Reservoir heterogeneity

@Hassan Z. Harraz 2019
Reservoir Heterogeneity
Prof. Dr. Hassan Z. Harraz
Geology Department, Faculty of Science, Tanta University
hharraz2006@yahoo.com
Spring 2019

Outlines
 Why study petroleum reservoirs ?
 Reservoir types and Reservoir characterizations
Siliciclastic Reservoir Facies
2) Carbonate Reservoir Facies
3) Fractured Reservoir Facies
 Introduction
 Styles of Geologic Reservoir Heterogeneity
 Classification of Heterogeneity/ Scales of Geologic Reservoir Heterogeneity
 Factors Causing Reservoir Heterogeneity
 Assessing Reservoir Heterogeneity
 Levels of Reservoir Heterogeneity
 Reservoir heterogeneity in sandstone bodies
 Diagenetic and Reservoir Quality and Heterogeneity Implications in Deltaic and Marine
Sandstones
 Hierarchical scales of Siliciclastic geologic reservoir heterogeneity (Levels)
 Fluvial Sandstone Reservoirs
 Scales of Fluvial Reservoir Heterogeneity
 Impact of Bioturbation on Reservoir Heterogeneity
 Carbonate Reservoir Heterogeneity
2

Why study petroleum reservoirs ?
A reservoir is a tank to be drained
 What is the most efficient way to empty the tank ?
 What do I need to know about its internal structure ?
 What obstacles am I likely to encounter ?
 One of the main objectives of reservoir
geology evaluation is to examine the impact of
reservoir heterogeneities on reservoir
behaviour.
3

Reservoir types and Reservoir characterizations
Depend on sedimentary facies, …..Three
types of reservoirs are existing, namely:-
1) Siliciclastic Reservoir Facies
4
What types of reservoirs exist ?
What kind of reservoirs ?

5
I- Siliciclastic reservoir facies
1- Not deep-water (Shallow water) 2- Deep-water
 Alluvial fan
• Meandering river
• Braided river
• Straight/anastomosing river
• Mixed aeolian/fluvial
• Lacustrine delta
• Shoreline/shelf
• Coastal plain
• Tidal flat
• Barrier-island/lagoon
• Shoreface-shelf
• Delta
• Glaciofluvial
• Lacustrine river-delta
• Lacustrine fan-delta
• Fluvial-dominated delta
• Wave-dominated delta
• Tide-dominated delta
• Mixed-influence delta
 Marine fan-delta
 Debris flow/ turbidite
• Pelagic
• Sublacustrine fan
• Gravel-rich slope/basin
• Mud-rich slope/basin
• Slope apron
• Submarine canyon
• Submarine-fan channel
• Submarine-fan lobe
• Submarine-fan lobe Fringe
• Submarine-fan channel levee
II- Carbonate reservoir facies III- Fractured reservoirs
 Marine embayment
 Sabkha/tidal flat
 Restricted shelf/lagoon
 Open shelf
 Nearshore bar/beach
 Offshore bar
 Platform/ramp margin shoal
 High-energy ramp
 Pinnacle reef
 Patch reef
 Reef mound
 Barrier reef
 Fringing reef
 Skeletal bank
 Mud-rich reef mound
 Mud-rich skeletal bank
 Karst-related detrital wedge
 Debris flow/turbidite
 Basement
 Volcanics
 Burial dolomite
 Carbonate sand
 Chert and siliceous shale
 Coal bed
 Foreslope carbonate
 Foreslope chalk
 Foreslope chert
 Microporous chert
 Microporous diatomite
 Microporous dolomite
 Microporous limestone
 Organic buildup
 Shale
 Shale and siltstone
 Shelf chalk
 Siliceous shale
 Muddy carbonate
 Muddy dolomite
 Karstic/ carbonate sand
 Karstic/ muddy
carbonate
 Karstic/ muddy dolomite
 Karstic/ organic buildup
 Low-resistivity sandstone
 Tight conglomerate
 Tight sandstone

1) Siliciclastic Reservoir Facies
 Traps: Structures, stratigraphy and
combination traps,
 Seals: shale and fault seals
 Strong variation in vertical succession
and horizontal distribution (Fluvial-
 Delta)
 From poor to high reservoir qualities
 Oil rim and oil leg
6

• Carbonate (mainly gas bearing)
 Reef carbonate structure
 High porosity and permeability
 Most widely investigated production issues include geological framework, rock
fabric, facies, and porosity and permeability distributions.
 Recognize the critical link between geological heterogeneity and reservoir quality
and performance.
 Finding the link between geological heterogeneity and reservoir quality often
becomes a matter of finding the appropriate data and sampling the heterogeneity at
the appropriate scale.
For example, many carbonates are characterized by abundant macrofauna and
macroflora that are larger than the scale of some sampling methods, such as 1” core
plugs, and may be better sampled with longer, whole cores or wireline logs. In this case,
geological heterogeneity must be sampled at greater scales to be valid.
In another example, very fine-grained mudstones may be extremely uniform in reservoir
quality at a scale much smaller than a 1” core plug. The recognition of the appropriate
scale of investigation is, therefore, critical to reservoir characterization efforts in
carbonates.
7

Carbonate Depositional Environments
8

Fractured Granite Basement
Complex fracture networks,
compartmented, highly heterogeneous
reservoir
Tend to be located around faults
systems.
Low porosity/permeability granite matrix
with high permeability macro-, micro-
fractures
Vugs, fractures essentially provide both
storage and paths for fluid flows.
Dual porosity and permeability
Difficult to determine OWC
9

Introduction
One of the main objectives of reservoir geology evaluation is to
examine the impact of reservoir heterogeneities on reservoir
behavior.
Reservoir heterogeneity is a function of the porosity/permeability
distribution due to lithologic variation during sedimentary deposition
which is further complicated by mechanical processes related to
deformation and chemical processes associated with diagenesis.
Fluid flow in reservoirs is affected by heterogeneity at a range of scales,
from submeter up to 10’s of meters, but the predominant control is
exerted by bedding, pore fluid changes, and diagenetic effects at the
meter-scale.
The geologic reservoir heterogeneity is defined as a variation in
reservoir properties as a function of space.
Reservoir heterogeneity:
variations in reservoir properties, on oil recovery.
severely affect the oil recovery by the impacts on the multiphase
flow.
The goal of reservoir heterogeneity studies is to understand the
extent, continuity, and volume of the reservoir and fluid migration paths.
10

Scales for reservoir heterogeneity
11

Styles of Geologic Reservoir
Heterogeneity
 There are essentially two styles of geological
reservoir heterogeneity:-
1. Vertical heterogeneity
2. Lateral heterogeneity
12

Style of Geological Reservoir Heterogeneity Matrix
Low Moderate High
Low
• Wave-dominated
delta
• Barrier core
• Barrier shoreface
• Sand-rich shoreface
• Delta-front mouth bar
• Proximal delta front
• Tidal deposits
• Mud-rich strand plain
• Meander belt
• Fluvially-dominated
• delta
• Back barrier
Moderate
• Eolian
• Wave-modified delta
(distal)
• Shelf bars
• Alluvial fans
• Fan delta
• Lacustrine delta
• Distal delta front
• Wave-modified delta
(proximal)
• Braided stream
• Tide-domainated delta
High
Basin-floor turbidites • Coarse-grained meander
belt
• Braid delta
• Back barrier
• Fluvially-dom. delta
• Fine-grained meander
belt
• Submarine fans
Lateral Heterogeneity
VerticalHeterogeneity

Classification of Heterogeneity
Scales of Geologic Reservoir Heterogeneity
A variety of types and scales of heterogeneity
are found in most reservoirs.
According to scale; (from the smallest to the
largest scale, Fig.3):-
i) Microscopic heterogeneity,
ii) Mesoscopic heterogeneity,
iii) Macroscopic heterogeneity, and
iv) Megascopic heterogeneity.
14

Scale of Geologic Reservoir Heterogeneity
15

Scales of Geologic Reservoir Heterogeneity
Fig. 3. Classification of heterogeneities in reservoirs according
to scale.
From the smallest to the largest, these are microscopic,
mesoscopic, macroscopic, and megascopic heterogeneities.
16

i) Microscopic or pore and grain-scale heterogeneities are related to pores and arrangement of grains, including
pore volume (porosity), pore sizes and shapes, grain-to-grain contacts that control permeability, and grain types.
Microscopic heterogeneities also can be subdivided according to features such as:-
a) Grain-size distribution,
b) Porosity,
c) Permeability,
d) Capillarity,
e) Grain-packing arrangements, and
f) Well log signature.
ii) Mesoscopic or well-scale heterogeneities can be recognized in the vertical dimension, such as in cores or well
logs. Such heterogeneities include bedding and lithologic types, stratification styles, and the nature of bedding
contacts.
iii) Macroscopic or interwell scale heterogeneities occur at the scale of well spacing. Such
heterogeneities include lateral bed continuity or discontinuity as a result of stratigraphic pinch-out,
erosional cut-out, or faulting. This is the most difficult scale of heterogeneity to quantify, because the
technologies required to image interwell-scale heterogeneities often exhibit resolutions that are too
coarse for one to observe the feature (subseismic). Cross-hole tomography, 4D (time-lapse) seismic,
and well tests can provide direct information on the presence or absence of such heterogeneities, but
the inherent resolution of definable features with 2D or 3D seismic often is too high to be able to resolve
important subseismic scale, interwell heterogeneities.
iv) Megascopic, or field-wide heterogeneities, such as overall geometry and large-scale reservoir
architecture (related to structure and/or depositional environment), normally can be delineated by 2D or
3D seismic, well tests, production information, and field-wide well log correlation. However, it is
important to note that the size of the depositional system that comprises a field normally exceeds the
size of the field itself. For this reason, regional mapping and field correlations should be extended
beyond the geographic confines of the field.
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What data are available ?
Data Type Use
Core (slabbed or oriented)
Sidewall cores
Cuttings
Thin sections
Facies, depositional environment
Paleocurrent directions
Mineralogy, lithology
Mineralogy, lithology
Paleontology (micro, macro, traces),
Palynology
Water depth, depositional environment,
time line; Paleocurrent direction,
lithofacies
Logs
FMS / FMI
SP,GR
Sonic, density, neutron
Paleocurrent directions, lithofacies
Lithology, curve shape analysis
Porosity, curve shape analysis
Repeat Formation Tester Pressure (sand body connectedness)
18

Overview of integrated analysis of petrophysical Logs
19

Reservoir heterogeneity: caused by depositional and
diagenetic variations.
Diagenesis exerts a strong control on the quality and
heterogeneity of most clastic reservoirs.
Variations in the distribution of diagenetic alterations
usually accentuate the variations in depositional porosity
and permeability.
Linking the types and distribution of diagenetic processes to
the depositional facies and sequence-stratigraphic framework
of clastic successions provides a powerful tool to predict the
distribution of diagenetic alterations controlling quality and
heterogeneity.
20
Factors Causing Reservoir Heterogeneity
Factors Causing Reservoir Heterogeneity
Depositional Diagenetic
• Lithology
• Bedding
• Lamination
• Detrital dolomite grains
• Microcrystalline dolomite
• Anhydrite or calcite nodules
• and patches
• Quartz overgrowth cement

Assessing Reservoir Heterogeneity
 Several technologies are available to address these issues, including diagnosis of:-
i) External shape of the reservoir,
ii) Internal geometry and architecture of the reservoir,
iii)Pore shape and geometry,
iv)Inter-grain and intra-grain contacts,
v) Cements and diagenetic paragenesis.
 External geometry of the reservoir includes the spatial location of the reservoir and surface analogs in terms
of
 regional setting and sequence stratigraphy;
 structural attitude and orientation, size and shape, and continuity.
 Internal geometry of the reservoir focuses on characterization, distribution and genetic controls on pay and
non-paying horizons, and documentation of continuity of fluid flow barriers and distribution of rock properties
(e.g., porosity, permeability, saturation [capillary] properties). The goal is to develop an understanding of the
three-dimensional distribution and continuity of the rock-pore-fluid system of the reservoir.
 Pore analysis and inter-grain analysis is used to diagnose specific reservoir problems, such as swelling clays,
pore throat constrictions, and passage of water versus petroleum.
 Maps of clays, fine clastic particles, and cements are important to reservoirs with restricted permeability and
limited susceptibility to secondary recovery by water injection.
 One of the new and most important technologies now available to analysis of reservoirs is high-resolution
correlation, which requires exceedingly detailed studies of cores and logs. This relatively new field of
reservoir heterogeneity analysis has been very effective in major oil company research programs. This
technology will be widely applied to the documentation of typical fields within plays.
21

Levels of Reservoir Heterogeneity
Flow patterns,
Drainage efficiency,
Vertical and lateral sweep efficiency,
Permeability (K), So/Sw, flow, formation damage
Hydrocarbon volume,
Areal distribution, and
Play trends.
22
 Heterogeneity strongly influences reservoir performance by
controlling fluid flow and recovery factors (Wardlaw and Taylor,
1976; Wardlaw and Cassan, 1979; Weber, 1982).

Levels of Reservoir Heterogeneity
23

Petrophysics: is the study of rock properties and rock
interactions with fluids (gases, liquid hydrocarbons, and
aqueous solutions).
Systematic theoretical and laboratory study of physical
properties of petroleum reservoir rocks, like:-
Lithology
Porosity
Compressibility
Permeability
Fluid saturations
Capillary characteristics
Rock stress
Fluid-rock interaction
24

Geological and Petrophysical Data
used to define Flow Units
25

26
 The property of reservoir is depending upon an:-
i) Interplay of tectonics,
ii) Sea level,
iii) Sediment supply,
iv) Physical and biological processes of sediment transport
v) Deposition, and
vi) Climate
Types and Distribution of Petrophysical Facies are
the most Significant Factors Causing Reservoir
Heterogeneity.
Shapes, sizes, Net-gross values, continuity, orientation, and other
reservoir characteristics are all a function of the nature of transport
and depositional processes, basin configuration, climate, tectonic and
eustatic sea-level fluctuations

 Reservoir heterogeneity refers to vertical and lateral variations in
porosity, permeability, and/or capillarity (Alpay, 1972; Evans, 1987;
Moraes and Surdam, 1993).
 Reservoir heterogeneity in sandstone bodies
 occurs at various extents and scales, ranging from micrometers
to hundreds of meters, and
 is commonly attributed to variations in depositional facies,
diagenesis, and structural features such as the presence of
fractures and faults (Figure 1) (e.g., De Ros, 1998; Schulz-Rojahn
et al., 1998).
 Elucidation and prediction of the reservoir heterogeneity are of
prime importance for the planning and execution of efficient
hydrocarbon production strategies (Hamilton et al., 1998; Barton et al.,
2004; Sech et al., 2009).
 The heterogeneity patterns of sandstone reservoirs, which determine the
volumes, flow rates, and recovery of hydrocarbons, are controlled by
geometry and internal structures of sand bodies, grain size, sorting,
degree of bioturbation, provenance, and by the types, volumes, and
distribution of diagenetic alterations.
27
Reservoir Heterogeneity in Sandstone Bodies

Figure 1. Types of reservoir heterogeneity in sandstone bodies that occur to various extents and scales,
ranging from micrometers to hundreds of meters, and is commonly attributed to variations in depositional
facies, diagenetic evolution pathways, and structural features, such as the presence of fracture network and
faults (modified from Weber, 1986).
28
Scales of Sandstone Reservoir Heterogeneity

 Variations in the pathways of diagenetic evolution
are linked to:
1) depositional facies, hence pore-water chemistry,
depositional porosity and permeability, types and
amounts of intrabasinal grains, and extent of
bioturbation;
2) detrital sand composition;
3) rate of deposition (controlling residence time of
sediments at specific near-surface, geochemical
conditions); and
4) burial thermal history of the basin.
29

Figure 3. Cartoon showing common diagenetic and related reservoir evolution pathways: (A)
porosity preservation; (B) porosity reduction (after Morad et al., 2010)
30

Table 1. Common Eogenetic Alterations in Sandstones and Their Controlling Parameters, Common Depositional
Facies in which They Occur, and Potential Impact on Reservoir Quality (after Morad et al., 2010)
31
Process Major Controlling Parameters Depositional Facies Impact on Reservoir Quality
Mechanical compaction and
pseudomatrix formation
(Figure 3A)
Abundance of ductile lithic
grains, mud intraclasts, or
glaucony
Turbidite, fluvial, and
deltaic sandstones
Rapid loss of porosity and permeability during burial
Dissolution and kaolinization
of framework feldspars
(Figure 3B, E)
Abundance of feldspars,
effective meteoric water flux
Fluvial, tidal, and deltaic
sandstones
Formation of intragranular and moldic pores
Cementation by K-feldspar
overgrowths
Formation of grain-coating
Fe-clays (e.g., odinite,
berthierine)
Abundance of K-feldspars
Low sedimentation rates
Fluvial, tidal, and deltaic
sandstones
Deltaic and shallow-
marine sandstones
Improvement of porosity and, to smaller extent, permeability
Rarely abundant enough to result in permeability deterioration
Transformation into chlorite during mesodiagenesis
Formation of grain-coating
microquartz (Figure 3D)
Alteration of felsic and mafic
volcaniclastic rock fragments
Abundance of siliceous bioclasts
Sediment provenance, coeval
volcanic activity, i.e., tectonic
basin setting
Shallow- and deep-marine
sandstones
All facies
Deep porosity preservation by inhibition of late quartz
cementation Inhibition of cementation by quartz overgrowths
Felsic grains: porosity-permeability loss due to cementation by
smectite, zeolites, microquartz, and opal
Mafic grains: formation of zeolites, calcite, and trioctahedral
smectite, which undergoes mesogenetic transformation into
chlorite, may contribute to porosity preservation
Dissolution of carbonate
grains
Extensive meteoric water flux,
which is enhanced by wet
climate and sand permeability
Deltaic and shallow-
marine sandstones
Increase in porosity due to formation of intragranular and
moldic pores
Cementation by calcite,
dolomite, siderite (Figure 3F)
Semiarid climate in fluvial
sandstones and availability of
carbonate grains in shallow-
marine sandstones
All facies Calcite cementation
Destruction porosity and permeability
Mechanical clay infiltration
(Figure 4A)
Mostly braided fluvial and
alluvial fans, subordinately
meandering fluvial and deltaic
settings
Fluvial and deltaic
sandstones
Reservoir compartmentalization
May inhibit cementation by quartz overgrowths, and thus
preserves porosity in deeply buried sandstones
Mesogenetic illitization of thin infiltrated smectite coats may
promote pressure dissolution

Table 2. Typical Mesogenetic Processes in Sandstones, Their Controlling Parameters, and Potential Impact on
Reservoir Quality (after Morad et al., 2010)
32
Process Major Controlling Parameters Impact on Reservoir Quality
Illite formation (Figure 4B) Availability of precursor clay minerals,
primarily kaolinite and dioctahedral
smectite
Permeability deterioration
Increase in water saturation
Enhancement of intergranular
pressure dissolution
Chlorite formation (Figure 6C, D) Availability of precursor grain-coating
berthierine or smectite
Inhibits quartz overgrowth
cementation in deep sandstone
reservoirs
Dickite formation (Figure 4A) Availability of precursor kaolinite;
mesogenetic acidic conditions
Prevention of illitization of
kaolinite and hence permeability
preservation
Albitization of K-feldspars
(Figure 4C)
Abundance of detrital K-
feldspar; high Na+ activities
Enhances illite formation by
supplying K+, hence causing
permeability deterioration
Albitization of plagioclase
(Figure 4C)
Abundance of Ca-rich
plagioclase; high Na+ activities
Provides Ca2+ and Al3+, which act
as sources of small amounts of
carbonate and clay mineral
cements
Quartz cementation and
pressure dissolution of quartz
grains (Figure 5A and B,
respectively)
Availability of monocrystalline
quartz grains with clean
surfaces, or of illite coatings
and micas, respectively
Substantial deterioration of
permeability and porosity
Dissolution of unstable grains and
calcite cement
Thermal maturation of organic
matter, which generates organic
acids and CO2
Enhancement of reservoir quality
through creation of secondary
intragranular and intergranular
porosity
Cementation by ankerite, Mg-
siderite, barite, and anhydrite
Flux of basinal fluids, primarily
along faults
Deterioration of reservoir quality

Table 3. Impact of Framework Grain Types on the Diagenetic and Reservoir-Quality and Heterogeneity
Evolution of Sandstones (after Morad et al., 2010)
33
Type of Framework Grains
Common Related Diagenetic
Alterations
Impact on Reservoir Quality Depositional/Tectonic Setting
Quartz Mesogenetic pressure dissolution
(silica exporters) and/or quartz
cementation (silica importers)
Preservation of reservoir porosity and
permeability to depth of about 3 km
Substantial loss of reservoir porosity
and permeability at depths greater
than 3 km
Intracratonic basins, wet climate,
granitic, felsic gneissic, and
quartzitic source rocks; more
common in eolian, fluvial, and
shallow-marine facies
Feldspars and plutonic rock
fragments
Eo- and mesogenetic dissolution,
resulting in the formation of
intragranular and moldic pores
Eogenetic kaolinization
Mesogenetic albitization
Creation of secondary porosity
Mesogenetic K-feldspar albitization
promotes illite authigenesis and
permeability deterioration
Rifts and pull-apart basins adjacent
to uplifted basement rocks;
common in all facies
Lithic: ductile (e.g., mud
intraclasts, glaucony,
mudrocks, low-grade
metamorphic)
Mechanical compaction and formation of
pseudomatrix
Severe loss of porosity and permeability Orogenic settings; intrabasinal
reworking
Lithic: chemically
unstable (e.g.,
volcanics)
Formation of smectite, chlorite, zeolites,
calcite, microquartz, and opal
Severe loss of permeability Basins adjacent to volcanic arcs or
plateaus
Lithic: chemically and
mechanically stable
(e.g., chert, quartzite)
No significant alterations; chert may be
subjected to partial dissolution
Preservation of reservoir porosity and
permeability
Basins adjacent to uplifted
continental crust, or
subduction complexes
Micas Enhanced pressure dissolution Reduction of porosity and permeability by
chemical compaction
Basins adjacent to uplifted
continental crust, or
orogenic arcs
Extrabasinal and
intrabasinal carbonate
grains
Extensive carbonate cementation and
chemical compaction
Deterioration of reservoir porosity and
permeability
Orogenic settings (extrabasinal) or
passive margins (intrabasinal)
Intrabasinal siliceous
bioclasts
Eogenetic dissolution resulting in
formation of microquartz rims
Preservation of reservoir porosity-
permeability to depth of about 3 km
Shallow- and deep-marine
sandstones

Table 4. Typical Diagenetic Alterations and Reservoir Quality and Heterogeneity Implications in Continental
Sandstones(after Morad et al., 2010)
34
Common Place of Occurrence Typical Diagenetic Alterations Reservoir Quality and Heterogeneity
Fluvial Deposits
Braided rivers: channel
deposits
Unstable silicate dissolution and kaolinite formation
under humid climatic conditions
Enhanced intragranular secondary porosity and limited
permeability reduction due to kaolinite precipitation in
intergranular pores
deposits
Possibly abundant and thick mechanically
infiltrated clay coatings and pore-filling aggregates
under semiarid climatic conditions
Weak to extreme intergranular porosity reduction; strong
heterogeneity where clay concentrations are laterally
extensive levels, forming baffles and barriers for fluid flow;
possible porosity preservation due to inhibition of quartz
cementation by thin coatings
deposits
Low-Mg calcite cementation as scattered, elongated
concretions
Scattered concretions may cause local porosity reduction but
overall limited impact on permeability
Braided/meandering rivers:
channel deposits
Tight calcite cementation along channel lags Strong intergranular porosity reduction and strong vertical
heterogeneity through generation of flow barriers and baffles
Meandering rivers: channel
deposits
Formation of pseudomatrix due to mechanical
compaction of mud intraclasts eroded from
floodplain deposits
Variable destruction of intergranular porosity and permeability,
depending on degree of compaction and quantity of ductile
grains
Meandering rivers: crevasse
splay deposits
Pressure dissolution of quartz grains enhanced along
intergranular contacts with mica flakes, and derived
quartz cementation
Strong deterioration of porosity and permeability due to
compaction and quartz cementation
Eolian Deposits
Eolian dunes
Formation of microcrystalline dolomite and
microcrystalline to poikilotopic anhydrite cements
Weak to moderate intergranular porosity deterioration and
heterogeneity generation along foresets or phreatic level
positions
Eolian dunes Thin mechanically infiltrated and possibly authigenic
smectitic clay coatings
Weak direct intergranular porosity and permeability reduction;
chlorite rims from transformed coatings may contribute to
porosity preservation through quartz overgrowths inhibition;
illitized coatings may enhance chemical compaction and
cause severe permeability reduction
Eolian dune, sand-sheet,
and interdune deposit
Formation of large gypsum (selenite) crystals along laterally
extensive levels and as crusts
Strong heterogeneity generation through formation of baffles
and barriers for fluid flow
Interdune deposits Precipitation of microcrystalline dolomite and pedogenesis
(semiarid climatic conditions)
Strong heterogeneity generation through formation of baffles
and barriers for fluid flow

Table 5. Typical Diagenetic Alterations and Reservoir Quality and Heterogeneity Implications in
Deltaic and Marine Sandstones (after Morad et al., 2010)
35
Depositional Facies Typical Diagenetic Alterations Reservoir Quality and Heterogeneity
Deltaic
Fluvial-dominated delta front Grain-coating and ooidal Fe-rich clays (primarily odinite and porosity
berthierine; subordinately smectite) in tropical river deltas
Permeability reduction; chloritized coatings preserve during burial through
inhibition of quartz cementation
Fluvial- and wave-dominated
delta fronts
Laterally extensive carbonate cementation associated with
layers rich in carbonate bioclasts or mud intraclasts (e.g., lags)
along flooding surfaces
Strong porosity heterogeneity and possible development of barriers and
baffles for fluid flow in-between amalgamated sandstone bodies
Wave-dominated delta front Carbonate cementation due to nucleation around carbonate
bioclasts incorporated during wave reworking
Loss of intergranular porosity and permeability due to extensive carbonate
cementation
Wave-dominated delta front Clean quartz-grain surfaces may result in extensive quartz
cementation
Loss of intergranular porosity and permeability at depths greater than
approximately 3 km
Tide-dominated delta front Formation of grain-coating clay minerals, primarily odinite and
berthierine but also smectitic clays
Permeability reduction; chloritized coatings preserve porosity during burial
through inhibition of quartz cementation
Shallow Marine
Foreshore and backshore
sandstones
Extensive Mg-calcite or aragonite cementation (beachrocks) Porosity and permeability reduction and strong vertical heterogeneity related to
laterally extensive cemented layers
Shoreface deposits Concretionary carbonate cement due to nucleation on
carbonate bioclasts; coalescence of concretions may
form cemented layers
Strong vertical heterogeneity related to coalescence of concretions and possible
development of barriers and baffles for fluid flow within amalgamated sandstone
bodies
Shoreface deposits Formation of opal, chalcedony, or microquartz cements (commonly as
rims) sourced from siliceous bioclasts
Permeability reduction and possible porosity preservation during burial due to
inhibition of quartz cementation
Shoreface deposits Cementation associated to carbonate, phosphate, and siliceous
bioclasts, as well as to carbonate intraclasts, peloids, and ooids
concentrated in storm layers; mud intraclasts compacted to
pseudomatrix
Strong heterogeneity related to the formation of laterally extensive layers cemented
or rich in pseudomatrix and possible formation of baffles and barriers for fluid flow
Deep Sea
Fan deposits Carbonate cementation due to nucleation around carbonate
bioclasts and other allochems, hydraulically concentrated
during gravity flow
Strong heterogeneity related to the formation of laterally extensive cemented layers
and possible formation of baffles and barriers for fluid flow
Fan deposits Formation of pseudomatrix by mechanical compaction of mud
intraclasts, eroded from slope during gravity flow
Strong heterogeneity related to the formation of laterally extensive layers
rich in pseudomatrix and possible formation of baffles and barriers for fluid
flow
Fan and levee deposits Carbonate cementation (commonly by calcite) along contacts
with interbedded mudrocks, marls, and calcilutites
Strong reduction of intergranular porosity at the base and top of sandstone
bodies; thin sandstones (e.g., fan fringes and levee deposits) may be
completely cemented by carbonate
Fan deposits Dissolution of siliceous bioclasts and formation of microquartz
rims around framework grains
Permeability reduction; porosity preservation during burial due to inhibition of
quartz cementation
Fan deposits Alteration of volcanic fragments and mafic minerals (e.g.,
biotite) and formation of smectite coatings or rims and of
derived chlorite rims during burial diagenesis
Generation of intragranular porosity and loss of intergranular porosity and
permeability by cementation and mechanical compaction; possible porosity
preservation during burial due to inhibition of quartz cementation
Fan deposits Dissolution of feldspars and other chemically unstable silicates, Enhanced intragranular secondary porosity

Figure 11. Diagram representing the distribution of major diagenetic processes and products and reservoir
heterogeneity aspects within the sequence-stratigraphic framework of clastic marginal and marine
successions(after Morad et al., 2010) .
36

Hierarchical scales of Siliciclastic geologic
reservoir heterogeneity (Levels)
Mesoscopic, macroscopic, and megascopic heterogeneities of
sandstone reservoirs can be subdivided further, according to the
scale of the feature.
For fluvial systems (as an example), these subdivisions, or
levels, are: -
a) Level 1: regional environments of deposition (i.e., continental,
mixed or marine);
b) Level 2: major type of deposit (continental: fluvial, eolian,
lacustrine or alluvial deposit, Fig. 4);
c) Level 3: more specific types of deposit (continental, fluvial:
meandering river, braided river, or incised valley fill) (Fig. 5);
d) Level 4: architectural elements of specific reservoir types
comprising 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) (Fig. 6).
37

38
Fig. 4: Level 2 environments include all of those within the
continental (as this example shows), mixed, or marine
environments (Level 1).
Fig. 5: Level 3 environments that may occur within each
Level 2 environment. In this example, Level 2 fluvial
environments and deposits occur as meandering river,
incised valley fill or braided river systems. Each system
has its own unique characteristics and trends. The three
photos are of modern surficial deposits
Fig. 6. Level 4 environments and deposits are
composed of smaller scale features which are part of
Level 3 deposits. In this example, the meandering river
(Level 3) is composed of a series of features. From
upper left to lower right, these features are a modern
meandering river and floodplain, a map reconstruction
of part of the modern Mississippi River showing point
bar (reservoir) sands isolated by mud plugs, the point
bar and cut-bank sides of a meander bend, cross
bedded, point bar sands along a trench wall, the ideal
vertical stratigraphy of a point bar deposit, and a 3D
model showing the complexities of the modern
Mississippi River example.

Fluvial Sandstone Reservoirs
Fluvial sandstone reservoirs contain some of the highest
percentages of unrecovered mobile oil within known
reservoirs (Tyler and Finley 1991) due to their inherently
complex internal depositional architectures.
Such architectures are difficult to ascertain from down-
hole data alone, and the rapid spatial and temporal
variations in lithofacies make sub-subsurface correlations
of fluvial strata extremely difficult (Miall 1996).
This lecture:
 insights heterogeneities at a baffle to permeability zonation
scale (1-25 m scale; Figure 2) and
provides insight into which fluvial architectures drive
heterogeneity on a sub-element scale in a low sinuosity
fluvial system.
39

Figure 2: The scales of
fluvial reservoir
heterogeneity, adapted from
Tyler and Finley (1991) and
Morad et al. (2010).
Note the scales of this study:
Fluid flow baffles within
genetic units and zonation
of permeability within
genetic units
40
Scales of Fluvial Reservoir Heterogeneity

Scales of Heterogeneity Within Fluvial Reservoirs
 Reservoir heterogeneity represents a change in reservoir properties between one rock unit and another.
These heterogeneities often result from variations in petrophysical properties across facies (Corbett et al.
2012).
 Such distributions of reservoir heterogeneities in fluvial systems are extremely complex (Miall
1996; Corbett et al. 2012).
 “Permeability Heterogeneities are the Principle Controls on Productivity throughout the Life of
a Reservoir” (Jordan and Pryor,1992).
 Some of the greatest challenges presented in the recovery of mobile oil are produced by reservoir
architecture and the relationships of fairway to baffle and barrier heterogeneities (Tyler and Finley 1991).
 Reservoir geologists commonly recognize four to six broad scales of reservoir heterogeneity across the Pore Scale,
Sedimentary Structure, Fairway and Baffle, and Genetic Unit to Fault Scale (Figure 2; Tyler and Finley 1991; Jordan and
Pryor 1992).
 Preliminary Recovery Models of Fluvial Reservoirs Operate at mega- to giga-scopic scale
heterogeneities within a reservoir (Figure 2; Tyler and Finley 1991).
 Given the high percentage of unrecovered oil in fluvial reservoirs higher resolution realisations must be made.
 This lecture looks at the scale of shale baffles within an architectural element.
 A classic example of this is in the study of point bars in highly sinuous fluvial systems and their
associated inclined heterolithic strata (Pranter et al., 2007).
 Heterolithic baffles of low sinuosity fluvial strata have received little attention by comparison.
This may be due to their high net-to-gross nature and homogeneity on the larger, lower,
resolution mega- to giga-scopic scale.
41

Impact of Bioturbation on Reservoir Heterogeneity
The degree of bioturbation is a reflection of sedimentation rate
(Wetzel,1984). Low sedimentation rates provide burrowing organisms
enough time to keep up with sedimentation and rework sediment thoroughly,
resulting in a high degree of bioturbation. Thus, intense bioturbation
commonly occurs below marine-flooding surfaces (Ramosetal.,2006).
Bioturbation has a particularly significant impact on the horizontal
permeability of sandstones (Dutton and Hentz, 2002; Taylor et al.,
2003).
Intense bioturbation is an efficient agent for vertical advective ionic transfer
from overlying seawater than by diffusion (Goldhaber et al., 1977).
Bacterial degradation of organic matter concentrated in bioturbation sites
commonly results in the local increase in carbonate alkalinity and, hence, in
the nucleation of microcrystalline calcite (Berner,1980) or dolomite
(Hendry et al., 2000). Further growth of calcite cement under these
conditions may occur by ionic diffusion from seawater (Berner,1968;
Wilkinson, 1991).
Accordingly, porosity and permeability deterioration due to
enhancement of carbonate cementation by bioturbation is common
in sandstones below TSs (Ruffell and Wach, 1998; Al-Ramadan et
al., 2005).
42

Carbonate Reservoir Heterogeneity
 Carbonates are characterized by different types of porosity
and have unimodal, bimodal and other complex pore size
distributions, which result in wide permeability variations for
the same total porosity, making difficult to predict their
producibility.
 Several porosity types coexist giving mixed log responses.
Carbonates are microscopically heterogeneous. Logs
respond differently to different components of porosity.
Integrated interpretation of the logs is necessary to quantify
heterogeneity with a limitation of the fact that various logs
respond differently due to different geometric responses of
the tools, investigating volume of the formation, and vertical
resolution.
43

Schematic Reservoir Layering Profile in a Carbonate Reservoir
44

References
Corbett, P.W., Hamdi, H. and Gurav, H., 2012. Layered fluvial reservoirs with internal
fluid cross flow: a well-connected family of well test pressure transient responses.
Petroleum Geoscience, 18, 219-229.
Jordan, D.W. and Pryor, W.A., 1992. Hierarchical levels of heterogeneity in a Mississippi
River meander belt and application to reservoir systems: Geologic note. AAPG
Bulletin, 76, (10), 1601-1624.
Miall, A.D., 1996. The Geology of Fluvial Deposits: Sedimentary Facies, Basin Analysis
and Petroleum Geology. Springer-Verlag, New York.
Mitten; A. J., Clarke; S. M., Pringle, J.K. and Richards, P., 2018. 2854396 Combining
Terrestrial Photogrammetry, Applied Sedimentology and Hand-Held Gamma Ray
Spectrometry to Characterise the Cretaceous Lower Castlegate Formation, Tuscher
Canyon, Utah, U.S.A. Conference Paper · May 2018.
Morad, S., Al-Ramadan, K., Ketzer, J.M. and De Ros, L.F., 2010. The impact of diagenesis
on the heterogeneity of sandstone reservoirs: A review of the role of depositional
facies and sequence stratigraphy. AAPG bulletin, 94, (8), 1267-1309.
Pranter, M.J., Ellison, A.I., Cole, R.D. and Patterson, P.E., 2007. Analysis and modelling of
intermediate-scale reservoir heterogeneity based on a fluvial point-bar outcrop
analogy, Williams Fork Formation, Piceance Basin, Colorado. AAPG bulletin, 91, (7),
1025-1051.
Tyler, N., and R.J. Finley, 1991, Architectural controls on the recovery of hydrocarbons
from sandstone reservoirs: Concepts in Sedimentology and Paleontology, v. 3, 1-5.
Weber, K. J., 1986, How heterogeneity affects oil recovery, in L. W. Lake and H. B. Carroll Jr., eds.,
Reservoir characterization: New York, Academic Press, p. 487– 544.
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Reservoir heterogeneity

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