fundamentals of reservoir geoscience

7/31/2019 Fundamentals of Reservoir Geoscience

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Reservoir Geoscience

PCB2013

Fundamentals of Reservoir Geology

Assoc. Prof. Swapan Kumar Bhattacharya

Petroleum Geoscience Depratment



Course outline(Reservoir Geoscience)

Dr. Swapan Kumar Bhattacharya (Course coordinator).

Coursework: (50%)

Test-1 (20%)

Quizz (10%)

Test/Lab. ( 5%) Exercises (5%)

Assignments (10%)----------------------------------------Final Examination: (50%)

REFERENCES:Stratigraphic reservoir characterization for petroleum geologists,geophysicists and engineers – Roger M. Slatt.



Learning Outcomes

Students should be able to :

• Interpret the Depositional Environment of

sedimentary rocks

• Analyse petrophysical properties and

subsurface facies from log and seismic

• Interpret Reservoir Distribution and Geometry

• Describe Hydrocarbon Distribution in a reservoir through

geophysical and geochemical studies

3



Learning Outcome-1

1. Interpret the Depositional Environment of

sedimentary rocks

Fundamentals Transport & Deposition Geological Control

(1) Classification (1) Physical Processes (1) Control of

(2) Texture (2) Bedforms & Sed. Str. environment on

(3) Structure (3) Py. & Sec. Structures reservoir limits

and reservoir

properties

4



Learning Outcome-2

Analyse petrophysical properties and subsurface facies from log and seismic

Poro- perm Clastic facies Carb. facies

5



Learning Outcome-3 &4

Interpret Reservoir Distribution and Geometry

&

Describe Hydrocarbon Distribution in a Reservoir

Reservoir Continuity Reservoir Continuity

(1) Geological Time (1) Basin Architecture

(2) Steno’s Principles (2) Sequence boundary

(3) Lithostratigraphy (3) Diagenesis related to

(4) Geological Events Unconformities(4) Slope & Basin floor fans

• Describe Hydrocarbon Distribution in a reservoir through

geophysical and geochemical studies: Res. Size, Shape &

modeling, Res. Geophysics.

6



A stratum (plural: strata ) is a layer of sedimentary rockwith internally consistent characteristics that distinguish it

from other layers. The "stratum" is the fundamental unit ina stratigraphic column and forms the basis of the study of

stratigraphy.

http://en.wikipedia.org/wiki/Sedimentary_rock

http://en.wikipedia.org/wiki/Stratigraphy

http://en.wikipedia.org/wiki/Stratigraphy

http://en.wikipedia.org/wiki/Sedimentary_rock



Sedimentary structures

Sedimentary structures are particularly important in the

interpretation of earth history. These rocks form at the

earth’s surface and as layer upon layer of sediment

accumulates, each records the nature of the environment atthe time the sediment was deposited (bed geometry).

Sedimentary structures are large-scale features of

sedimentary rocks such as parallel bedding, cross bedding,

ripples, and mudcracks that are best studied in the field.



http://en.wikipedia.org/wiki/File:The_River_Cam_from_the_Green_Dragon_Bridge.jpg



Detrital/Clastic sedimentary rocks can be classified by grain size

differences (size & shape)

PAB 1023 Petroleum Geoscience 13

Conglomerate

Breccia

Sandstone

Shale

All these rocks have clastic textures – the rocks are composed ofparticles (fragments) that are cemented together



Chemical and biochemical sedimentary rocks


Limestones – composed of calcite

Travertine Coquina



Chemical sedimentary rocks


Evaporites

Gypsum

Rock salt

Chert (silica)

Flint

Agate




Coal forms from plantmaterial in continentalsediments

On the continental

shelves, the organic remains of marine plants and animals are buried in

sediment, and become oil and gas




1. Original Horizontality- all sedimentary rocks are

originally deposited horizontally. Sedimentary rocks

that are no longer horizontal have been tilted from their

original position."Strata either perpendicular to the horizon orinclined to the horizon were at one time parallel tothe horizon." Steno, 1669

Principles of Stratigraphy





2. Lateral Continuity- sedimentary rocks arelaterally continuous over large areas."Material forming any stratum were continuousover the surface of the Earth unless some other

solid bodies stood in the way." Steno, 1669

3. Superposition"...at the time when any given stratum was beingformed, all the matter resting upon it was fluid,and, therefore, at the time when the lower stratumwas being formed, none of the upper strataexisted." Steno, 1669.





4. Cross-Cutting Relations

"If a body or discontinuity cuts across a stratum,it must have formed after that stratum.“

5. Law of Inclusions- this law states that rockfragments (in another rock) must be older thanthe rock containing the fragments.

6.Law of Faunal Succession- This law wasdeveloped by William Smith who recognized thatfossil groups were succeeded by other fossilgroups through time.




There are three

Types of rocks



The Earth’s crust consists of three kinds of rocks:

• Igneous rocks solidify from magma (or molten rock)

• Sedimentary rocks form from materials that are eroded fromother rocks

• Metamorphic rocks are rocks that have changed due to being

heated and/or compressed.




Formation of Sedimentary Rocks(Processes)

Deposition - occurs

when geologic agentcan no longer transportmaterial


• The basic processes involved in the formation of a clastic (granular)

sedimentary rocks are: weathering (erosion), transportation, deposition,

compaction (lithification) and diagenesis.

Weathering -mechanical or chemical break downof rock

Transportation -

movement of sedimentby gravity, wind, water (geologic/geomorphicagents)

Compaction (lithification) - pressureof overlying sediments packs grains andsqueezes connate water from pores

Cementation - pore spaces fill with abinding agent, typically - calcite, quartz, ironoxide, precipitated from circulating water.

Crystallization ordiagenesis - new mineralsgrow, or existing crystalsgrow larger as time passes -helps hold rock together.

Sedimentary rocks

http://en.wikipedia.org/wiki/Weathering

http://en.wikipedia.org/wiki/Erosion

http://en.wikipedia.org/wiki/Transportation_%28geology%29

http://en.wikipedia.org/wiki/Deposition_%28geology%29

http://en.wikipedia.org/wiki/Compaction

http://en.wikipedia.org/wiki/Compaction

http://en.wikipedia.org/wiki/Deposition_%28geology%29

http://en.wikipedia.org/wiki/Transportation_%28geology%29

http://en.wikipedia.org/wiki/Erosion

http://en.wikipedia.org/wiki/Weathering



Weathering


• Mechanical or physicalweathering is the breakdown of

rock into particles without

producing changes in the

chemical composition of the

minerals in the rock.

• Ice is the most important agent of mechanical weathering.

• Water percolates into cracks and

fissures within the rock, freezes,

and expands. The force exerted

by the expansion is sufficient to

widen cracks and break off pieces of rock.

Inside Lower Antelope Canyon, looking out with the sky near the top of the frame. Characteristic layering in the sandstone is visible.

• Heating and cooling of the rock, and the resulting expansion and contraction, also

aids the process.

• Mechanical weathering contributes further to the breakdown of rock by increasing

the surface area exposed to chemical agents.

http://upload.wikimedia.org/wikipedia/commons/a/aa/Lower_antelope_3_md.jpg




Products of weathering

Primary Residual Dissolved

Minerals Minerals Ions

Feldspar Clay minerals K+, Ca+2, Na+

Aluminum hydroxide

Fe-Mg minerals Hematite & Mg+2

Limonite

Quartz Quartz Silica

Primary Solids that Ions that are carriedMinerals remain in soil away in water

----------Detrital/Clastic sediments------ Chemical & biochemicalsediments



Introduction


• Sedimentary rock is one of

the three main rock groups

(along with igneous and

metamorphic rocks)

• It is formed in three main

ways:

• by the deposition of theweathered remains of

other rocks (known as

'clastic' sedimentary

rocks);

• by the deposition of the

results of biogenic activity;

and

• by precipitation from

solution.

http://en.wikipedia.org/wiki/Igneous_rock

http://en.wikipedia.org/wiki/Metamorphic_rock

http://en.wikipedia.org/wiki/Weathered

http://en.wikipedia.org/wiki/Clastic

http://en.wikipedia.org/wiki/Solution

http://en.wikipedia.org/wiki/Solution

http://en.wikipedia.org/wiki/Clastic

http://en.wikipedia.org/wiki/Weathered

http://en.wikipedia.org/wiki/Metamorphic_rock

http://en.wikipedia.org/wiki/Igneous_rock




Sediments - loose debris that has not been lithified

• Sedimentary rocks include common types such as conglomerate,

sandstone, siltstone, shale, chalk, limestone, coal and etc.

• Sedimentary rocks cover 75% of the Earth's surface. The sedimentary

rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 5% of the total. As such, the sedimentary sequences we see represent only a thin layer over a crust consisting mainly of igneous and metamorphic rocks.




Classification ofSedimentary Rocks




Detrital/Clastic sediments & rocks are classified by grain size

The grain size indicates the energy of the transporting agent.Turbulent water carries large particles, wind carries fine dust



29

Sedimentology

Fd is the frictional force acting on the

interface between the fluid and the particle

R is the radius of the spherical object

v is the particle's velocity

η is the fluid's viscosity

V s is the settling velocity

http://en.wikipedia.org/wiki/Viscosity

http://en.wikipedia.org/wiki/Viscosity

http://upload.wikimedia.org/wikipedia/commons/a/ae/Stokes_sphere.svg




DistanceProvenance

P o t e n t i a l Grain size distribution

Sedimentology



Formation


• Sedimentary rocks are formed from overburden

pressure (burial) as particles of sediment are

deposited out of air, ice, or water flows carrying the

particles in suspension.

• As sediment deposition builds up, the overburden (or

'lithostatic') pressure squeezes the sediment intolayered solids in a process known as l ithification ('rock formation') and the original connate (water)

fluids are expelled.

• (The term diagenesis is used to describe all the chemical, physical, and biological changes, including cementation , undergone by a sediment after its initial deposition and during and after its lithification,exclusive of surface weathering.)

http://en.wikipedia.org/wiki/Rock_%28geology%29

http://en.wikipedia.org/wiki/Overburden_pressure


http://en.wikipedia.org/wiki/Sediment

http://en.wikipedia.org/wiki/Settling

http://en.wikipedia.org/wiki/Suspension_%28chemistry%29

http://en.wikipedia.org/wiki/Lithification


http://en.wikipedia.org/wiki/Connate_fluids


http://en.wikipedia.org/wiki/Diagenesis

http://en.wikipedia.org/wiki/Cementation

http://en.wikipedia.org/wiki/Cementation

http://en.wikipedia.org/wiki/Diagenesis








http://en.wikipedia.org/wiki/Suspension_%28chemistry%29

http://en.wikipedia.org/wiki/Settling

http://en.wikipedia.org/wiki/Sediment




http://en.wikipedia.org/wiki/Rock_%28geology%29




A sedimentary rock comprises of (1) grains (2)

cement & matrix and (3) voids

Lithification and diagenesis (as they proceeds)both reduces porosity and permeability and thus

degrades the reservoir quality. Lithification reducesporosity by compaction only whereas, diagenesisreduces porosity by compaction, cementation,dissolution and deposition of new minerals.

GrainMatrix/Cement Pore




Change of

porosity

with depth

Sedimentary Processes




Sedimentary Processes

1

•Detrital/Clastic sediments arefragments of primary orresidual minerals.

•Chemical and biochemical

sediments are precipitatedfrom water.

4



Coarsely Crystalline

• Precipitated from water orrecrystallized from finematerial:

– Carbonates

• Limestone (calcite)CaCO3

• Dolomite, CaMg(CO3)2

– Evaporites

• Gypsum, CaSO4*2H20

(cement, wallboard)• Anhydrite, CaSO4

• Salt, NaCl, KCl


Fine-Grained

(Cryptocrystalline)

Some limestones

Chert - quartz-rich

Flint – fine quartz

Chalcedony – banded quartz

Whole Fossil Rocks

Formed from calcite fossils

or material with high organiccontent such as coal

Fossiliferous limestone -fossils sand-size or coarser

Coal (peat, lignite)

Oil shale

Fish fossil




Fish fossil

AmmonitesCoral fragments

Marine life



Lithification – change from

sediment to sedimentary

rock

Processes are:

compaction, cementation,

recrystallization


Typicalcements arecalcite, silica

andhematite/iron

oxides

Diagenesis



Textures & Structures

• Texture is the size, shape and arrangement

(packing & fabric) of the component elements

of a sedimentary rock.

• Structures deal with larger features of the rock

such as bedding, tracks, trails etc.




Size Classification

• > 256mm: Boulder• 256mm – 64mm: Cobble

• 64mm – 4mm: Pebble

• 4mm – 2mm: Granule

• 2mm – 1/16mm: Sand

• 1/16mm – 1/256mm: Silt

• < 1/256mm: Clay


Wentworth Scale



Shape Classification

Class 1. b/a >2/3 c/a<2/3 Oblate

Class 2 b/a >2/3 c/a>2/3 Equiaxial

Class 3 b/a <2/3 c/a<2/3 Triaxial

Class 4 b/a <2/3 c/a>2/3 Prolate






b/a

0

2/3

1

c/a 2/3 1

Tabularor

Oblate

Triaxial/Bladed

Equiaxial

Prolate



Various textural properties of clastic sediments and sedimentary rocks





Roundness: Sharpness of edges and corners.

It is defined as the average radius of curvatureof the corners of the grain image divided by the

radius of the maximum inscribed circle.

Angular 0 to 0.15Sub-angular 0.15 to 0.30Subrounded 0.30 to 0.50Rounded 0.50 to 0.70Well Rounded 0.70 to 1.00



Sedimentary Structures

Inorganic Organic


Mechanical / Primary Chemical / Secondary

A:Planar Bedding Structures

1.Laminations

2. Cross Bedding

3. Graded Bedding

B. Linear Bedding Structures

1. Striations

2. Sand Lineations3. Spatulate Casts

4. Ripple Marks

C. Bedding Plane Irregularities

A. Solution Structures

1. Stylolites

2. Corrosion Zones

3. Vugs

B. Accretionary Structures

1. Nodules

2. Concretions3. Crystal aggregates

4. Veinlets

5. Color Banding

A: Petrifactions

B: Miscellaneous

1. Boring

2. Tracks & Trails

3. Casts & Mould

4. Faecal Pellets



Sedimentary Structures

Inorganic Organic


Mechanical / Primary Chemical / Secondary

C. Bedding Plane Irregularities

1.Wave & swash Marks

2. Pits & Prints (rain)

3. Cut-outs, Scoops

D. Deformed & Distorted

Bedding.

A. Solution Structures

1. Stylolites

2. Corrosion Zones

3. Vugs

B. Accretionary Structures

1. Nodules

2. Concretions3. Crystal aggregates

4. Veinlets

5. Color Banding

A: Petrifactions

B: Miscellaneous

1. Boring

2. Tracks & Trails

3. Casts & Mould

4. Faecal Pellets



CONCLUSION

• Layering and bedding are the main basic principle toidentify sedimentary rocks by naked eyes.

• Sedimentary rocks divided into two main categories,i.e. clastic (detrital) or non-clastic (chemical).

• The sedimentary structures appear indicating theirdepositional environment.

• The importance of sedimentary rocks in O & G

Industry.


http://en.wikipedia.org/wiki/Image:Igammonite.jpg



Formation


• Sedimentary rocks contain important

information about the history of the Earth.• They contain fossils (the preserved

remains of ancient plants and animals ).• Sedimentary rocks can contain fossils

because they form at temperatures &

pressures that don't destroy fossilremnants, unlike igneous & metamorphic

rocks,.

• The composition of sediments provides

us with clues as to the original rock.

• Differences between successive layers

indicate changes to the environment

which have occurred over time or

Paleogeography.• Uniformitarianism - The present is the key to

the past – the most important concept.

Large scale features of sedimentary rocks

http://en.wikipedia.org/wiki/History_of_the_Earth

http://en.wikipedia.org/wiki/Fossil

http://en.wikipedia.org/wiki/Image:Igammonite.jpg

http://en.wikipedia.org/wiki/Fossil











Large-scale features of sedimentary rocks 1) Stratification or bedding – each layer (stratum or bed) of

sediment indicates the kind of environment in which the

sediment was deposited



Types of Sediment


Detrital / Clastic – mineral & rockfragments

Chemical – halite (NaCl) crystalsthat precipitate from water

Biochemical – shells made ofcalcite (CaCO3) by organismsthat extract the ions from water



Weathering


• Chemical weathering is the breakdown

of rock by chemical reaction. In this

process the minerals within the rock arechanged into particles that can be easily

carried away.

• Air and water are both involved in many

complex chemical reactions.

• The minerals in igneous rocks may be

unstable under normal atmosphericconditions, those formed at higher

temperatures being more readily

attacked than those which formed at

lower temperatures.

• Igneous rocks are commonly attacked by

water, particularly acid or alkaline

solutions, and all of the common igneous

rock forming minerals (with the exception

of quartz which is very resistant) are

changed in this way into clay minerals

and chemicals in solution.

Igneous rocks weathered into sandy –clayey soils and gravels, which are the main source of modern sediments



Factors which influence clastic depositional systems.



Classification of heterogeneities in reservoirsaccording to scale. From the smallest to thelargest:

Microscopic.

Mesoscopic.

Macroscopic.

Megascopic.



Microscopic or pore/grain-scale heterogeneities are related to pores andarrangement of grains, including pore volume (porosity), pore sizes and

shapes, grain-to-grain contacts that control permeability, and grain types.

Mesoscopic or well-scale heterogeneities can be recognized in thevertical dimension, such as in cores or well logs. Such heterogeneitiesinclude bedding and lithologic types, stratification styles, and the natureof bedding contacts.

Macroscopic or interwell scale heterogeneities occur at the scale of wellspacing. Such heterogeneities include lateral bed continuity ordiscontinuity as a result of stratigraphic pinchout, erosional cutout, orfaulting.

Megascopic or field wide heterogeneities, such as overall geometry andlarge-scale reservoir architecture (related to structure and/ordepositional environment), normally can be delineated by 2D or 3Dseismic, well tests, production information, and field-wide well logcorrelation.



Figure: Classification of heterogeneities in reservoirs according to scale.From the smallest to the largest, these are microscopic, mesoscopic,macroscopic, and megascopic heterogeneities



Level 4: architectural elements of specific reservoir typescomprising a continental (Level 1), fluvial (Level 2),meandering river deposit (Level 3) composed of floodplain,point bar, cut bank, mud plug, fining-upward, and cross-

bedded elements (Level 4).

For fluvial systems (as an example), these subdivisions, orlevels, are:

Level 1: regional environments of deposition (i.e.,continental, mixed or marine);

Level 2: major type of deposit (continental: fluvial, eolian,lacustrine or alluvial deposit);

Level 3: more specific types of deposit (continental, fluvial:meandering river, braided river, or incised valley fill);



The above discussion refers only to stratigraphic and

sedimentologic features of reservoirs and not to tectonic orstructural features.

Tectonic features include folds, faults, fractures, diapirs

(salt and shale), microfractures, and stylolites (chemicalcompaction)



Figure: Tectonic features at both seismic and subseismic scales, including faults, folds,diapirs and fractures. These features, both large and small, can influence reservoirperformance.




Reservoir Characterization

“The principal goal of reservoir characterization

is to outsmart nature to obtain higherrecoveries with fewer wells in better

positions at minimum cost throughoptimization”.

Halderson and Damsleth (1993)





(Reprinted with permission of Institut Francais du Petrole.)





If a proper reservoir characterization is conducted for a field and it leads

to an incremental improvement in production beyond what was

anticipated, then there is economic value to the characterization. For

example, if the characterization of a field that was originally estimated to

contain 100 MMBO recoverable improves that field‟s recovery by anadditional 5%, an extra 5 MMBO is produced.

Production improvements can come about through a better

understanding of the geologic complexities of the field which may result

either from sound geologic evaluation and/or new technologies applied

to the field from improved reservoir characterization.

Recovery in many mature fields has improved by using 3D seismic

to image fine-scale stratigraphic and structural





(A) Simplistic perception of a continuous reservoir sandstoneundergoing waterflood.(B) Stratigraphic and structural complexities between wells thatcan affect the waterflood.





This field was to be subjected to an expensive tertiary recovery project andthe positions, orientations and number of faults were quite important.



68


Schematic illustration of multilateral horizontal wells drilling into sandstones(yellow/white), exhibiting stratigraphic pinchout, offset stacking andcompartmentalization by shales, and good lateral and vertical continuity andconnectivity. Horizontal drilling is ideally suited for stratigraphically andstructurally complex reservoirs.



69


(A) Net pay thickness determined from well control only and (B) from3D seismic and well control The 3D seismic clearly shows the highdegree of compartmentalization of the reservoir sandstone. Note thatsome sandstone thicks have not been penetrated by wells (black dots),so represent untapped parts of the total reservoir.




Reservoir CharacterizationStatic reservoir properties are those rock and fluid properties that normally

do not change during the life of a field. They are the result of primary

depositional processes coupled with postdepositional burial, diagenesis,

and tectonics

Static properties include:

• stratigraphy

• geometry

• size

• lithologies

• structure

• initial porosity and permeability

• temperature.




Reservoir CharacterizationDynamic properties are those that do change significantly during the life of

a field. For example, fluid saturations, compositions, and contacts, as well as

reservoir pressure, change as the field is produced. Porosity and permeabilitycan change as the reservoir pressure changes over time or as injected fluids

react with formation minerals (either to precipitate new minerals that fill pore

spaces or to dissolve minerals and thereby provide new pore spaces).

Dynamic properties include:

• fluid saturations

• fluid contacts• production and fluid-flow rates

• pressure

• fluid compositions, including gas-to-oil ratio (GOR) and water-to-oil ratio

(WOR)

• acoustic (seismic) properties.

Acoustic properties, which are measured and documented as seismic attributes, are dependent

upon porosity, fluid type and content, and the nature of the reservoir rock. Seismic attributes are

dynamic, because fluid type and content change during oil and gas production. By comparing

seismic attributes at different times in the life of a field, it is possible to indirectly measure fluid

movement in the reservoir.




Reservoir CharacterizationThe three main stages of field development are

exploration (through to discovery),appraisal, and

production.

In exploration, one starts with a conceptual geologic model, which may be

based on geologic knowledge of the area, including basin evolution,

structure, and stratigraphy.

Conventional 2D or 3D seismic-reflection analysis normally is thenext phase of exploration, because subsurface seismic profiles and 3D

volumes can provide a regional-scale image of the area being explored If

the seismic data reveal potential drill sites, and a well is drilled, the well is

logged with conventional logging tools to determine rock and fluid properties

in the wellbore.



Fig. 2.2. Diagram showing some of the different data types used in the

study of reservoirs. Clockwise from upper left are conventional welllogs, a conceptual geologic model, 2D and 3D seismic reflection data(shallow and deep), outcrops, cores and borehole image logs, andgeologic reservoir models. Not shown are geochemical andbiostratigraphic data, which also are important in reservoir

characterization.



Fig. 2.3. Exploration, discovery, development, and

production stages in the evolution of a reservoir overtime (steps 1 –9). Geophysics, geology, and petroleumengineering all play dominant roles at different times inthe life of a field.



Computers and the computing environment (see pages

35-37)

Computers are essential tools for reservoir characterization. Most

organizations provide adequate computing environments for their

staff, from secretaries to geoscientists and engineers.

Some individuals and very small companies still prefer to hang

cross sections on walls with a piece of string as a datum, or tointerpret paper copies of seismic lines and well logs.



A - Seismic-reflection and subsurface imaging

1- Two-dimensional (2D) seismic

2- Three-dimensional (3D) seismic

3- Four-dimensional (4D) seismic

4 - Cross-well seismic investigation



1- Two-dimensional (2D) seismic (see pages 38 – 39)

Seismic-reflection acquisition, or “shooting”, provides an image

of the subsurface that is not as detailed as the true geology, but

that is adequate for imaging large- to medium-scale geologic

structures and stratigraphy. Seismic-reflection analysis has

become the dominant tool used in hydrocarbon exploration, and

with some resolution limitations, it is becoming widely used for

characterizing reservoirs.



2- Three-dimensional (3D) seismic (see pages 42 – 44)

In the early 1980s, the technology for acquiring and processing

seismic data had improved enough that the costs were reduced and

it became practical and economic to shoot 3D, rather than 2D

seismic surveys.

The advantage of a 3D survey is obvious – a three-dimensional

image of the subsurface is much more useful for exploration and

field development than is one or more 2D vertical images. Three-

dimensional seismic is designed to image a large area of thesubsurface, including up to and beyond the size of a reservoir, both

areally (horizontally) and stratigraphically (vertically) (Fig. 2.13).



Fig. 2.13. Graph showing the vertical resolution of a reservoir on the horizontal axis andthe horizontal coverage of the reservoir on the vertical axis. Cores can exhibit

sedimentary features down to the scale of a millimeter or less, but the areal coverage is

very small (15 cm, or 6 in, diameter). At the other extreme, 3D surface seismic covers a

large area of a reservoir, but the features must be on the order of tens of meters to be fully

resolved and imaged. Various other tools measure properties between these two end

members.



3- Four-dimensional (4D) seismic (see page 44)

4D seismic measures dynamic reservoir properties. The underlying

principle of 4D seismic is that acoustic properties of reservoir

strata will change as a function of change in fluid content and type

within the rock’s pore spaces.

4 - Cross-well seismic investigation (see pages 51 – 53)

Cross-well seismic is one of the only methods that canimage subseismic-scale lateral and vertical attributes at

interwell spacings. The method involves placing a seismicsource string down one well and a set of geophonereceivers down another well, and then shooting theseismic to obtain images between the wells (Fig. 2.26).



Fig. 2.26. Cross-well seismic provides an energy source that

generates sound waves within a borehole. Receivers arranged in a

series are placed down another well to detect the sound waves.Behavior of the waves from source to receiver can be related back

to rock and fluid properties, such as lateral and vertical

variations in porosity, at much higher resolutions than can be

acquired from conventional surface seismic reflection.



Classification of sandstones on the basis of their mineral composition

Geological Controls on Reservoir Quality





Porosity:

Permeability:

Capillarity:

Tortuosity:

Direct Measurement of porosity:

Pore spaces can be examined directly in sedimentary rocks,

either in a hand sample or by cutting a “thin section” (0.030

mm thick) of the rock. To prepare a thin section, a small slab

of rock is cut from the larger sample and is placed in

a chamber containing colored epoxy (normally, either blue or

red). Then, either pressure or a vacuum is applied to thesealed chamber until the epoxy fills the pores and pore

throats. This procedure is done at elevated temperature to

lower the viscosity of the epoxy. The rock is allowed to cool,

and the thin section is cut. Figure 5.1 shows a thin section of

a sandstone with pale gray-pink quartz grains.






Thin-section photomicrograph of a quartz-rich sandstone. Quartzsand grains are pale colored and the blue /dark-gray is dyed epoxyin pore space. Brown/black material is fine cement crystals andclay minerals that partially fill pore spaces and pore throats(narrow spaces between grains). Some of the quartz grains exhibitquartz cement rims. Sand grains are about 0.150 mm in diameter.

Using a thin section, it is possible

to estimate the area of pore

space relative to the area of rockby “point counting” pore spaces

using a polarizing microscope. In

practice, 200 or 300 equally

spaced points on a thin section

are counted as “grain”, “cement”,

or “pore”, and the arealpercentage of pore space is

calculated.






Pores and pore throats, as well as grain boundaries, also can be observed with

a scanning electron microscope. is a high-magnification, 3D scanning

electron photomicrograph of quartz grains comprising a sandstone. Clay mineral

crystals have accumulated in the pore throats of the sandstone and on the

faces of the quartz grains.




Scanning electron photomicrograph of clay mineral particles lininga pore throat between two sand grains and bridging the grains.Such bridging reduces permeability and porosity of sandstones.White scale bar is 10 microns in length.




Cartoon illustrating 3.75 cm (1.5 in) long, horizontal core plugs extractedfrom a full-diameter core for routine core analysis. The photograph of threepieces of core show where core plugs were extracted. The plug from coreA is representative of the entire piece of homogeneous sandstone. The

plug from core B is representative only of the thin sandstone from which itwas extracted. The plug from core C crosscuts different sandstone(light-colored) and shale (dark-colored) laminae, so it is representative onlyof the average reservoir quality of the combined laminae.




In recent years, a rapid, reliable method has been developed to measure

permeability over a very small sample of rock using a laboratory

minipermeameter. This instrument measures the rate of flow of air from asmall-diameter tube (approximately 1-mm aperture) into and through the rock.

This air-flow rate can then be related to rock permeability through calibration.

A core-plug permeability value has been obtained on the slab of finely

laminated sandstone; a single core-plug permeability is 19 md(millidarcys). However, the core plug was cut through several laminae, so

the value is not really representative of the heterogeneous permeability

resulting from this fine-scale lamination. Individual spot values of

permeability shown on the diagram were taken with a minipermeameter.

There are at least two orders of magnitude of variation in permeability

within the laminae (from <0.5 to 38.5 md). The spot measurements provide a more accurate representation of the vertical permeability‟s

heterogeneity than does the single core-plug measurement.




Diagram of a slab of corethrough which a core plug

permeability value of 19 mdhas been obtained. Individualspot values of permeabilityshown on the diagram weretaken with aminipermeameter. There are

at least two orders ofmagnitude of variationin permeability within thelaminae (from <0.5 to 38.5 md). Diagram is after Weber (1987).(Reprinted with permission ofthe Society of SedimentaryGeology (SEPM).)




Unconsolidated sands in cores present a special problem, because insertion

of the plugging tool into the sands will destroy the in situ arrangement and

packing of the grains and thus the porosity and permeability. An example of

plugs taken from unconsolidated cores is shown in Fig. 5.8. The extent to

which the original reservoir-quality values are modified by the plugging

process is not known, but undoubtedly this modification is quite variable and

not systematic.

A similar problem is encountered when one is attempting to obtain reservoir

quality measurements on sidewall cores. Forceful insertion of the plug into

the borehole wall can severely modify the in situ rock fabric. Any

measurements of porosity and permeability from sidewall cores should be

considered suspect.




Variations in porosity as a function of cubic and rhombohedral packingof sand grains. With cubic packing, each pore is outlined by four grain-to-grain contacts (in 2D view). In the tighter rhombohedral packing,each pore is defined by three grain-to-grain contacts, so within a givenarea (or volume, in 3D space), the porosity and permeability will bereduced. (Figure provided by T. Cross.)




Relatively coarser-grained sandstones exhibit higher permeabilities

than do relatively finer-grained sandstones, siltstones, and shales.More poorly-sorted sandstones exhibit lower permeability than do

better-sorted sandstones, because, in the former, small grains can

infiltrate into pore throats of adjacent grains.




A commonly observed

trend in sandstones is

the increase of

permeability with

increasing porosity

(Fig).

Such a porosityversus permeability

relation can usually

be related to grain

size and sorting (Figs.5.12 and 5.13).




Figure illustrating the

relationship of porosityand permeability for setsof sands and muds ofdiffering grain size. Aconstant sorting isassumed for individual

core plugs measured. Forany given grain size, thereis a clearly positivecorrelation betweenporosity and permeability.




Median grain size versuspermeability (log –log)crossplot for the samesamples shown in Fig. 5.13.Grain-size analysis wasconducted on the plugs

after the porosity andpermeability measurementshad been made. There is aclear trend of increasingpermeability with anincrease in grain size of the

plug sample.




Surface area of the sphere of the same volume as the fragment /

surface area of the object



102

• Neutron logs (NL or GRN) measure the

h d i t ti i

• A lower neutron log reading (fewerenergetic back scattered neutrons)




hydrogen ion concentration in a

formation.

– In clay-free formations where

porosity is filled with water orhydrocarbons the neutron log

measures liquid filled pores (the

only significant occurrence of

hydrogen).

– The neutron log measures energy

loss when neutrons emitted from

the tool collide with other

particles in the formation.

– The maximum energy loss during

a neutron collision occurs when

– A neutron collides with a particleof equal mass, that is a hydrogen

atom.

energetic back scattered neutrons)indicates abundant formationhydrogen.

– Clay rich formations containhydrogen in the crystalstructure ofthe clay mineralsand give anomalous values forliquid filled pore volume.

• Neutron log excursions (decreasingin value from right to left) indicate

higher proportions of hydrogen inthe Formation

– either increased liquid filledporosity or

– higher shale content.

• Neutron log excursions increasing

from left to right indicate

– less porosity and/or

– less shale

• Natural Gamma Ray (γ-ray) Logs

Decay of radioactive



104

– Decay of radioactive

elements produces high

energy gamma ray

emissions – Radioactive elements (K, U,

Th) are normally

concentrated in shaley rocks

while most sandstones are

very weakly radioactive. – Because radioactive

material is concentrated in

shale, shale has high gamma

ray log readings

– Clay-free sandstone andcarbonate rocks have low

gamma ray log readings


fundamentals of reservoir geoscience

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