shale characterization

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Shale Characterization

By

Dr. Rahmat Ali Gakkhar

December, 2013

Exploration Department


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Purpose of the course:

To introduce terminology and fundamental concepts for the description and

interpretation of Shale.

Shale Characterization


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Source Books / Articles

Sedimentology and Sedimentary Processes by Virginia T. McLemore 2008.

Petroleum geochemistry and geology by John M Hunt 1996.

Basics and Application of Rock-Eval/TOC Pyrolysis by NUEZ-BETELU, L & BACETA, J. I. 1994

Presentation of Schlumberger 2011.

PhD Thesis 2010

Different Research Papers


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Introduction


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What is the relative stability of minerals?

Bowens Reaction series

shows the sequence in which

minerals crystallize from a

cooling magma.

Introduction


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Mineral stability can also be shown using Bowens 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 themost 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.

Introduction


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Sedimentary rocks make up only

7.9% of the Earthscrust.

More than 70% of the surface of the

Earth is covered by sediments or

sedimentary rocks.

Introduction


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Shale's abundance is dominant, 35% of the surface of the Earth is covered by it.

Introduction


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Sediment

Sediment= loose, solid particles and can be:

Terrigenous= fragments from silicates (igneous and/or metamorphics).

Biogenic= fossils (carbonate - reefs; silicates - forams).

Chemical= precipitates (halite, gypsum, anhydrite, etc).

Note:with chemical sedimentary rocks, evaporation > precipitation and/or

supersaturation in closed basins (lakes or oceans).


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Classified by particle size

Boulder

Cobble

Pebble

Sand

Silt

Clay

Sediment


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Classified by particle size

Boulder- >256 mm

Cobble- 64 to 256 mm

Pebble- 2 to 64 mm

Sand- 1/16 to 2 mm

Silt- 1/256 to 1/16 mm

Clay-


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Grain Size1

(mm)

Sediment name Rock Name Adjectives

> 2 Gravel Rudite Cobble, pebble, well sorted,etc.

0.0625-2 Sand Arenite Coarse, medium, well sorted,

etc.

< 0.0625 Mud Mudstone or

Lutite

Silt or clay

1For the purposes of this general classification we will assign the rock or sediment name

shown if more than 50% of the particles are in the range shown.

More detailed classification schemes will limit terms on the basis of different proportions of

sediment within a given grain size.

A simple classification of terrigenous clastic rocks and sediment is based on thepredominant grain size of the material:

Classification of Sediment Based on Grain Size


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Grade Scales

Grade scales define limits to a range of grain sizes for a given class (grade) of grain size.

They provide a basis for a

terminology that describes grain

size.

Sedimentologists use the Udden-Wentworth Grade Scale.

Sets most boundaries to vary by

a factor of 2.

e.g., medium sand falls between

0.25 and 0.5 mm.

Grain Texture


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Sedmentologists often express grain size in units call Phi Units (f; the lower case Greekletter phi).

Phi was originally defined as: )(log 2 mmdf

To make Phi dimensionless it

was later defined as:

Phi units assign whole numbers to the boundaries between size classes.

Od

mmd )(log 2f

Where dO= 1 mm.

Grain Texture

Grain Size


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Grain size (diameter) and grain-shape depend on:

Transport media: Rivers (pebbles bounce on river bottom, sand moved in traction, and silt/clay suspended

in water column);

Oceans and lakes (near-shore and deep-water systems);

Glaciers (sediment moved on glacier bottom); glaciers sort poorly (meaning there isa large spread of grain sizes in glacial deposits)

Wind (sand dunes) winds sort well (meaning grain sizes are very similar);

Grain Size of Sediment


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Grain Size of Sediment

Distance from parent (source) rock: the longer the distance traveled,generally the smaller and the more well-rounded the grains (due to higherkinetic energy).

Mineral hardness: the harder the parent rock, the longer it will take thesediments to erode (example: silicates are more resistant to weathering anderosion than feldspars, and this is why beaches are often comprised of sand,not feldspar-rich sediments).


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Provenance of a Sediment

TheProvenanceof a sediment is inferred from aspects of compositionthat reflect

The source rock

Tectonic and

Climatic characteristics of the source area for the sediment.

Provenance: where something originated.


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The source rock of a sediment and the tectonic setting are closely linked:

The tectonic setting determines

The relative abundance of different types of rock that is available for weathering and

The production of clastic sediment.

e.g., An arkosic sandstone (rich in feldspars) would have a source area that isrich in granites.

An exposed craton (e.g., the Indian/Canadian Shield)?

A mountain chain adjacent to a convergent margin (e.g., modern Andes)?

i) Tectonic setting


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e.g., a sandstone with abundant volcanic and low grade metamorphic rock fragments (Island

arc setting).

Quartz arenite: sedimentary source rocks; uplifted sediments in an orogenic belt.

Two very different tectonic settings.

i) Tectonic setting



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ii) ClimateClimate exerts a strong control on the type of weathering that takes place in the

source area of a sediment; this, in turn, influences composition.

Cold, arid climate:

Predominantly physical weathering, producing abundant detrital grains (unalteredmineral grains and rock fragments).

Sandstones produced in such settings will be relatively immature, depending onthe source rocks.

Provenance of a Sediment Climate of Sediment


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Warm, humid climate:

Chemical weathering predominates.

Unstable minerals removed from the sediment that is produced byweathering.

Will produce a more maturesediment than a cold climate.



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Overall, there is a reduction in the proportion of feldspar in sands towards the south.

Several factors at work:

Source rocks: in the north are more

granitic source rocks whereas in the

south the major source rocks are

sedimentary rocks.



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Climate: colder in the north so that physical weathering is important, producing immaturesediment.

Warmer in the south so that chemical

weathering produces a more maturesediment.

Many sediments were produced during

glaciation which only breaks down

source rocks by physical processes.



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Transport distance:Transported sediment over long distances, increasing the maturity of the

sands.

Provenance of a Sediment Transport Distance

After lithification the sediments

turn into sedimentary rocks


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Types of Sedimentary Rocks


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Clastic Sedimentary Rocks

Shale, Claystone, Siltstone and Sandstone

Non Clastic Sedimentary Rocks

Limestone, Evaporites (Rock Salt Gypsum)

Types of Sedimentary Rocks


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The Classification of Clastic Sedimentary Rocks Based on Type

A very basic classification of

all sedimentary rocks is

based on the type of material

that is deposited and the

modes of deposition.


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DETRITAL SEDIMENTARY ROCKS:

a) All detrital rocks are clastic

b) Sand and silt are predominantly quartz

c) Finer-sized particles of clay minerals

d) Conglomerates

e) Breccias


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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.

DETRITAL SEDIMENTARY ROCKS


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Classification of Fine Grained Sedimentary Rocks

Shale: The general term applied to this class of rocks (> 50% of particles are finer

than 0.0625 mm).

Lutite: A synonym for "shale".

Mud: All sediment finer than 0.0625 mm. More specifically used for sediment inwhich 33-65% of particles are within the clay size range (68% of particles fall within the silt size range (0.0625

0.0039 mm).

Clay: All sediment finer than 0.0039 mm.Silt- 1/256 to 1/16 mm

Clay-


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Siltstone: A rock composed largely of silt size particles (68-100% silt-size)

Mudstone: A bocky shale, i.e., has only poor fissility and does not split finely.

Argillaceous

Sediment: A sediment containing largely clay-size particles (i.e., >50%).

Argillite: A dense, compact rock (poor fissility) composed of mud-sizesediment (low grade metamorphic rock, cleavage not developed).

Psammite: Normally a fine-grained sandstone but sometimes applied to rocks of

predominantly silt-size sediment.

Classification of Fine Grained Rocks


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Lutite terms are based onproportion of clay, degree ofinduration and thickness ofstratification.


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Terminology Related to Stratification and Fissility (Parting)


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Sedimentary Environments


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Sketch of Sedimentary Environments


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Alluvial Fans

Alluvial fans are sedimentary deposits that typically form at the margins of a drybasin.

They typically contain coarse boulders and gravels and are poorly sorted.

Fine-grained sand and silt may be deposited near the margin of the fan in thevalley, commonly in shallow lakes.

These lakes may periodically dry, and evaporite deposits may result.


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Alluvial fans are fan shaped deposits of water-transported material at break in slope.

Consequently, alluvial fans tend to be coarse grained.

Alluvial Fans


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Eolian Deposits

Wind is an effective sorting agent and will selectively transport sand.

Gravel is left behind and dust-sized particles are lifted high into the atmosphereand transported great distances.

Windblown sand forms dunes that are characterized by well-sorted grainsshowing large-scale cross bedding.


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Eolian Structures (Thar Desert Pakistan)


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Glacial Deposits

Glaciers do not effectively sort the materials that they transport.

Common type of resulting deposit is an unstratified accumulation of boulders,gravel, sand, and fine silt for which the term "till" is usually applied.


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Rivers

Fluvial environmentsinclude braided and meanderingriver and streamsystems.

River channels, bars, levees, and floodplains are parts (or subenvironments) ofthe fluvial environment.

Channel deposits consist of coarse, rounded gravel, and sand.


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Rivers

Bars are made up of sand or gravel.

Levees are made of fine sand or silt.

Floodplains are covered by silt and clay.
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Flood Plains

Rivers commonly meander across a flat flood plain before reaching the seaand depositing a considerable amount of sediment.

Rocks formed in a flood plain environment are commonly lenses of "fluvial"sandstone deposited in the meander channel enclosed in a shale depositedon the flood plain.


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Flood Plains

Indus Flood Plains


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Lakes

Lacustrine Environments(or lakes) are diverse; they may be large or small, shallow ordeep, and filled with terrigenous, carbonate, or evaporitic sediments.

Fine sediment and organic matter settling in some lakes produced laminated oil shales.


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Delta

Deltas are large accumulations of sediment that are deposited where a riverempties into a standing body of water.

They are one of the most significant environments of sedimentation and includea number of subenvironments such as stream channels, flood plain beaches,bars, and tidal flats.

The deposit as a whole consists of a thick accumulation of sand, silt, and mud.

Because of the abundance of vegetation in geologically young deltaicenvironments, coals of various ranks commonly are associated with theseclastic sediments.


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Swamps

Swamps (Paludal enviro nments) Standing water with trees. Shale and Coalare deposited.

Marine Environment


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Shoreline

Beaches, bars, and spits commonly develop along low coasts and partly enclose

quiet-water lagoons.

Sediments are well washed by wave action and is typically clean, well-sorted quartzsand.

Behind the bars and adjacent to the beaches, tidal flats may occur where fine silt andmud are deposited; evaporites may be present locally.

Barrier islands

Transitional Environment

Marine Environment

Marine Environment


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Shoreline Lagoons

Lagoonsare bodies of water on the landward side of barrier islands.

They are protected from the pounding of the ocean waves by the barrierislands, and contain finer sediment than the beaches (usually silt and mud).

Lagoons are also present behind reefs, or in the center of atolls.

Marine Environment

Marine Environment


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Shoreline Tidal flats

Tidal flatsborder lagoons. They are periodically flooded and drained by tides(usually twice each day).

Tidal flats are areas of low relief, cut by meandering tidal channels.

Laminated or rippled clay, silt, and fine sand (either terrigenous or carbonate)

may be deposited.

Intense burrowing is common.

Marine Environment


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The con t inental shelfis the flooded edge of the continent.

The cont inenta l s lope and cont inenta l r iseare located seaward of thecontinental shelf.

The abyssal plainis the deep ocean floor. Marine Environment

Marine Environment

Marine Environment


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Shallow Marine

Shallow seas are widespread along continental margins and were even moreextensive during many periods of the geologic past.

Sediments deposited in these shallow marine waters from extensive layers ofwell-sorted sand, shale, limestone, and dolomite, that commonly occur in acyclic sequence as a result of shifting depositional environments related to

changes in sea level.

When the rate of evaporation exceeds the rate of water supply, chemicalsdissolved in the water may be concentrated and precipitated as beds ofgypsum, halite, and more complex salts.

Marine Environment


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Shallow Marine Environment

Marine Environment


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Organic Reef

An organic reef is a structure built of the shells and secretions of marineorganisms.

The framework of geologically young reefs typically is built by corals and algae,but the reef community includes many types of organisms.

A highly fossiliferous limestone commonly is the result of these organisms in therock record.

Marine Environment

Marine Environment


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Organic Reef

Reworking of reef-derived sediments by wave and biological activitiescommonly results in a complex group of sedimentary facies that may bereferred to as the reef tract.

Reefs are wave-resistant, mound-like structures made of the calcareousskeletons of organisms such as corals and certain types of algae.

Marine Environment


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Sketch of Marine Sedimentary Environments


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Development of Organic Reef

Marine Environment


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Deep Ocean The deep oceans contain a variety of sediment types. Adjacent to the

continents, a considerable amount of sediment is transported from thecontinental margins by turbidity currents.

As the current moves across the deep-ocean floor its velocity graduallydecreases, and sediment carried in suspension settles out.

The resulting deposit is a widespread layer of sediment in which the size ofgrains grade from coarse at the base to fine at the top.

Marine Environment

Marine Environment


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Deep Ocean

Such deep-sea deposits are characterized by sequences of graded beds ofthese "turbidites".

Distant to the continents, dust transported by eolian processes mayaccumulate as muds.

In sediment-starved parts of oceans away from the continents, siliceous oozeformed of the tests of microorganisms called radiolaria accumulate.

These sediments form the radiolarian cherts of the rock record.

Marine Environment


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Sketch of Marine Sedimentary Environments

Depositional Environments


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The faciesconcept refers to the sum of characteristics of a sedimentary

unit, commonly at a fairly small (cm-m) scale.

Facies and Depositional Environments


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The facies concept refers to the sum of characteristics of a sedimentaryunit, commonly at a fairly small (cm-m) scale. The characteristics areas follows:

Lithology

Grain size

Sedimentary structures

Color

Composition

Biogenic content



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Generally faciesare divided into three types:

Lithofacies (physical and chemical characteristics)

Biofacies (macrofossil content)

Ichnofacies (trace fossils)



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Walthers Law (1894) states that two different facies foundsuperimposed on one another and not separated by an unconformity,must have been deposited adjacent to each other at a given point oftime.

Facies associations constitute several facies that occur incombination, and typically represent one depositional environment(note that very few individual facies are diagnostic for one specificsetting).



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Facies successions(or facies sequences) are facies associations witha characteristic vertical order.

Facies analysisis the interpretation of strata in terms of depositionalenvironments (or depositional systems), commonly based on a widevariety of observations.

Description of Facies and Depositional Environments


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Shale

Sh l


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The term shale is applied to those rocks,

With grains less than 1/16 mm,

That are fissile, or

Split into thin sheets,

Without regard to silt vs. clay.

Shale

Sh l


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To distinguish siltstones from claystones is very difficult (common name isshale),

The rock how it breaks or splits depends upon gross textures.

Fissile rocks owe their character to parallel alignment of platy grains.

Shale

Environments of Shale Deposition


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Accumulation of mud begins with the chemical weathering of rocks.

This weathering breaks the rocks down into clay minerals, and

Other small particles which often become part of the local soil.

p



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Rainstorm might wash tiny particles of soil from the land, and

Transport into streams, giving the streams a "muddy" appearance.

The stream slows down or enters a standing body of water such as a lake, swampor ocean.

p



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The mud particles settle to the bottom.

If undisturbed and buried this accumulation of mud might be transformed into asedimentary rock known as "mudstone".

This is how most shales are formed.

p


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Composition of the Average Shale

Shale is a rock composed mainly of clay-size mineral grains.

They are usually clay minerals such as illite, kaolinite and smectite.

Shale usually contain other clay-size mineral particles.


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Shale consists of inorganic minerals and organic matter. Themineralogy of shale consists of:

Clay Minerals,

Quartz,Chert,

Feldspar,

Carbonates,

Iron Oxides,

Organic Matter.

Composition of Shale

Clay and Clay Bound Water


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Clay : < 4 micron size fraction of rocks/soils that is composed of hydrous

layered alumino silicate minerals.

Clay Bound WaterAn intrinsic property of a clay type:

Adsorbed water on the clay surface (internal and external).

It occurs as molecules hydrating the cations and as physio-sorbedmolecules.

Excludes the volumetrically continuous phase in the interstitial pores;also excludes capillary bound water.

Clay and Clay Bound Water


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Understand the Clay Water Interface

Pores and Clay Platelets


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Pores and Clay Platelets

Clay Bound Water Content & Surface Areas of Clay Minerals


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Clay Bound Water Content & Surface Areas of Clay Minerals


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Parameters used for shale gas estimation

PREVALENT DATA ON CLAY TYPES


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ALL CLAYS ARE NOT SAME

ALL CLAYS DO NOT SWELL

ALL CLAYS ARE NOT SMECTITE/BENTONI TE

Clay Mineral Composition of the Average Shale


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Clay Mineral Composition of the Average Shale

Illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)],

Kaolinite Al2Si2O5(OH)4 ,

Smectite: (Na, Ca)(Al,Mg)6(Si4010)3(OH)6-nH20 ,

Chlorite: (Mg,Fe,Li)6AlSi3O10(OH)8

The main clay minerals of shale are as under:

Illite


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Illite

Illite is a non-expanding, clay-sized,micaceous mineral.

Illite is a phyllosilicate or layeredalumino-silicate.

The interlayer space is mainlyoccupied by poorly hydratedpotassium cations responsible for theabsence of swelling.

Illite (K,H3O)(Al, Mg, Fe)2(Si, Al)4O10[(OH)2,(H2O)]

Illite


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Illite

Illite occurs as an alteration product of muscovite and feldspar in weathering andhydrothermal environments.

It is common in sediments, soils, and argillaceous sedimentary rocks as well as

in some low grade metamorphic rocks.

The iron rich member of the illite group, glauconite, in sediments can be

differentiated by petrography & X-ray analysis.

Kaolinite


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Kaolinite

It is a soft, earthy, usually white mineral,produced by the chemical weathering ofaluminiumsilicateminerals like feldspar.

Kaolinite has a low shrink-swell capacity.

Al2Si2O5(OH)4O

OH

Al

Si

Kaolinite
http://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Silicatehttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Shrink-swell_capacityhttp://en.wikipedia.org/wiki/Shrink-swell_capacityhttp://en.wikipedia.org/wiki/Shrink-swell_capacityhttp://en.wikipedia.org/wiki/Shrink-swell_capacityhttp://en.wikipedia.org/wiki/Feldsparhttp://en.wikipedia.org/wiki/Silicatehttp://en.wikipedia.org/wiki/Aluminiumhttp://localhost/var/www/apps/conversion/tmp/scratch_10//upload.wikimedia.org/wikipedia/commons/a/a0/Kaolinite_strcutural_model_VA.jpg


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Kaolinite

Kaolinite clay occurs in abundance in soils that have formed from the chemicalweathering of rocks in hot, moist climatesin tropical rainforest areas.

Towards progressively cooler or drier climates, the proportion of kaolinitedecreases, while

The proportion of other clay minerals such as illite (in cooler climates) orsmectite(in drier climates) increases.

Such climatically-related differences in clay mineral content are often used toinfer changes in climates in the geological past, where ancient soils have beenburied and preserved.

Montmorillonite
http://en.wikipedia.org/wiki/Illitehttp://en.wikipedia.org/wiki/Smectitehttp://en.wikipedia.org/wiki/Smectitehttp://en.wikipedia.org/wiki/Illite


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Montmorillonite

Montmorillonite is a very softphyllosilicate group of minerals.

Montmorillonite, a member of the smectitefamily.

Montmorillonite is the main constituent ofthe volcanic ash weathering product,bentonite.

The water content of montmorillonite isvariable and it increases greatly in volume

when it absorbs water

Smectite: (Na, Ca)(Al,Mg)6(Si

40

10)3(OH)

6-nH

20

Chlorite


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Chlorite

The chloritesare a group of phyllosilicate minerals.

Chlorite minerals are ubiquitous minerals within low and medium temperature

Metamorphic rocks,

Some igneous rocks,

Hydrothermal rocks and

Deeply buried sediments.



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Other constituents might include:

Organic particles,

Carbonate minerals,

Iron oxide minerals, sulfide minerals and

Heavy mineral grains.

Other Constituents" in the rock are often determined by the shale's environment of

deposition and often determine the color of the rock.


Colors of Shale


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Like most rocks, the color of shale is often determined by the presence of specificmaterials in minor amounts.

Just a few percent of organic materials or iron can significantly alter the color of arock.

Colors of Shale

Black Shale


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A black color in sedimentary rocks almost always indicates the presence of organicmaterials.

Just one or two percent organic materials can impart a dark gray or black color tothe rock.

Black color almost always implies that the shale formed from sediment deposited inan oxygen-poor environment.


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Oxygen in Shale Environment


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Any oxygen that entered the environment quickly reacted with the decaying organicdebris.

If a large amount of oxygen was present the organic debris would all have decayed.

An oxygen-poor environment also provides the proper conditions for the formation ofsulfide minerals such as pyrite.

yg

Gray Shale


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Gray shales sometimes contain a small amount of organic matter.

Calcareous materials or simply clay minerals in shale result in a gray color.

R d B d Y ll Sh l


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Shales that are deposited in oxygen-rich environments often contain iron oxideor iron hydroxide minerals such as hematite, goethite orlimonite.

Just a few percent of these minerals distributed through the rock can producethe red, brown or yellow colors.

The presence of hematite can produce a red shale.

The presence of limonite or goethite can produce a yellow or brown shale.

Red, Brown and Yellow Shale


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Green shales are occasionally found.

This should not be surprising because some of the clay minerals and micas

that make up much of the volume of these rocks are typically a greenish color.

Green Shale

Colour of Shale as Environmental Indicator


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Colour of Shale as Environmental Indicator


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The different colours of these shale samples tell us something about theconditions at their environment of deposition.

The black colour of the left specimen is due to preserved organic matter in ananoxic or anaerobic environment, whereas

The red/brown sample on the right reflects oxidizing conditions that have turnedthe

iron content red.

Hydraulic Properties of Rock


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Hydraulic properties are characteristics of a rock such as permeability and porositythat reflect its ability to hold and transmit fluids such as water, oil or natural gas.

Hydraulic Properties of Rock

Hydraulic Properties of Shale


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Shale has a very small particle size so the interstitial spaces are very small.

In fact they are so small that oil, natural gas and water have difficulty movingthrough the rock.

Shale can therefore serve as a cap rock for oil and natural gas traps and it also

is an aquiclude that blocks or limits the flow of underground water.




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Although the interstitial spaces in a shale are very small they can take up asignificant volume of the rock.

This allows the shale to hold significant amounts of water, gas or oil but not beable to effectively transmit them because of the low permeability.




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Some of the clay minerals that occur in shale have the ability to absorb or adsorblarge amounts of water, natural gas, ions or other substances.

This property of shale can enable it to selectively and tenaciously hold or freelyrelease fluids or ions.

The oil and gas industry overcomes these limitations of shale by using horizontaldrilling and hydraulic fracturing to create artificial porosity and permeability withinthe rock.



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Expansive Soils


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These soils swell on wetting and when they dry out they shrink.

Expansive soils contain minerals such as smectite clays that are capable ofabsorbing and release large amounts of water.

When they absorb water they increase in volume.

This change in moisture content is usually accompanied by a change in volume.

Expansive Soils

Expansive Soils


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Expansions of ten percent or more are not uncommon.

The more water they absorb the more their volume increases.

This change in volume can exert enough force on a building or other structure

to cause damage.

Buildings, roads, utility lines or other structures placed upon or within thesematerials can be weakened or damaged by the forces and motion of volumechange.

Expansive Soils

Key Factors to Assess the Shale for Gas


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Key Factors toAssessthe Shale for Gas

The key factors to assess the shale gas in a basin are followings:

Lithology (mineralogy) of the formation

Organic matter richness

Maturity of organic matter

Formation thickness

Formation depth

Assessment of Rock Mineralogy


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Lithology (Mineralogy) of a rock can be assessed by different means:

with Petrographic Microscope,

XRD and XRF

SEM and

with Spectral Gamma Ray Log

Assessmentof Rock Mineralogy


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Introduction: Gamma Ray Logs


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The value of high gamma ray in shaleis due to:

the uranium fixed in organic matter,

the thorium in clay minerals and potassium content (principally illite).

Sandstone, limestone and dolomite generally have low level of radioactivity(gamma ray).

Introduction: Gamma Ray Logs


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The high gamma ray insandstonecan be attributed to:

Clay minerals

Potassium feldspars,

Mica and

Heavy minerals.

Spectral Gamma Ray Log: Applications


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Estimation of clay mineral volume (types).

The recognition of fractures filled by uranium salts.

The potassium-thorium crossplot is useful for:

The recognition of clay minerals.

Distinction of micas and K-feldspars.




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Micas have higher content of K than clays.

Illite has higher potassium than mixed layer clays or smectite.

Kaolinite has very little or none of potassium.




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Uranium has an insoluble tetravalent state (U+4) that is fixed under reducing

conditions.

Uranium can be transformed to the hexavalent state (U+6) which is soluble.

Thorium has a single insoluble tetravalent state (Th+4) which is geochemicallyassociated with uranium and becomes a useful standard for comparison.




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The Th/U ratio is useful in the recognition of geochemical facies.

The Th/U ratio is an indicator of redox-potential.




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Th/U7

Reducing Oxidizing

Adams and Weaver (1958) proved the following results:

When Th/U ratio was less than two (i.e. uranium-rich) the depositional environmentwas reducing marine.

When Th/U ratio was greater than seven (uranium-poor), due to uranium mobilizationthrough weathering or leaching indicating an oxidizing, possibly terrestrial origin.

Insoluble U+4 SolubleU+6

Insoluble Th+4 Insoluble Th+4

2 7


Gamma Ray Logs in Carbonates


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Incarbonates the high gamma rayis due to:

Shaly horizons

Uranium mineralization (diagenetic processes within fracture system)

Interpretation of Spectral Gamma Ray Data inCarbonates


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K Th U Explanation

LowLow Low Pure carbonate, no organic matter or oxidizing

environment.

LowLow High Pure carbonate, organic matter reducing

environment.

LowHigh Low Not a carbonate, or shaly carbonate with rarer low

K high Th clay minerals no organic matter, oroxidizing environment.

LowHigh High Not a carbonate, or shaly carbonate with rarer low

K high Th clay minerals organic matter, reducingenvironment.

HighLow Low

Glauconite carbonate, no organic matter, oroxidizing environment. Also consider K-evaporite.

HighLow High

Algal carbonate, or glauconite present, organicmatter, reducing environment.

High HighLow Shaly carbonate, no organic matter or oxidizing

environment.

HighHigh High Shaly carbonate,organic matter, reducing

environment.

Significance of Depositional Environment


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An important criterion is the depositional environment of the shale, particularlywhether it is marine or non-marine.

Marine-deposited shales tend to have lower clay content and tend to be high inbrittle minerals such as quartz, feldspar and carbonates.

Brittle shales respond favorably to hydraulic stimulation.

g p


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Mineralogy and Depositional Environment by Spectral Gamma Ray Log


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133/362



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Sardhi

0.5

0.1 2 7 100

Th/U

Th/K

1

Feldspar

Glauconite

Mica

Illite

Smectite

Mixed layerclays

Kaolinite-

Chlorite

50

10

Bahu We ll #01

Fixed U Leached U

OxidizingReducingOxidizingReducing

Warccha

50

10

0.5

0.1 2 7 100

Th/U

Th/K

1

Feldspar

Glauconite

Mica

Illite

Smectite

Mixed layerclays

Kaolinite-

Chlorite

Bahu We ll #01

Fixed U Leached U


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Chart showing the clay mineralogy derivedfrom Th/K ratios in different froamtions inBahu, Amir Wali and Ali Sahib wells

Mineralogy & Depositional

Environment Assessment bySpectral Gamma Ray Logs


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Mineralogy by XRD and SEM Case Studies


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CLASTICS OF THE ALI SAHIB WELL


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Lithological Log (Profile) of Core 04 of Ali Sahib Well


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AGE FORMATION LITHOLOGYLithofaciesno.

1

2

1886.00M

1886.90M

1893.45M

JU

R

A

S

S

IC

SHIN

WA

RY?

SANDSTONE: TRANSPARENT,WHITE,LOOSEANDFRIABLE,FINETOMEDIUMGRAINED,SUBANGULARTOSUBROUNDED,QUARTZOSE,MODERATELYWELLSORTED,ANDPOORLYCEMENTED,OCCASIONALLYPYRITIC,SLIGHTLY

TONONCALCAREOUS,

CALCIMETRY: CACO3 =2%VISUALPOROSITY- PRIMARYFAIRTOGOOD,WHICHIS INTERGRAULAR.

Scale 1.5 cm = 1 m

As33

As34

As35

As36

As37

As38

As39

As40

As41

AS37

Sandstone

Claystone

Claystone: Brick red todirty brown, soft tomoderately firm, partlysticky and hydrophylic,slightly to non

calcareous


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Main Objectives


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BY assessing lithological elements in detail, to delineate the provenance,weathering and depositional patterns in Jurassic age.

Pre-microprobe Anylysis


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General lithological description of Core (Red Claystone).

Measurement of porosity and permeabilty

Calcimetry.

XRD.

The XRD Of The Sample


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2 - T h e t a

2 . 0 0 8 . 0 0 1 4 . 0 0 2 0 . 0 0 2 6 . 0 0 3 2 . 0 0 3 8 . 0 0 4 4 . 0 0 5 0 . 0 0 5 6 . 0 0 6 2 . 0 0

A S 3 7 - t1 8 9 0 . 2 0 C 4

Kaol

Kaol

Kaol

Feldspar

HemaKaol

Kaol

KaolKaol

Hema HemaKaolHema

Pre-microprobe Anylysis Conclusion


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Brick red to dirty brown,soft to moderately firm, partly sticky and hydrophylic,slightly to non calcareous.

The general lithological description (red claystone) and XRD pattern, we mayeasily conclude that sample consists of only Kaolinite and Hematite withsmall traces of Feldspar (oxidized claystone).

The measurement of porosity, permeabilty and calcimetry is not to bediscussed here.

Zircon with Backscattered Electron


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Zircon

Kaolinite

AS 37 1890.20 m C 4

Zircon with CathodeluminicenceZoning can be seen


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AS 37 1890.20 m C 4

Zircon With CathodeluminicenceFracture / Pores and Zoning in The Grain is

Prominent


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Zoning

AS 37 1890.20 m C 4

Fracture/ Pores

Ilmenite (FeTiO3) with Fractures


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AS 37 1890.20 m C 4

Fracture

Ilmenite Grains with a Lot of Pores and Fractures


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Fracture

Pores

AS 37 1890.20 m C 4

Kaolinite Al2Si2O5(OH)4, Ilmenite FeTiO3(below left) and Hematite Fe2O3 (right)


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Ilmenite

Hematite

Kaolinite

AS 37 1890.20 m C 4


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ORGANIC MATTER CONTENT BASICS

Organic Shale


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Deposited in environment with little or no oxygen.

Animals cantsurvive and organic mush accumulates.

Where sediment contains more than 5% organicmatter (by volume), it eventually forms a rock known

as a black shale.


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Organic Matter Content Basics


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To know if a sedimentary rock may have any petroleum potential, laboratoryanalyses of the organic compounds contained in the rock are necessary.

In the subsurface, hydrocarbons are produced by thermal alteration of the

organic matter at temperatures between 50 and 175 C through a long period oftime.

Organic Matter Content Basics


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The substitution of the natural conditions by unnaturally high temperatures inthe laboratory is necessary so that hydrocarbons can be produced overpractical periods of time.

Laboratory pyrolysis of the organic matter in sedimentary rocks aims to parallelthe changes in the subsurface (BAJOR et al., 1969) and provides a usefultechnique for characterizing organic matter.


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Types of Organic Matter



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Total Organic Content (TOC) is the basic quantitative parameter that must beused when determining the petroleum generation potential of a stratigraphicunit.

However, although organic matter content in sediments is usually estimated bya determination of organic carbon, the limiting element in the petroleum forming

reaction is not carbon but hydrogen.

The reason for analyzing carbon, however, is that only the hydrogen bonded inorganic molecules is active in the petroleum forming processes.


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The amount of organic hydrogen is essentially controlled by the nature of the organicmatter present in the sediment, and thus,

The kerogen has been broadly classified in four types (I, II, III, and IV Fig next slide)depending on the relative content of organic hydrogen.

These four kerogen types correspond to distinct biological source materials.



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SOURCE ROCK

Definition of Source Rocks


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Rocks that generate petroleum are "potential source rocks" and only can be classed as"source rocks" after commercial quantities of petroleum have migrated out of them.

A more detailed definition of "potential source rock" was given by DOW (1977):

"A unit of rock that has the capacity to generate oil or gas in sufficient quantities to formcommercial accumulations but has not yet done so because of insufficient thermalalteration".


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WHOLE SAMPLE ROCK-EVAL PYROLYSIS TECHNIQUE

Whole Sample Rock-eval Pyrolysis Technique


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The rock-eval pyrolysis technique is based on the methodology described by ESPITALIet al. (1977, 1985), ESPITALI (1986), PETERS (1986), and RIEDEGER (1991).

This technique provides data on the quantity, type, and thermal maturity of the associatedorganic matter.



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This a simple and quick technique that can be carried out in the laboratory and in the wellsite since it does not require especially expensive or highly sophisticated equipment.

Furthermore, the pyrolysis analyzer uses whole rock samples that do not need anyprevious treatment.



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Following this technique each sample is pulverized and 100.0 mg (+/- 0.1 mg) were

weighed into stainless steel crucibles.

These crucibles have a fritted (glass is finely porous glass through which gas or liquidmay pass) or screened top and bottom which allow the passage of helium carrier gas andair for oxidation through the sample.



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The samples are, one at a time, placed in an oven, and the generated data is sent to aconnected computer set.

After a sample is automatically placed in the oven, this is closed and the air purged with aflow of helium.



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Analysis of each sample requires about 20 min.

The sample is first heated under an inert atmosphere of helium at 300 C for 3-4 min andthen paralyzed at 25 C/minute to 600 C, followed by posterior cooling down for the nextsample to be run.



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The heating of organic matter in the absence of oxygen yields organic compounds.

In the first stage of pyrolysis when the sample is maintained at 300 C the free organiccompounds (bitumen) already present in the rock are distilled.



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In the second stage of increase heating to 600 C the insoluble organic matter (kerogen)is cracked down into pyrolytic products.

Flame ionization and thermal conductivity detectors sense any organic compounds andCO2 generated during the two stages.



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In the first stage the helium gas flow sweeps the volatile products out of the oven to asplitter.

The first half of the split effluent is sent to water scrubber and then to a CO2trap.



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The second half is directed into a hydrogen flame ionization detector where hydrocarbonsvolatile at 300 C are detected and quantitatively measured.

After 2 minutes the oven increases its temperature at a rate of 25 C/ minute up to 600C.

Once this temperature has been reached the CO2 trap is dumped onto a thermalconductivity detector and the amount of thermally evolved organic CO2is measured.



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The pyrolysis values collected on the computer are presented in a table that includesvalues such as Tmax, S1, S2, S3, PI, S2/S3, TOC, HI, and OI.

All these values are indicative of the level of maturity of the organic matter, the type ortypes of organic matter, and the amount of hydrocarbons already produced or that can beproduced from a studied rock sample.



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The first value, Tmax, represents the temperature at which the maximum amount of

hydrocarbons degraded from kerogen are generated (Fig. below next slide ).

Tmax does not represent the actual burial temperature of the rock but rather a relativevalue of the level of thermal maturity.

If the rock has not been subject of oil generation then the organic matter has been littlealtered and, therefore, if heated during pyrolysis it will produce hydrocarbons.

In turn, if the organic matter is more mature it will take more temperature to make itproduce hydrocarbons since it has already been impoverished.



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The second value is S1 and represents milligrams of hydrocarbons that arethermally distilled from one gram of rock.

The S1 peak is measured during the first stage of pyrolysis at the fixedtemperature of 300 C.

As rocks are buried they are subjected to increasing temperatures andhydrocarbons start to be generated. These hydrocarbons form the S1peak.



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The third value is S2 and indicates the milligrams of hydrocarbons generated

from degrading the kerogen in one gram of rock during the second stage ofpyrolysis.

Tmaxis the temperature at the maximum of the S2peak.

The larger the S1peak the deeper (up to a point) and more mature the organicmatter is.



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This increase occurs at the expense of the S2peak which not only decreases insize but also moves to higher temperatures as the less thermally stable materialhas already broken down during natural maturation leaving a thermally morestable kerogen residue in the rock.

If there is very little organic matter in the rock (below 0.3 wt. %) a very limitedamount of hydrocarbons can be produced and thus, the S1and S2peaks will bevery low and form a wide gentle hump.



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The pyrolysis analyzer, then, will have difficulties to pick the highest point of thehump and will present unreliable, scattered data.

Both the ratio S1/S2and Tmaxindicate the level of maturity of the organic matter.



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The forth value is called S3 and expresses the milligrams of carbon dioxide

generated from a gram of rock during temperature programming up to 390 C.

Next comes the production index (PI) which is defined as the ratio S1/(S1+S2).



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PI is an indication of the amount of hydrocarbon which has been producedgeologically relative to the total amount of hydrocarbon which the sample canproduce.

The S2/S3 ratio is the sixth value and represents a measure of the amount of

hydrocarbons which can be generated from a rock relative to the amount oforganic CO2released during temperature programming up to 390 C.



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HI versus Tmaxof samples from Ali Sahib and Amir Wali wells. W: Ditch cuttings of the Amir Wali Well, CW: Core ofthe Amir Wali Well, S: Ditch cuttings of the Ali Sahib Well, CS: Core of the Ali Sahib Well.

AUTHORS: Gakkhar et al.Source-rock Potential and Origin of Hydrocarbons in the Cretaceous & Jurassic S ediments of the Punjab Platform (Indus Basin) Pak istan



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S2

/S3

ratios are considerably lower for Type III kerogen than for Type II andType I because terrestrially derived organic matter contains substantially moreoxygen than the other types of organic matter.

The Pyrolyzed Carbon (PC) is defined as the ratio (S1+S2)/100 and is anotherorganic type indicator.

Type I kerogen yields PC values of about 80 %, Type II of about 50 %, andType III between 10-30 %.



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After pyrolysis is complete the sample is transferred to an oxidation oven where

is heated to 600 C in the presence of air.

There the residual organic matter generates CO2 which is quantitativelymeasured by passing the effluent over a series of traps and catalysts.

The amount of this carbon is added to S1and S2 to obtain the Total OrganicCarbon (TOC) content that is given in weight percentages.



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Two other obtained values are the Hydrocarbon Index (HI) and Oxygen Index(OI).

HI is defined as the ratio S2/TOC, and represents the quantity of pyrolysableorganic compounds from S

2relative to TOC in the sample.

OI is defined as S3/TOC and corresponds to the quantity of carbon dioxide fromS3 relative to TOC.



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Petroleum generation results from the transformation of sedimentary organic

matter in the subsurface under the influence of both temperature and geologictime.

This transformation can be ascribed to the thermal cracking of the kerogenwhich releases micropetroleum into the pore system of the source rock

(TISSOT and WELTE, 1984; HUC, 1990).



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Rock-eval pyrolysis permits rapid evaluation of the organic matter type, quantityand maturity and, thus, yields information on the petroleum-generative potential.

However, a minimum amount of organic matter is needed to obtain reliableresults.

This technique is based on the production of hydrocarbons from a rock sampleby steadily heating it.



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However, if the amount of organic matter present in the rock is very small or iscompletely burned little information can be obtained.

If high thermal maturation values (expressed as high, R0, vitrinite reflectancevalues) are suspected other techniques should be more appropriate to the studyof the organic matter.



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The amount of organic matter is expressed by the TOC values which represent

the total amount of organic carbon present in the rock.

For shales, usually a TOC of 2.0 % is considered to be good, and a

TOC value higher than 4 % is considered as very good.

For limestones even lower values are good.

The Tmax value represents the temperature at which the largest amount of



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hydrocarbons is produced in the laboratory when a whole rock sample

undergoes a pyrolysis treatment.

The production of these hydrocarbon by pyrolysis is linked to the amount ofhydrogen the rock still contains.

The more mature the rock is the lower amount of hydrogen it contains and thehighest amount of energy it needs to liberate hydrocarbons.

The thermal maturation level is deduced from the Tmax values.



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In general, Tmax values lower than 435 C indicate immature organic matter(organic matter).

Tmax values between 435 C and 455 C indicate "oil window" conditions

(mature organic matter).



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Values between 455 and 470 are considered transitional.

A Tmax higher than 470 C represents the wet-gas zone and over matureorganic matter (PETERS, 1986).

The thermal maturation level for oil-prone type I kerogen is often higher than forthe other types of kerogen (TISSOT et al., 1978).

T d t f l ith S k l th 0 2 HC/ k b



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Tmax data for samples with S2 peaks less than 0.2 mg HC/g rock may be

inaccurate because the S2 peak becomes so broad and low that there is nosharp top and, thus, the analyzer takes any point as the top of the peak.

For this reason if there is very little organic matter in the rock, the S2peak isvery low and broad, and Tmax values are scattered and unreliable.



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Tmaxvalues may also be depressed by the presence of resinite from fossil treeresin or they may be increased by the presence of other organic compoundssuch as gilsonite.

For the same maturation level, carbonate-rich rocks usually yield lower Tmax

values than clay-rich samples.


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The type of organic hydrogen is controlled by the nature of the organic matter



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The type of organic hydrogen is controlled by the nature of the organic matter.

Aquatic organic matter has a high hydrogen content whereas terrestriallyderived organic matter has a low hydrogen content and a variable high oxygencontent.



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The type I kerogen has a mono specific algal origin and presents the highesthydrogen content.

Thus, this type of kerogen usually gives the highest HI values.

In the HI versus OI pyrogram of PETERS (1986) this type of kerogen occurs inthe upper left owing to its high hydrogen and low oxygen contents.



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Type II kerogen is originated mainly from phytoplanktonic organisms and has arelatively high hydrogen content but not as high as type I.

Therefore, HI values for this type of kerogen are intermediate and occupy thecentral part of the both pyrograms.

This is usually the oil forming kerogen.

The HI versus Tmax diagram also will provide information about the maturation



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The HI versus Tmax diagram also will provide information about the maturation

level, and, thus, the oil generation expectancies.

Type III of kerogen corresponds to terrestrially produced organic matter,especially material from higher plants.

The majority of the terrestrial plant material has less hydrogen than the aquaticplant material.

In addition, the terrestrial plant organic matter is transported (usually by fluvial



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add t o , t e te est a p a t o ga c atte s t a spo ted (usua y by u a

systems) for relatively long distances before it is deposited in subaquaticbasins.

During transport the terrestrial organic matter is partly degraded andimpoverished in hydrogen.

Due to its nature and the degradation suffered during transport, terrestrialorganic matter and, therefore, its sedimentary counterpart, the type III kerogen,present low HI values.

Type III kerogen commonly plots on the lower part of both pyrograms



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Type III kerogen commonly plots on the lower part of both pyrograms.

This type of kerogen usually is a source of gas rather than oil.

The maturation level will offer information on the gas forming capability of thestudied sample.

Type IV of kerogen represents an extreme of type III and contains very little



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Type IV of kerogen represents an extreme of type III and contains very little

hydrogen.

If plotted on the pyrograms it occurs on the bottom of the diagrams.

The only difference with type III is that type IV organic matter usually has highTmax values or lacks the S2peak.

Type IV kerogen behaves as oxidized kerogen.

The Production Index (PI) is also in part indicative of the degree of thermalmaturity (Peters 1986)

Interpretation Of Rock-eval Pyrolysis Data


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maturity (Peters, 1986).

In general, PI values below 0.4 indicate immature organic matter; PI valuesbetween 0.4 and 1.0 indicate mature organic matter; and PI values above 1.0are indicative of overmature organic matter.

Outcrop samples commonly show depletion in S1 and S2 and high S3 valuesdue to weathering



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due to weathering.

PI is defined as the ratio S1/(S1+S2), and, hence, depletion of S1and S2mayinduce changes on actual PI values.

Immature sediments commonly yield poorly separated S1and S

2peaks which

can lead to anomalous results.

O id i i h f f d d i f i



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Oxidation is the most common form of degradation of organic matter.

Oxidation removes hydrogen and adds oxygen to the kerogen, and therefore, HIvalues are usually lower and OI values higher for outcrop samples than forfresh-core samples.


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Mechanism of Shale Gas Storage


At a given temperature and pressure, the gas sorption capacities of organic rich


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At a given temperature and pressure, the gas sorption capacities of organic rich

shales are primarily controlled by the organic matter richness, but

The significantly influenced by the type and maturity of organic matter,

Mineral composition (specially clay content),



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Moisture content,

Pore volume and structure,

Resulting in different ratios of gas sorption capacities (GSC) to total organiccarbon content for different shales.


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Under geological conditions (assuming hydrostatic pressure gradient and constant

thermal gradient),

the GSC increases initially with depth due to the predominant effect of pressure,

passes through a maximum, and then

decreases because of the influence of increasing temperature at greater depth.


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Notionally, the Gas Storage Capacity (GSC) of an organic rich shale is controlledby:

1. Characteristics of organic matter (richness, type and maturity),

2. The composition of matrix minerals,



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3. The volume and structure of pores,

4. The content of moisture, and

5. The pressure and temperature regimes


Natural gas stored in organic rich shales may exist in three forms:


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g g y

a) free gas in pores and fractures,

b) adsorbed gas in organic matter and inorganic minerals,

c) dissolved gas in oil and water

Two models are proposed to predict the variation of GSC and total gas content



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p p p g

over geological time as a function of burial history.

High contents of free gas in organic rich shales can be preserved in relativelyclosed systems.

Loss of free gas during post generation up lift and erosion may result in undersaturation (total gas contents lower than the sorption capacity) and is the majorrisk for gas exploration in marine organic rich shale.

Characteristics of Organic Matter (Richness,Type and Maturity)

Overmature organic rich shales have high degrees of organic matter conversion


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and, therefore, have good potential to have high gas contents and high gas flowrates (Jarvie et al).

Barnett Shale of USA is the primary source rock for conventional oil and gas,and has produced approximately 2 billion barrel of oil and 7 tcf of gas since theproduction began in the early 1900s (Hill et al., 2007a & 2007b).

Gas contents for the overmature shales are indeed significantly higher thanthose for shales within oil window (Ro< 1.3 %),

Characteristics of Organic Matter (Richness,Type and Maturity)


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In contrast, the most richest marine shales in South China had made a littlecontribution to the conventional oil and gas so far found because mosthydrocarbons generated and expelled had been lost during intensivepostgeneration tectonic motions (Ma et al., 2004).

Same is the case with Pakistan particularly in Upper Indus Basin.

However, the Cretaceous Shales in the Lower Indus Basin has contributed a lotas a source rock and assumed to be gas filled.


Understanding the relative proportions of gas stored in these different forms is


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Understanding the relative proportions of gas stored in these different forms iscritical to an accurate assessment of shale gas resources (Zang et al., 2012).

Adsorption is the process of molecules accumulation on the surface of material(adsorbent) and is a consequence of surface energy minimization (Zang et al.,2012).

Because of the differences in chemical structures, and/or specific surface areas,different kerogen types or coal maceralshave different gas sorption capacities.


Zang et al., 2012 showed that the gas sorption capacities of kerogenes


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decrease in the following order:

type III> type II > Type I.

They attributed the differences in gas sorption capacities among differentkerogen types to changes in chemical structures and stated that aromatic rich

kerogens have stronger affinity for methane than kerogen containing aliphaticorganic matter.


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The thermal conversion of kerogen to petroleum results in the formation of an



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increasingly aromitized carbon rich residue and generates organopores(microscale and nanoscale within organic matter in shales (Slatt and O Brien,2011).

The increased GSC for high maturity may be caused by the combination ofincrease in aromitization (Zang et al., 2012), increase in organoporosity andsurface, and decrease in pore surface hetrogeneity with increasing maturity

(Ross and Bustin, 2009).

Quartz and carbonate mineralshave low internal surface areas and, therefore,have low GSC ((Ross and Bustin, 2007).

Clay may adsorb gas to their internal structure (Valzone et al., 2002).



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Different clay minerals have different micropores volumes and surface areasand therefore, have different gas sorption capacities.

On dry basis, illite and montmorillonite have larger sorption capacities thankaolinite.



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For example, the GSC for illite at 30oC and 7 MPa (1015 psi) is as much as 3cm3 /g.

Clay minerals are hydrophillic, and the existence of moisture will significantlyreduce the adsorption capacities of clay minerals.

As a result, mineral play a relatively less significant role in a gas adsorptionwithin organic rich shales (Zang et al., 2012).

Gas in organic rich shales may be stored in matrix pores, organopores, or



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fracures.

The tengas producing have an average porosities between 3% and 14%.

Shales and mudstones in different sedimentary basins exhibit widely varyingrelations between porosity and burial depth and may have a porosity of as

much as 15% at depth deeper than 5 km (Mondol et al 2007).



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Organic rich shales are dominated by nanometers pore scales but may havepores up to the micrometer or sub-millimeter scale.

Pores in organic rich shales are classified as micropores (less than 2 nm),mesopores (2-50 nm) and macropores (greater than 50 nm).

Pore volume and structure of organic rich shales have an important influence on



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gas sorption capacities.

Micropore volumes increase with increasing TOC and Al2O3contents in certainshales.

The microporous surface areas display an increasing trend, whereas

mesoporous surface areas display a decreasing trend as TOC contentsincrease (Chalmer and Bustin, 2008 a, b).


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Fractures are common in organic rich shales (Slatt and Abousleiman, 2011) and



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may potentially provide a large amount of surface area.

However, the role of fracture surface area in gas adsorption in organic richshales is still poorly understood.



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The gas sorption capacities of coals can apparently be reduced by 60 % to 90% as compared to dry state.

The effect of moisture contents on the gas sorption capacities of organic richshales are quite different. Because organic rich shales have much highercontent of minerals and hydrogen rich organic matter ( for marine shales of lowmaturity level).

Clay minerals are hydrophyllic, and organic matter is hydrophobic. Therefore, a



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selective sorption of methane and moisture in organic shales must exist.

Moisture is preferentially adsorbed on clay minerals, whereas methane may bepreferentially adsorbed on the surface of organic matter.

Moisture acts as a dilutent to gas sorption.

Under moisture equilibrated conditions, moisture moisture may make manymicroporous sorption sites unavailable to methane by filling pore throats or



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microporous sorption sites unavailable to methane by filling pore throats oroccupying sorption sites.

The GSC under the moisture-equilibrated state is less than 60 to 70 % of thatunder dry state for organic rich shales.

When pressure is greater than 1 MPa (145 psi), the GSC under moisture-equilibrated conditions is at least 25% lower than that under dry conditions.



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equilibrated conditions is at least 25% lower than that under dry conditions.

This confirms that the presence of moisture significantly reduces the gassorption capacities of organic rich shales.

A general correlation between TOC content and GSC has been observed foralmost all organic rich shales which suggests that TOC content is the most



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g ggsignificant control on GSC.

However, the ratios of GSC to TOC content for different shales varyconsiderably.



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Certain organic rich shale display no correlation between gas sorptioncapacities and TOC contents as Gordondale Formation of Canada having 12 %TOC and lower than 1.0 m3/ton.

This formation consists of high quartz and calcite contents 60 %- 90% of themineral phase; and

Relatively low thermal maturity (Ro < 1.2 %) and has low micropore volumesassociated with organic matter and clays, which accounts for the low GSC-to-TOC content ratios.

The level of kerogen aromitization, high micropore volumes associated withboth high-maturity organic matter and clay minerals, and decreased pore



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g y g y , psurface heterigeneity accounts for the high ratios of GSC to TOC content.

Varying kerogen types (type I, II, II/III, and III), varying thermal maturity (Tmax416oC to 476oC), and varying clay mineral contents 14 % to 88 % show mediumratio of GSC to TOC content.

Adsorption is an exothermic process, and therefore, the gas sorption capacities



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of organic rich shales decrease with increasing temperatures.

The temperature dependence of the sorption capacity is controlled by theisosteric heat of sorption which, in turn, depends on the surface coverage.

The isosteric heats of sorption range from 10 to 22 kj/mol for types I to IIIkerogens (Zang et al., 2012).


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Using equations 1 and 2, the effect of temperature on pressure on GSC is



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expressed as / max.

The temperature and pressure conrolled gas sorption give important insight intothe variation of gas sorption capacities of organic rich shales under geologicalconditions.


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CONVENTIONAL VERSUS

UNCONVENTIONAL RESERVOIRS

Conventional Versus Unconventional Reservoirs


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The main differences are :


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- low-permeability structure itself

- response to overburden stress

- impact of the low-permeability structure on effective permeability relationshipsunder conditions of multiphase saturation, or

- understanding of multi-phase, effective permeability to gas at varying degreesof water saturation under conditions of overburden stress

Why Unconventional


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With the increased global demands on oil and gas,

Operators conducting more advanced drilling operations, such as

Horizontal and high-pressure/high-temperature (HP-HT) drilling into

unconventional resources.

Unconventional gas resources offer significant gas production growth potentialin the coming years, currently accounting for 43% of the US gas production.



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However, economical production of Unconventional is very challenging as itexists in reservoirs with nano to micro-Darcy range permeability and lowporosity.

But has a huge potential for production in the future.

Poor permeability results in lower gas production rates from Unconventionalreservoirs.

In order to economically develop Unconventional resources an advancedtechnology has to be developed and implemented.



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Most of the Unconventional reservoirs (Shale Gas /Tight Gas) are characterizedby being thick where their gas production rates can be enhanced by hydraulicfracturing.

The used technology to drill, complete and stimulate Unconventional reservoirsis quite complex and the results are often unexpected and unforeseen.

The appropriate completion methods and stimulation techniques in these reservoirs



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are dependent on many parameters and variables, such as:

Depth,

Pressure,

Temperature,

Capillary and

Overburden pressures and

The number of sand layers.

The total scope of gas resources was viewed as a triangle for the first time byMaster (1979) as shown in Figure Resource Pyramid.



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This figure shows that the natural gas resources are distributed log-normally innature with respect to formation permeability of tight gas sands.

The triangle peak represents the conventional gas, which is relatively easy to

extract, with a small available supply.

There is much larger supply of unconventional gas, which makes up the base of

th t i l b t it i diffi lt t t t



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the triangle, but it is more difficult to extract.

As development of gas continues, oil and gas industries are moving down thetriangle and developing more unconventional gas resources that are difficult tobe exploit but they are large in size.

The tight sand gas reserves distribution is well-matched with the scheme of the

t i l h i Fi ( b lid )

CONVENTIONAL VERSUS UNCONVENTIONAL RESERVOIRS


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resource triangle shown in Figure (above slide).

This figures confirms the fact that significant improvement in technology orchanges in the gas market are required before the gas in the resourcescategory can be produces at an economic level.

100 md

1000 md

Resource Pyramid

D

em

Incr

D

ev

Cont


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Figure showing availability of gas in different reservoir

0.0001 md

0.001 md

0.1 md

1 md

100 mdemand

creasingCost

evelopmentTechno

logy

ntinousdemandanddevelopment

Ti ht d d b t 6 T f f i th U it d St t hi h i



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Tight sands produce about 6 Tcf of gas per year in the United States which is27-30% of the total gas produced.

As of January, 2009, the U. S. Energy Information Administration (EIA)estimates that 310 Tcf of technically recoverable tight gas exists within the U.S.



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Worldwide, more than 7,400 TCF of natural gas is estimated to be containedwithin tight sands (Rogner, 2006) with some estimates as large as 30,000 TCF.

According to Holditch et al. (2007) large resources of unconventional gasreservoirs exists worldwide.

Kawata and Fujita (2001) summarized the work of Rogner (1996) the world withtotal unconventional resources of 32,560 Tcf,


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Driving the Development of Unconventional

The related developed technology in the United States over the past 3 to 4


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The related developed technology in the United States over the past 3 to 4decades will be available for application around the world.

New technology is rapidly becoming a worldwide commodity through efforts ofmajor service companies.

The global need for energy, particularly natural gas, will continue to be anincentive for worldwide unconventional gas resource development.


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If technology can be developed well enough to provide a better estimate offormation permeability, porosity and water saturation, the development ofunconventional reservoirs can

shale characterization

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