • •

Quartz, orthoclase feldspar and plagioclase feldspar are main minerals. Feldspar shows the alteration. Mica and calcite are common as accessory minerals. Grain boundaries and fractures on quartz and feldspar reveals evidence of intense weathering. • Q, Quartz, F, Feldspar, C, Calcite, M, Mica, B, Biotite.

Major minerals Mica and Clay minerals

Most clay minerals are too small to identify without x-ray difraction Sheet silicate

“Sheety” Cleavage of Mica Cleavage

Chip Clark

Examples: kaolinite, illite, chlorite

mica

Accessory minerals (1-2%) mafic, heavy minerals stable, non opaque, Z,R,Ts Mafic = magnesium (mg2+) and iron (fe3+) = dark black/green in color examples = hornblende, amphibole, magnetite Heavy minerals = mineral density > 2.9 gm/cm3

garnet

stable, non opaque, Z,R,Ts Z = zircon R = rutile T = tourmaline Last to chemically weather

quartz

hornblende

rock fragments igneous (IRF & VRF)) sedimentary (SRF) metamorphic (MRF) Pieces of ancient source rocks that were NOT physically or chemically weathered Rock fragments make up 1520% of framework grains in sandstones; are most common in gravel-sized particles of conglomerates; are rare in shales.

Volcanic lithic grain (Lv) - This grain, seen under plain light, is nicely rounded and internally you can see many, euhedral (i.e. with sharp angular crystal faces) microphenocrysts of plagioclase (note the twinning on these euhedral grains). This is a classic Lv grain, probably from a volcanic arc.

Chemical cements silicate, carbonate, iron oxide, sulfate

framework grains MATRIX matrix

Cements are precipitated from solutes (cations, anions, and complex molecules that are produced during chemical weathering.

FRAMEWORK GRAINS

cement CEMENT

silicate cement – chert, opal, feldspar, zeolites carbonate – calcite, dolomite (mg), siderite (fe) iron oxide – hematite, limonite, geothite sulfate – anhydrite, gypsum, barite

UKURAN BUTIR

http://www.eos.ubc.ca/courses/eosc221/sed/sili/siligsize.html#size

Detrital Sediments & Sedimentary Rocks gravel & conglomerate

• gravity flows • fluid flows

• fluid flows • gravity flows

• suspension & flocculation • gravity flows

sand & sandstone

clay, silt & mudstone

Sorting - Well-sorted sediment indicates prolonged reworking by wind or water; poorly sorted sediment may indicate rapid deposition, or deposition by ice or mass movement.

Sorting • Sorting is another important way to distinguish the different types of sandstones and reflects the maturity of the rock. • Mature sandstones are well sorted and contain few variations of mineral composition. • Immature sandstones are not well sorted and contain a variety of minerals.

MORFOLOGI BUTIR : SHAPE

MORFOLOGI BUTIR : SPHERICITY

Angularity/Roundness/Shape – Well rounded sediment also indicate prolonged reworking by transporting agent; the shape of grains often indicates the transport system, but also may be related to the type of mineral or rock fragment

Angularity/Roundness/Shape • • •

Gravel-sized clastics are classified by the angularity / roundness /shape of the mineral grains. Rounded particles reveal that the particles have been exposed to rounding agents such as moving water. Angular particles suggest that the fragments are close to their source, are immature, and have not been exposed to rounding agents.

Maturity

Maturity

Maturity

Mineral stability can also be shown using Bowen’s Reaction series: The earliest minerals to crystallize are the least stable.

Quartz is the most stable of the common mineral; it resists chemcial weathering and is the most common mineral in most sedimentary rocks. Potassium feldspar is also common but Muscovite is relatively soft and breaks down during transport. The stability of rock fragments varies with their mineralogy.

The most “mature” sediment would be made up of 100% quartz grains. With increased transport and number of times through the rock cycle the less stable minerals are lost. The “average” igneous and metamorphic rocks contain 60% feldspars.

The “average” sandstone contains 12% feldspars. This reflects the fact that many sandstones are made up of particles that have been through several passes of the rock cycle.

Dott, 1964

Dott, 1964

KEMAS / FABRIC

Petrography of siliciclastic sedimentary rocks: POROSITY

Comparison of Compositions of Clastic and Carbonate Rocks Clastic Rocks

Sand Grains

Carbonate Rocks

Fossils Allochemical Pelloids Oolites Grains Intractlasts

Quartz Feldspar Rock Fragments Average Sandstone

Average Mudrock (Shale)

Average Sparry Limestone Average Micritic Limestone

Clay Matrix

Chemical Cement

Illite Kaolinite Smectite

Quartz Calcite Hematite

Microcrystalline Matrix

Chemical Cement

Calcite

Calcite

FOUR COMPONENTS OF SANDSTONE Geologist’s Classification

1. 2. 3. 4.

Framework Matrix Cement Pores

Note different use of “matrix” by geologists and engineers

Engineering “matrix”

PORE FRAMEWORK

CEMENT

(QUARTZ)

MATRIX

FRAMEWORK (FELDSPAR)

0.25 mm

Terms : porosity- pore space between grains permeability- measure of how connected these pores are with one another both are a function of grain size, grain shape, and any cements examples : detrital or clastic rocks have very good porosity and permeability limestones, igneous and metamorphic rocks have poorer porosity and permeability Permeability: sandstones have greater permeability than other lithologies Test: pour water on sandstone, shale and siltstone-watch result…. Definition: aquifer/ reservoar=a permeable layer transporting groundwater The best aquifers / reservoar are well sorted, well rounded sands and gravels

Porosity : • • • •

•

Primary (formed during deposition) Secondary (diagenetic): formed by dissolution of framework grains, cement or matrix, fracturing Total porosity – percentage of all pore space in a rock Effective porosity excludes isolated pores and pore volume occupied by water adsorbed on clay minerals or other grains Decreases with increasing depth of burial

Permeability:

• ability of a sandstone to transmit fluids

Porosity and Permeability  What

is meant by the terms porosity and permeability? Porosity: Amount of pore space between grains: Porosity = Volume of pore space x 100% Total volume of rock

Porosity

Vp 

Vp

V V  Vp  Vr

Vr Usually = 0.05 – 0.40

POROSITY DEFINITION

Porosity: The fraction of a rock that is occupied by pores • Porosity is an intensive property describing the fluid storage capacity of rock

Vb  Vma Porosity     Vb Vb Vp

Pore Volume Porosity  Bulk Volume Bulk Volume  Matrix Volume  Bulk Volume 8 r3  4 / 3  r3    1  47.6% 3 2 3  8r

PORE-SPACE CLASSIFICATION

•

•

•

Total porosity, t =

Total Pore Volume Bulk Volume

Effective porosity, e =

Interconnected Pore Space Bulk Volume

Effective porosity – of great importance;

Direction of fluid flow

contains the mobile fluid

Non-interconnected Pore spaces

Dead-end pores

Permeability= measure of how connected pores are…sands are more permeable than shales.

CONTROLS ON POROSITY Packing - how grains or clasts are packed together. Sorting - particle size distribution. Clast porosity - porosity within clasts or fragments. Fracturing porosity - secondary porosity added by stress and deformation. Solution porosity - secondary porosity added by solid-fluid interaction. Primary porosity is often destroyed. Geological processes (e.g., cementation, compression/extension, consolidation, burial), continuously change effective reservoir porosity through time. The origin of porosity is often complex. Geological history determines porosity.

FACTORS THAT AFFECT POROSITY PRIMARY • Particle sphericity and angularity • Packing

• Sorting (variable grain sizes)

SECONDARY (diagenetic) • Cementing materials • Overburden stress (compaction) • Vugs, dissolution, and fractures

GRAIN PACKING IN SANDSTONE Line of Traverse (using microscope)

4 Types of Grain Contacts Packing Proximity

Sutured Contact

A measure of the extent to which sedimentary particles are in contact with their neighbors

Long Contact

Packing Density

Tangential Contact

Cement

Matrix (clays, etc.)

A measure of the extent to which sedimentary particles occupy the rock volume Concavo-Convex Contact

This Example Packing Proximity = 40% Packing Density = 0.8 (modified from Blatt, 1982)

Grain contacts

• point, concavo convex, sutured contacts, free floating

CUBIC PACKING OF SPHERES Porosity = 48%

RHOMBIC PACKING OF SPHERES Porosity = 27 %

Packing of Two Sizes of Spheres Porosity = 14%

Grain-Size Sorting in Sandstone

Very Well Sorted

Well Sorted

Moderately Sorted SORTING

Poorly Sorted

Very Poorly Sorted

TYPES OF TEXTURAL CHANGES SENSED BY THE NAKED EYE AS BEDDING Sand Shale

Slow Current Fast Current

Change of Composition

Change of Size River

Eolian Beach Fluvial

Change of Shape

Change of Orientation

Change of Packing

Effect of sorting on porosity Well sorted :  = 32%

Poorly sorted :  = 17%

Grains of two sizes :  = 12.5%  Poor sorting results in reduction in porosity ()

Permeability: • Definition: The ability of a liquid to pass through a rock. • Permeable = water can pass through, varies from high to low. • Impermeable = water cannot pass through. • The larger the pore spaces the more permeable the rock will be especially if the pore spaces are well connected. • A moderately porous rock may not necessarily be permeable, as the pore spaces may not be connected. • E. g. shale has a porosity of 18% similar to sandstone but clay minerals hold onto the water and so it is impermeable.

Permeability

Permeability

Permeability k [D, mD] - ‘capacity of rock to transmit fluid’ - function of open space and its interconnection - depends on properties of rock formation

Post Depositional Changes:

• 1. 2. 3. 4.

These can affect porosity and permeability: Faulting increases the permeability Jointing increases the permeability Cementation decreases permeability Metamorphism will crystallise the rock and reduce the porosity.

Porosity <> Permeability

• pores – – – – –

size number unconnected open cement

PERMEABILITY

Permeability

cement > unconnected

Oil or gas reservoir

Limestone reservoir

Sandstone reservoir

Petrographic Classification of Sandstones Most sandstone classifications are based on the composition of the rock. It is based on the relative proportions of: Martrix (fine-grained - <0.03mm - material that is associated with the sand grains). Quartz Feldspar Rock fragments (sand grains that are made up crystals of two or more different minerals).

To classify sandstones the first step is to determine composition of the rock. Point counting is a method whereby a thin section on a petrographic microscope is examined by stepping across the thin section at equal intervals and identifying the material (quartz, feldspars, rock fragments or matrix) that lies immediately beneath the cross hairs. Counting 250 to 300 grains will accurately yield the proportion of each component.

KLASIFIKASI  Berdasarkan proses dominan yang mempengaruhi: Sedimen Klastika terrigen (silisiklastika atau epiklastika); Sedimen biogen, biokimia dan organik; Sedimen kimiawi dan Sedimen volkaniklastika. Sedimen klastika terrigen

Sedimen biogen, biokimia & organik

Sedimen kimiawi

Sedimen volkaniklastika

Konglomerat/ breksi, batupasir dan mudrocks

Batugamping, rijang, fosfat, batubara dan “oil shale”

Sedimen evaporit dan “ironstone”

Ignimbrit, aglomerat, tuf

 Berdasarkan tekstur: Tekstur Klastika dan Tekstur Non-Klastika.  Tekstur Klastika: umumnya terdiri dari mineral allogenik, berlaku sekala Wentworth, besar butir ditentukan oleh (jenis pelapukan, macam transportasi, waktu/jarak transportasi), 3 unsur tekstur (butiran, matrik, semen), sorting/pemilahan, rounding/kebundaran.

 Tekstur Non-Klastika: umumnya terdiri dari mineral autigenik, umumnya memperlihatkan gejala diagenesa pada P&T tertentu (porositas menjadi rendah/hilang), dicirikan oleh porositas rendah dan ditandai tekstur mozaik, besar butir (5 mm kasar; 1 mm  5 mm sedang;  1 mm halus), mikrokristalin (sulit dibedakan).

 Pengaruh diagenesa: ada rekristalisasi; tidak ada perubahan mineral; butiran bertambah besar berupa mozaik (porositas mengecil/hilang); bila ada replacement umumnya tidak memperbesar besar butir malahan memperkecil/ memperhalus.  Pengaruh pelarutan: menambah porositas (muncul pori-pori baru); membentuk tekstur khas (stylolitik) batas-batas mineral sangat bergerigi/ tidak beraturan.

BATUPASIR Bahan penyusun utama : • Kuarsa/silika (kuarsa, opal & kalsedon) • Felspar (K-felspar & plagioklas) • Fragmen batuan Bahan penyusun tambahan : • Mineral berat (Zirkon, Turmalin) • Mineral ringan lainnya (karbonat: kalsit, dolomit, siderit; mineral lempung: kaolin, montmorilonit, zeolit; glaukonit, serisit, klorit). • Kolofan (Collophane): mineral fosfat.

Sand and sandstone classification: based on quartz, feldspar and lithic grain contributions (Pettijohn at al., 1987).

Petrographic Sandstone Classification

Folk, 1974

McBride, 1963

Sandstone (note large number of quartz grains)

Grains subangular to subrounded, sandstone is poorly sorted Plagioclase grain

Photomicrograph of quartz rich sandstone (Arenite)

Batupasir yang lain: •Green sand: batupasir banyak mengandung glaukonit. •Phosphatic sandstone: batupasir banyak mengandung mineral fosfat.

•Calcarenaceous sandstone: batupasir yang tersusun oleh detrital kuarsa dan karbonat (dalam bentuk pecahan cangkang atau oolit). •Calcareous sandstone: batupasir dimana karbonat berfungsi sebagai semen. •Calclithites: batupasir dimana komponen litik berasal dari rombakan batuan karbonat. •Ilacolumite: Batupasir banyak mengandung sekis (Fig. 732, hal. 247, Pettijohn, 1975).

Non-Terrigenous Sediments and Rocks

Petrography of Carbonate Rocks: Carbonate Particles

Most Common non-Terrigenous Sedimentary Rocks • Carbonates (>50% primary carbonate minerals) • Limestone (CaCO3) • Chemical • biochemical

• Dolomite (CaMg(CO3)2) • Chemical

Batuan karbonat disebut sebagai Batugamping apabila penyusun utamanya terdiri dari kalsit >90% dan dolomit jika teridiri dari dolomit > 90 persen (Boggs,1987).

The Origin of Carbonate Sediments • Most form as biogenic particles (essentially the only source) in • • • •

warm (tropical; 30oN to 30oS latitude), shallow (shelf; within the photic zone), (mostly <10-20 m) marine water Also accumulate in deepwater limestone (fine-grained) made up of skeletons of pelagic microorganisms such as Globigerina

Controls on Carbonate Accumulation • Temperature (climate) -Tropics & temperate regions favor carbonate production: true of ancient too! • Light – Photosynthesis drives carbonate production • Pressure – “CCD” dissolution increases with depth • Agitation of waves - Oxygen source & remove CO2 • Sea Level – fluctuating change many condition

Carbonate Rocks Constituents • Allochems Non skeletal – Skeletal grain

• Micrite • Micro Crystalline calcite

• Cement (sparry calcite)

KOMPONEN UTAMA BATUAN KARBONAT

Allochem (Grain) Allochem merupakan komponen batuan karbonat berupa partikel/butiran yang berukuran lebih dari atau sama dengan pasir. Macam-macam grain (Allochem) adalah: Non Skeletal Grain Merupakan grain atau butiran dalam batuan karbonat yang bukan berasal dari cangkang atau rangka organisme karbonatan. Macammacam non skeletal grain antara lain: · Intraclast & Extraclast · Ooid/oolith/coated grain · Pisoid/pisolit · Pelloid/pellet Skeletal Grain Skeletal grain merupakan fragmen karbonat yang berasal dari bagian keras oraganisme/cangkang/tubuh organisme.

Komponen

Carbonate Rocks Constituents •

The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. 1. 2.

3.

4.

Intraclasts (rock fragments): • formed, transported and redeposited within the basin Skeletal particles (bioclasts): • whole microfossils, whole megafossils, broken shell fragments • algae, forams, corals, bryozoans, brachiopods, gastropods, pelecypods, ostracods, etc. Ooliths: concentrically laminated carbonate structures, including: • oolites -concentrically laminated structures less than 2mm in diameter, thought to be abiogenic in origin • pisolites - same as oolites, but greater than 2mm in diameter • oncolites - spheroidal stromatolites (> 1-2 cm) Peloids: • silt to fine grained sand sized carbonate particles with no distinctive internal structure; most thought to be fecal pellets

KOMPONEN (GRAIN)

KOMPONEN (GRAIN)

Carbonate Rocks Constituents •

The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. •

Intraclasts (early lithified carbonate fragments): •

irregularly-shaped grains that form by syndepositional erosion of partially lithified sediment.

Extraclasts

Extraclasts

Extraclasts

Carbonate Rocks Constituents •

The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. • Skeletal particles (bioclasts): • whole microfossils, whole megafossils, broken shell fragments • algae, forams, corals, bryozoans, brachiopods, gastropods, pelecypods, ostracods, etc. • Standard microfacies (fossil fragment type -> environment)

Foraminifera

Foraminifera

After Scholle

Brachiopods

Bryozoan

GEOL 325 Lecture 4: Carbonates

Bryozoan

Trilobite Remains

Ostracod Remains

Calcispheres

Red Calcareous Algae

Carbonate Rocks Constituents •

The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. •

Ooliths: concentrically laminated carbonate structures, including: • oolites -concentrically laminated structures,less than 2 mm in diameter, thought to be abiogenic in origin • pisolites - same as oolites, but greater than 2 mm in diameter • oncolites - spheroidal stromatolites (> 1-2 cm)

Ooids

Aragonitic Ooids

Aragonitic Ooids

After Scholle, 2003

Calcitic & Aragonitic Ooids Great Salt Lake

After Scholle

Carbonate Rocks Constituents •

The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. • Pelloids: • silt to fine grained sand sized carbonate particles with no distinctive internal structure; most thought to be fecal pellets

Pellets

Lime Mud or Micrite

Carbonate Rocks Constituents • Micrite: • microcrystalline carbonate particles of clay (<1-4 micron) size (subtranslucent matrix) formed by • chemical or biochemical ppt • abrasion of allochems

• implies deposition in a low energy environment just like in terrigenous mudstones

Lime mudstone/Micrite

Carbonate Rocks Constituents •

The sand-sized grains that occur in carbonate rocks are called allochemical particles or allochems. The interpretation of the depositional setting of carbonates is based on grain types, grain packing or fabric, sedimentary structures.

Carbonate Rocks Constituents • Cement: • sparry (twinkling crystalline) orthochemical material formed in interstitial pore spaces of “grainy” carbonate sediment • cement in pores indicates original void space

Carbonate Cement Fabrics • Crust or rims coat grains • Syntaxial overgrowth – optical continuity with skeletal fabric • Echinoid single crystals • Brachiopod multiple crystals

• Blocky equant • Meniscus

Meniscus Cement

Radial Ooids – Marine Cement

GEOL 751 Lecture 6: Cementation & Diagnesis

Gastropod – Aragonite Marine Cement

C. G. St. C. Kendall

Gastropod – Cement Fill of Test

C. G. St. C. Kendall

Carbonate Mineralogy • Aragonite – high temperature mineral • Calcite – stable in sea water & near surface crust • Low Magnesium Calcite • High Magnesium Calcite • Imperforate foraminifera • Echinoidea

• Dolomite – stable in sea water & near surface • Carbonate mineralogy of oceans changes with time!

Klasifikasi

Klasifikasi

Klasifikasi

Autochthonous Carbonates A - Reef rock types (Embry & Klovan, 1971) B - Roles of organisms in the construction of reef carbonates (Tsien, 1994) C – Dependence of reef rock types on environmental constraints of benthic communities (After Wood, 1993)

Allochthonous versus Autochthonous Carbonates

Reef complexes are essentially living buckets, in which the walls of the bucket are composed of boundstone of living corals and related species, the shallow inner lagoon is filled with fine grainstones and mudstone, while the steep outer flanks are composed of coarser grainstones and rudestone, formed from talus off the reef front.

Boundstones

Carbonate Depositional Environments • Generic rimmed carbonate shelf platform – basin margin

217

Petrografi Batubara Petrografi batubara adalah ilmu yang mempelajari komponen-komponen organik (maceral) dan anorganik (mineral matter) secara mikroskopik. Seperti pada petrografi mineral, petrografi batubara memerikan komponen-komponen penyusun batubara secara kualitatif dan kuantitatif untuk mengetahui asal mula dan genesa pembentukkan batubara.

• Ditinjau secara petrografi, batubara dapat dikelompokan menjadi a. lythotype, b. microlithotype c. maceral. Maceral pada batubara adalah analog dengan mineral pada batuan (anorganik). Maceral terbagi atas tiga group, yaitu; 1. Vitrinite, 2. Liptinite 3. inertinite.

Coal Origin

231

Grup vitrinit berasal dari tumbuh-tumbuhan yang mengandung serat kayu (woody tissue) seperti batang, akar, dahan dan serat daun. Vitrinite umumnya merupakan bahan penyusun utama batubara (>50%). Melalui pengamatan mikroskop refraksi, grup vitrinit memperlihatkan warna coklat kemerahan sampai gelap, semakin tinggi peringkat batubara semakin gelap warna maseralnya, demikian pula sebaliknya. Melalui pengamatan miskroskop refleksi, grup vitrinit memperlihatkan warna pantul lebih terang, mulai dari abu-abu tua sampai abu-abu terang , semakin tinggi peringkat batubara semakin terang warna pantul yang dihasilkan

Grup liptinit berasal dari organ tumbuhan (ganggang/algae, spora, kotak spora, kulit luar (kutikula), getah tanaman (resin) dan serbuk sari /pollen). Grup liptinit memiliki kandungan hidrogen paling banyak dan kandungan karbon paling sedikit bila dibandingkan dengan grup maseral lainnya. Di bawah miskroskop refleksi menunjukkan pantulan berwarna abu-abu sampai gelap, mempunyai reflektivitas rendah dan flouresens tinggi (Teichmueller, 1989).

Grup inertinit diperkirakan berasal dari tumbuhan yang sudah terbakar (charcoal) dan sebagian lagi diperkirakan akibat proses oksidasi dari maseral lainnya atau proses decarboxylation yang disebabkan oleh jamur atau bakteri (proses biokimia). Dengan adanya proses tersebut kelompok inertinit memiliki kandungan oksigen relatif tinggi, kandungan hidrogen rendah, dan ratio O/C lebih tinggi dari pada grup vitrinit dan liptinit. Grup inertinit memiliki nilai reflektensi tertinggi diantara grup maseral lainnya. Dibawah miskroskop refleksi , inertinit memperlihatkan warna abu-abu hingga abu-abu kehijauan, tetapi pada sinar ultra violet tidak menunjukan flouresens

Material organik berasal dari berbagai macam tumbuhan dan sebagian bercampur dengan sedimen anorganik selama tahap pembentukan gambut, oleh karena itu jenis batubara (coal type) ditentukan pada tahap biokimia yang dapat dipergunakan untuk mengetahui lingkungan pengendapan batubara, terutama berdasarkan material organiknya. Penentuan jenis batubara (coal type) dapat secara mikroskopis dan makroskopis.

Type batubara berkaitan dengan jenis tumbuhan asal (komposisi maceral), lingkungan pengendapan dan pengubahan bahan organic yang terjadi pada proses pembatubaraan tahap biokimia (biochemical coalification stage) Rank batubara adalah posisi relatif pembatubaraan atau derajat kematangan pengubahaan (transformation) bahan organic pada proses pembatubaraan (geochemical coalification) yang dimulai dari seri tahapan gambut - batubara lignit – subbituminus dan bituminous – antrasit dan meta antrasit sampai akhirnya ke semigrafit dan grafit.

Coal Identification

239

SUBSTANSI BATUBARA

SUBSTANSI MACERAL

Fusinite

Fusinite

Resinite Resinite

Cutinite cutinite

Sporinite Sporinite

Macrinite

marcenite

Telinite Telinite

Sclerotinite

sclerotinite

Fusinite dengan Fusinite dengan bogenstructur 242

Bogen struktur

0

100 mikron

Maseral Telocollinite,posisi sinar reflektan, perbesaran 20x (nomor conto 5).

0

100 mikron

Framboidal pirit, posisi sinar reflektan, perbesaran 50x (nomor conto 6).

a.

0

100 mikron

b.

0

100 mikron

Maseral Resinite, nomor contoh 1 a.Posisi sinar flourecense, perbesaran 20x b.Posisi sinar reflektan, perbesaran 20x

Penentuan rank batubara

dapat dilakukan dengan cara mengukur

reflektan vitrinite.

Pemilihan vitrinite sebagai maceral penentu rank batubara adalah karena maceral ini umumnya selalu hadir dalam betubara dengan proporsi terbanyak dan memiliki perubahan reflektan dan relative linier terhadap pembatubaraan jika dibanding maceral – maceral lainnya.

Metoda penggunaan reflektan vitrinite dalam menentukan rank batubara memiliki keunggulan dibanding cara konvensional (analisa kimia) oleh karena : memiliki ketelitian tinggi dan akurat, tidak memerlukan contoh dalam jumlah besar serta lebih mudah dan cepat.

Semakin matang suatu kerogen maka akan memiliki nilai pantulan (reflectance, Ro) yang besar. Kerogen pada tahapan diagenesis hanya sedikit memantulkan cahaya sehingga memiliki nilai Ro di bawah 0,5% (immature). Ketika struktur kerogen semakin memadat dan teratur dalam proses katagenesis akan semakin memantulkan cahaya. Untuk kerogen pada proses katagenesis, akan memiliki kisaran nilai vitrinite reflectance sekitar 0,6 – 1,3% dan mencapai maksimum pada nilai sekitar 0,8 – 1,0% dimana pembentukan minyak bumi maksimal. Sedangkan untuk proses metagenesis, memiliki nilai Ro di atas 2%

CONTOH ANALISA PETROGRAFI BATUBARA UNTUK MENGETAHUI KOMPOSISI MASERAL BATUBARA ANALISA MASERAL KELOMPOK MASERAL

% VOL

VOL (mfb)

SUB KELOMPOK MASERAL

MASERAL

VOL % (mfb VOL )

Textinite Telovitrinite (Humotelinite) 33.4 VITRINITE (HUMINITE)

80.0

80.0

INERTINITE

2.0

17.0

E-ulminite Telocollinite Attrinite Densinite

Detrovitrinite (Humodetrinite) 45.4 Desmocollinite Gelovitrinite (Humocolinite) 1.2

LIPTINITE (EXINITE)

Texto-ulminite

15.2

Telo-inertinite

1.8

Detro-inertinite

Corpogelinite Porigelinite

33.4 35.6 9.8 1.2

Eugelinite Sporinite Cutinite Resinite Liptodetrinite Alginite Suberinite Fluorinite Exsudatinite Bituminite Fusinite Semifusinite Sclerotinite Inertodetrinite

1.0

5.6 9.6 1.8

Micrinite Gelo-inertinite MINERALS MATTER

1.0

TOTAL

100

Macrinite Carb Pyrite Clay

1.0