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Exam Calendar
20/6/2011, ore 9.00, Aula E, Dip. Scienze della Terra
30/6/2011, ore 9.00, Aula E, Dip. Scienze della Terra
15/7/2011, ore 9.00, Aula E, Dip. Scienze della Terra 29/7/2011, ore 9.00, Aula E, Dip. Scienze della Terra
09/9/2011, ore 9.00, Aula E, Dip. Scienze della Terra
29/9/2011, ore 9.00, Aula E, Dip. Scienze della Terra
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PETROLEUM
GEOLOGY
TEXT BOOKS:
Geology of Petroleum, second edition, by A.I. Levorsen
Petrolio, M. Pieri
Petroleum Geology eTextbook
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Summary
Origin and accumulation of Organic Matter (OM)
Preservation of OM
Transformation of OM to kerogen Diagenesis, catagenesis, metagenesis
Migration
Trapping
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How did petroleum most likely form?
Petroleum is primarily made from plankton. Phytoplankton(plants) and zooplankton (animals) are microscopic organisms
that live in water, both freshwater lakes and the oceans. These
creatures die and sink to the bottom of the ocean where they
accumulate in the sediment along with organic materials that are
washed into the lake or ocean by rivers and streams.
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THE PRODUCTION AND ACCUMULATION
OF ORGANIC MATTER
Organic matter is defined as material
comprised solely of organic molecules in
monomeric or polymeric form, that are derived
directly or indirectly from the organic part of
organisms. deposited or preserved in
sediments
The phytoplankton is the largest producer of
organic carbon in water. The contribution of
larger marine organisms is negligible.
The production of organic matter occurs in the
euphotic zone (the first 60-80 m of water depth).This production can reach 600g/mq/yr, for a
total amount of about 60 billion tones a year.
Only the 0.1 to 0.01% of the organic matter
produced becomes fossil fuel.
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CHEMICAL COMPOSITION OF LIVING MATTER
The organic matter that accumulates in sedimentsbelongs to the following groups of chemical
constituents:
Lipids
Protein
Carbohydrates
Lignin (higher plants)
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Environments where bioproductivity and depositional
environment favour the accumulation of organic rich
sediments.
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DIAGENESIS(from 0 to several hundreds of metres, about 60 C)
CATAGENESIS(several kilometres, about 150 C)
METAGENESIS(more than 5-6 km, more than 150-200 C)
TRASFORMATION OF ORGANIC MATTER
Three distinct stages can be recognized based onthe degree of alteration of the organic matter:
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In high porosity rocks: aerobic bacteria utilize the oxigen
occurring in the water to degrade the organic matter
(oxidation) and generate H2O, CO2.
In low porosity rocks:
anaerobic bacteria mustacquire oxygen via a sulfate
reduction process, which is a
relatively slower process
(fermentation) and generateCO2 e CH4 (biogenic gas).
Through the disintegration of
organic matter, reactive
molecules are produced thatcombine to formgeopolymers
DIAGENESIS(from 0 to several hundreds of metres, about 60 C)
(kerogen).
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Trasformed inbiogenic gas through
fermentation
DIAGENESIS(from 0 to several hundreds of metres, about 60 C)
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CATAGENESIS (several kilometres, about 150 C)
The onset of petroleum generation and the thermal degradation of kerogen
marks the beginning of catagenesis. The kerogen is a macromolecule composed
primarily of carbon (C) and hydrogen (H) and to a lesser extent, oxygen (O),sulfur (S) and nitrogen (N).
The classification of different
types of kerogen is based on the
values of the atomic ratios H/Ce O/C. During the
catagenesis, the kerogen is
heated and becomes relativelyricher in carbon releasing
lighter molecules of CO2, H2O
and crude oil or natural
gas, collectively known ashydrocarbons.
Diagram of van Krevelen
Mixed terrestrial and
marine source materialthat can generate
abundant oil and gas
Woody terrestrial source
material that typically
generates gas
Rare, produces high
amounts of oil
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Thermal maturation is the natural
transformation of kerogen into
petroleum in response to increasedthermal stress, which is due to an
increase in burial depth
throughout the geological time.
The oil window is a
temperature dependant interval inthe subsurface where oil is
generated and released from the
source rocks. The oil window is
often found in the 65-125 degree
Celsius interval (aprox. 2-4 kmdepth), while the corresponding
gas window is found in the 100-
200+ degree Celsius interval (3-6
km depth).
THE OIL WINDOW
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CATAGENESIS (several kilometres, about 150 C)
METAGENESIS
Graphite
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POROSITY
Pore volume X 100Total volume of the rock
Total porosityEffective
Non-effettive
The percentage of pore volume or void space, or that volume within rock that can contain
fluids. Porosity can be a relic of deposition (primary porosity, such as space betweengrains that were not compacted together completely) or can develop through alteration of
the rock (secondary porosity, such as when feldspar grains or fossils are preferentially
dissolved from sandstones).
Effective porosity is the interconnected pore volume in a rock that contributes to fluid
flow in a reservoir. It excludes isolated pores.
Total porosity is the total void space in the rock whether or not it contributes to fluid flow.Thus, effective porosity is typically less than total porosity.
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POROSITY
PrimaryIntergranular
Intragranularsedimentation
TYPE ORIGIN
Secondary
*Diagenetic
**Vuggy
***Moldic
Fractures
replacement
solution
tectonic
*Diagenetic. The geochemical process where magnesium [Mg] ions replace calcium [Ca] ions in calcite, forming the
mineral dolomite. The volume of dolomite is less than that of calcite, so the replacement of calcite by dolomite in a
rock increases the pore space in the rock by 13% and forms an important reservoir rock. Dolomitization can occur
during deep burial diagenesis.
**Vuggy porosity is pore space that is significantly larger than grains or crystals
***Moldic porosity. A type of secondary porosity created through the dissolution of a preexisting constituent of a
rock, such as a shell, rock fragment or grain. The pore space preserves the shape, or mold, of the dissolved material
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PRIMARY POROSITY IN CLASTIC ROCKS
Effect of sorting onto the interparticle porosity: Poorly sorted sedimentsare less
porous than well sorted sediments. Sorting is an important characteristic of
siliciclastic reservoir rocks because as the degree of sorting decreases, the interstitialarea in between the large grains becomes increasingly infilled with finer material.
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Make a comment to this slide.
How does porosity change?
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A diagrammatic representation of the basic fabric-selective porosity types used in
the Choquette and Pray (1970) carbonate porosity classification. What is meant by
fabric selectivity is that the porosity is controlled by the grains, crystals, or other
physical structures in the rock and the pores themselves do not cross those primary
boundaries.
Choquette & Pray (1970) basic, fabric-selective porosity types
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A diagrammatic representation of the basic non-fabric-selective or variably fabric-
selective porosity types used in carbonate porosity classification. These are all porosity
patterns that actually or potentially can cross-cut primary grains and depositional
fabrics. They also include porosity types that potentially can be much larger than any
single primary framework element.
Choquette & Pray (1970) basic nonfabric-selective or
variable porosity types
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Primary porosity (interparticle). The primary porosity in this sandstone
partially filled with quartz cement during diagenesis. The remaining primaryporosity, filled with blue epoxy during sample preparation, has not been altered
by diagenesis. The field of view is approximately 0.65 mm wide
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Shelter porosity. A type of primary interparticle porosity created by the
sheltering and umbrella effect of relatively large sedimentary particles
which prevent the infilling of pore space by finer clastic particles.
I t d i t ti l it
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Inter- and intra-particle porosity
Intra-particle porosity
Inter-particle porosity
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Moldic porosity. Moldic porosity is a type of secondary porosity
that preserves the shape of the precursor grain, such as these
fragments of fossils. Pores were filled with blue epoxy during
sample preparation.
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Close-up photograph ofvuggy porosity These
features indicate pervasive karstification.
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The whitish-blue dolomite dolomite crystals are replacing the pink-
stained calcite through the process of dolomitization. The field of view is
approximately 2 mm wide.
Dolomitization. The geochemical process where magnesium [Mg] ions replace
calcium [Ca] ions in calcite, forming the mineral dolomite. The volume of
dolomite is less than that of calcite, so the replacement of calcite by dolomite in
a rockincreases the pore space in the rock by 13% and forms an important
reservoir rock. Dolomitization can occur during deep burial diagenesis.
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Fractureporosity
A type of secondary porosity produced by the tectonic fracturing
of rock. Fractures themselves typically do not have much
volume, but by joining preexisting pores, they enhance
permeability significantly. In exceedingly rare cases, nonreservoir
rocks such as granite can become reservoir rocks if sufficient
fracturing occurs
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Fractureporosity
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PRIMARY, SECONDARY, and
TERTIARY MIGRATION
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Petroleum System Elements
Source Rock - A rock with abundant hydrocarbon-prone organic matter.
Reservoir Rock - The hydrocarbons are contained in a reservoir rock.
This is a porous rock. The oil collects in the pores within the rock. The
reservoir must also be permeable so that the hydrocarbons will flow to
surface during production.
Seal or Cap-Rock - A rock through which oil and gas cannot moveeffectively. Attributes which favour a rock as a seal include a small pore
size, high ductility, large thickness, and wide lateral extent.
Trap - The structural and stratigraphic configuration that focuses oil andgas into an accumulation.
Migration RouteAvenues in the rock through which oil and gasmove from source rock to trap (faults, fractures, permeable rocks).
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Diagram of a cross section of a petroleum bearing
basin, illustrating the five key components: source
rock, seal, reservoir, trap, and migration route
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MIGRATION
Oil and gas migrate as:
SMALL GLOBULES or DROPS (larger than a micron)
TRASPORTED BY TERMOGENIC GAS (only light oils)
IN WATER SOLUTION (particularly gas)
Migrate because of:
Hydrodinamic pressure
Compaction
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Transformation of kerogen into products of lower density produces an increase
of volume and, therefore, of pressure and generates a network of microfractures
PRIMARY MIGRATION
(within the source rock up to the contact with a different, more permeable rock
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PRIMARY MIGRATION
(within the source rock up to the contact with a different, more permeable rock)
Overpressure zones
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SECONDARY MIGRATION
(through an integrated system composed of permeable rocks, fractures, and faults)
Oil and gas are less dense than water and, following expulsion from the source
rock, they rise towards the Earths surface unless movement is arrested by aseal. Seals tend to be fine-grained or crystalline, low-permeability rocks, such
as mudstone/shale, cemented limestones, cherts, anhydrite, and salt (halite).
Seals can also develop along fault planes, faulted zones, and fractures. The
presence of seals is critical for the development of petroleum pools. In the
absence of seals, hydrocarbons will rise to the Earths surface as oil seeps, and
be destroyed.
oil seep
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Fault
Faults can act as both conduits
(migration pathways) and seals,
depending on the hydraulic conditions,
the rock properties of the faults, and the
properties of the rocks juxtaposed
across the faults. The consideration of
faults as seals follows the same
reasoning as for cap-rock seals above,
i.e. the sealing capacity of a fault relatesto its membrane strength and hydraulic
strength. Fault seals fail when the
pressure of the petroleum can exceed
the entry pressure of the largest pores
along the fault plane.
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CONTROLLED BY:
buoyancy pressure (spinta di galleggiamento)
capillarity pressure (pressione capillare)
hydrodynamic pressure (pressione idrodinamica)
Water density is in the order of 1-1.2; that of oil is di 0.7-1; that of
gas < 0.001
Thebuoyancy pressure increases in proportion to the density
difference between water and oil and with the diametre of
globules so that very small globules may have a compulsion
insufficient to lift.
SECONDARY MIGRATION
(through an integrated system composed of permeable rocks, fractures, and faults)
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SECONDARY MIGRATION
(throug a system composed of permeable rocks integrated with fractures and faults)
The termination or
continuation of movement is
determined by an interplay
between the driving force
(buoyancy pressure) and the
resistive force (capillarity
pressure).
When the upper and lower radii (r) within the distorted
globule globule are equal to one another, the capillarity
force is overcome and the globule can rise due to buoyancy
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The balance between buoyancy and capillary pressure of a
sealing rock is given by:
2(1/rt1/rp) = ho g (w-o)
(Capillarity pressure) (Buoyancy pressure)
= interfacial tension between oil and water (dine/cm)
rt = globule radius outside the pore (cm)
rp = globule radius inside the pore (cm)
ho = height of the column of oil (cm)
g = acceleration of gravity (cm/s2)
w = water density (g/cm3)
= oil density (g/cm3)
SECONDARY MIGRATION
(through an integrated system composed of permeable rocks, fractures, and faults)
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A pressure vs. depth plot, illustrating
a typical water gradient (aquifer)
supporting a petroleum column,
whose steeper gradients lead to a
pressure difference (Pb, buoyancy
pressure) at its maximum beneath the
seal
SECONDARY MIGRATION
(through an integrated system composed of permeable rocks, fractures, and faults)
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Fluid content
The pores of a reservoir rock are filled by water, oil and gas which
may be still or moving.
Variations in pressure,
temperature, density
and volume can put in
motion these fluidsalong gradients that
will move from places
with high potential
energy to places of
lower energy.
A. di fondo
A. marginale
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The buoyancy pressure is
proportional to the height of the
column of oil that accumulates
under the cover. The maximum
height of the column of oil that can
be held in a combination ofreservoir and rock cover is called
critical height(ho).
The seal capacity determines the
height of a petroleum column that
can be trapped beneath it, and the
seal will be breached when the
buoyancy pressure (Pb) exceeds the
seal capillary entry pressure (Pd).
SECONDARY MIGRATION
(through an integrated system composed of permeable rocks, fractures, and faults)
ho=2 (1/rt1/rp)
g (w-o)
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The distance traveled by the oil from the source rock to the
reservoir varies from case to case and, in the case of large fields,
may be of the order of tens of kilometers.
During migration, the composition of the mixture that makes up
the oil changes (splitting), becoming enriched of the lightest and
mobile fractions (saturated hydrocarbons and, to a lesser extent,
aromatic hydrocarbons)
SECONDARY MIGRATION
(through an integrated system composed of permeable rocks, fractures, and faults)
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Tertiary migration includes leakage,seepage, dissipation, and alteration of
petroleum as it reaches the Earths surface.
Tertiary MIGRATION
(through an integrated system composed of permeable rocks, fractures, and faults)
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PETROLEUM TRAPS
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Petroleum System Elements
24803
Source RockSource Rock
Top Seal RockTop Seal Rock
Reservoir RockReservoir Rock
Anticlinal TrapAnticlinal Trap
(Organic Rich(Organic Rich)
(Impermeable)
PotentialMigration Route
(Porous/Permeable)
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The exact definition ofconventional and unconventional
reservoirs is vague. In economic terms, a conventional
reservoir is one in which a reasonable profit can be made at low
oil or gas prices and without requiring large volume stimulation
prices and without requiring large volume stimulationtreatments. Likewise, an unconventional reservoir can be
described as one that requires the higher oil and gas prices and
large volume treatments before a reasonable profit can be made.
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Trap is the term to describe the body, bounded by seals and containing reservoir, in which
petroleum can accumulate as it migrates from the source rock to the Earths surface. There are
many different trap geometries. These can be grouped into two categories:
Static traps:
Structural (anticlines, faults)
Stratigraphic (primary and secondary)
Traps that combine both elements (mixed)
Dynamic traps:
the closure is provided by the idrodynamic flux that contrasts the buoyancy pressure
Structural traps are created by tectonic, diapiric, compactional, and gravitational processes.
Almost the entire worlds discovered petroleum is in traps that are largely structural.
Stratigraphic traps are formed by lithological variations or property variations generated by
alteration of the sediment or fluid through diagenesis. Much of the worlds remainingundiscovered petroleum will be found in stratigraphical traps.
Purely hydrodynamic traps are rare. Such traps rely on the flow of water through the reservoir
horizon to drag the petroleum into a favourable trapping configuration, such as the plunging
nose of a fold.
.
TRAP CLOSURE
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Closure.The vertical distance from
the apex of a structure to the lowest
structural contour that contains the
structure. Measurements of both theareal closure and the distance from
the apex to the lowest closing
contour are typically incorporated
in calculations of the estimated
hydrocarbon content of a trap.
In map view (top), closure is the
area within the deepest structural
contour that forms a trapping
geometry, in this case 1300 ft [390
m]. In cross section A-A', closure is
the vertical distance from the top of
the structure to the lowest closing
contour, in this case about 350 ft
[105 m]. The point beyond which
hydrocarbons could leak from or
migrate beyond the trap is the spillpoint.
TRAP CLOSURE
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Crest
Spill plain
Closure
Splill point
Trap volume
The trap capacity
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The trap capacity
The following properties must be known or estimated in order
for the petroleum volume to be calculated:
1. Net to gross
2. Reservoir effective porosity
3. Permeability
4. Petroleum saturation
5. Reservoir thickness
6. Trap closure
H d b T T
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Hydrocarbon Trap Types
American Petroleum Institute, 1986
Salt DomeFault
Unconformity
Pinchout
Anticline
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The Basic Trap
Gas
Oil
Water
Petroleum Accumulates in Anticlines
St t l t
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Structural traps
Piercement
Fold
Fault
Combination
fold/fault
F ld d i t t t l t
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Fold-dominant structural traps
Fold dominant structural traps
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Fold-dominant structural traps
Fault dominant structural traps
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Fault-dominant structural traps
Fault dominant structural traps
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Fault-dominant structural traps
Fault dominant structural traps
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Fault-dominant structural traps
Combination fault fold structural traps
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Combination fault-fold structural traps
Combination fault fold structural traps
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Combination fault-fold structural traps
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Piercement structural traps
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Piercement structural traps
A salt dome begins to form when a small
part of a wet salt layer rises. That causes
other salt in the layer to flow horizontally
and then up into a rising plume. If the salt
is abundant and saturated with water,
friction offers little resistance, and salt willcontinue to feed into the rising plume. The
upturned (or bowl-shaped) layers next to
the salt dome can become traps in which
oil collects, so understanding salt domes
has great economic value.
Piercement structural traps
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Piercement structural traps
Piercement structural traps
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Piercement structural traps
Combination fault fold structural traps
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Combination fault-fold structural traps
Combination fault-fold structural traps
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Combination fault-fold structural traps
Combination fault-fold structural traps
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Combination fault-fold structural traps
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Stratigraphic traps: due to lateral changes of
permeability produced by lateral changes in grain-
size
primary
secondary
Secondary stratigraphic traps form as a
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Oil-water contact
Oil-gas contact
y g p p
consequence of cementation or dissolution, but,
usually, they are due to the occurrence of
unconformities
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Primary and secondary stratigraphic traps
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Dynamic traps
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Dynamic traps
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dZ/dl=w/(w-o) x dh/dl
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An oil play - A particular
combination of reservoir, seal,
source and trap associated withproven hydrocarbon
accumulations.
An oil pool - A subsurface oil
accumulation.
An oil field - Consists of one or
more oil pools.
Principles of seismic
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surveying: (A) onshore,
with diagrams showing the
various methods of
producing the signal, and
(B) offshore
Principles of seismic surveys. P = P-wave, S = S-wave
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3D Seismic Image of a continental margin
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g g
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Raster Box display
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Three Planes display
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Three Planes display
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Random Profile display
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Random Profile display
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Random Profile display
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Random Profile display
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Random Profile display
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Primary recovery
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Reservoir fluids (gas, oil, water) are under high pressure (geostatic
and hydrostatic pressure) and elevated temperature, any drop inpressure, such as opening the borehole to near atmosferic pressure,
will result in an increase in volume, producing flow. Removing a
volume of fluid will also lead to drop in pressure. However, the
amount of pressure drop depends upon the type of fluid. Gas is
highly compressible, so removing a small amount of gas will not
appreciably affect reservoir pressures. In contrast, oil is not very
compressible, so removing oil will create a measurable drop in
reservoir pressure, unless the volume removed is replenished by
another fluid (e.g., water).
y y
The natural expansion of reservoir fluids is the
primary energy source for initial production
The recovery efficiency depends on the nature of mechanisms
providing pressure to the pool.
Primary recovery
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Simple expansion: Oil and water are not very
compressible, so that drop in reservoir pressure, does
not generate a significant increase in oil volume.
y y
5-6% liquids, more than 90% gas
Primary recovery
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y y
There are two basic types of primary drive
mechanisms: gas drive and water drive.
Gas drive. There are two types of gas drive
mechanisms: gas cap and solution gas (depletion)drive. Both mechanisms function through the
expansion of gas and the volumetric displacement of
oil. The difference between them is the presence or
absence of an initial gas cap.
Primary recovery
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y y
Thegas cap drive is a reservoir containing free-gas in the highest point of the
trap, as a gas cap. Reservoir pressure is maintained by expansion of the gaswithin the gas cap.
Thegas solution (depletion) mechanism lacks an initial free gas cap. A pressure
drop, due to initialwithdrawal of oil from the reservoir causes gas to come out of
solution . The dissociation and expansion of gas drives the oil. The moviment of
the gas within the reservoir will be generally upwards towards the crest of the
trap to form a gas cap.
20-40% of liquids
Very high for gas, 5-30% liquids
Gas cap expansion
Primary recovery
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y y
The water drive: the water drive mechanism occurs because the
natural hydrodynamic flow into the structure maintains pressurebeneath the pooled oil, driving the oil upwards. A natural water
drive mechanism occurs when the underlying aquifer is large and
capable of undergoing recharge.
35-70% liquids
Secondary recovery
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Over the lifetime of the well the pressure will fall, and at some point
there will be insufficient underground pressure to force the oil to the
surface. After natural reservoir drive diminishes, secondary recoverymethods are applied. They rely on the supply of external energy into
the reservoir in the form of injecting fluids to increase reservoir
pressure, hence replacing or increasing the natural reservoir drive with
an artificial drive. Secondary recovery techniques increase the
reservoir's pressure by water injection, natural gas reinjection and gas
lift, which injects air, carbon dioxide or some other gas into the bottom
of a production well, reducing the overall density of fluid in the
wellbore. Typical recovery factors depend on the properties of oil and
the characteristics of the reservoir rock. On average, the recovery factorafter primary and secondary oil recovery operations is between 30 and
50%
y y
Secondary oil recovery
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Gas injection
y y
Gas injection is presently the most-commonly used approach to enhanced recovery. Unwanted gas is
injected into the gas cap or oil-bearing stratum under high pressure. That pressure pushes the oil into
the pipe and up to the surface. In addition to the beneficial effect of the pressure, this method
sometimes aids recovery by reducing the viscosity of the crude oil as the gas mixes with it.
Gases commonly used include CO2, natural gas or nitrogen.
Secondary oil recovery
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Water flood
y y
Tertiary (enhanced) oil recovery
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The third stage of hydrocarbon production during which sophisticated techniques that alter the
original properties of the oil are used. Enhanced oil recovery can begin after a secondary
recovery process or at any time during the productive life of an oil reservoir. Its purpose is not
only to restore formation pressure, but also to improve oil displacement or fluid flow in thereservoir.
The three major types of enhanced oil recovery operations are chemical flooding (alkaline
flooding or micellar polymer flooding), miscible displacement (carbon dioxide [CO2] injection or
hydrocarbon injection), and thermal recovery (steam flood or in situ combustion). The optimal
application of each type depends on reservoir temperature, pressure, depth, net pay, permeability,
residual oil and water saturations, porosity and fluid properties
y y
Chemical techniques: utilize reagents that change the
physical properties of the produced fluid or the
displacement fluid.
Miscible processes: Miscible fluids are utilized to
produce oil that could potentially become residualoil. This is achieved by injecting a fluid that mixes
with the produced fluid. Typical miscible fluids are
organic-based solvents, CO2Thermal techniques: reduce the viscosity of oil
within the reservoir .
Thermally enhanced oil recovery methods (TEOR) are
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Thermally enhanced oil recovery methods (TEOR) are
tertiary recovery techniques that heat the oil, thus reducing its
viscosity and making it easier to extract.
Steam injection is the
most common form ofTEOR.
In-situ burning is another
form of TEOR, but
instead of steam, some of
the oil is burned to heatthe surrounding oil.
Hierarchy
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Meander belt(field)
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Pool
(Meander scroll)
Pool
Total volume in the
order of tens ofbillions of m3
Meander scroll(pool)
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Total volume in the
order of fewbillions of m3
Channel, point bar and crevasse-splay
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Total volume in the order of
several tens of millions of m3
5 orders of migrating and overlapping bedforms of varying type and dimensions are present
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Channel, point bar and crevasse-splay
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Total volume in the order of
several tens of millions of m3
Lobe sheet Level 4 heterogeneity
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Bedding unit
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Mud-laminae between and within bedsets are the result of flow separation and variation
in flow regime.
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Sech et al. (2009)
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Hierarchical orders of internal complexityobserved in the Torre Saracena section.
The main feature of the first-order units is
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alternation of thicker and thinner packages of
dunes. Each unit is, in turn, internally characterized
by alternating simple and compoundsecond-order foreset units. Tidal bundles are the
main features of the third-order units, whilst
bioclastic and siliciclastic packages of laminae
typify fourth-order units (see text
for explanation). Longhitano et al. (2010)