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Department of Petroleum Technology, University of Karachi
Maturation and expulsion
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Conversion of Kerogene to Oil and Gas
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With increasing burial by later sediments and increase in
temperature, the kerogen within the rock begins to breakdown.
This thermal degradation orcracking releases shorter
chain hydrocarbons from the original large and complexmolecules found in the kerogen.
The hydrocarbons generated from the source rock are
expelled, along with other pore fluids, due to thecontinuing effects ofcompaction and start moving
upwards towards the surface, a process known as
migration.
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Maturation and expulsion
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Rock Pyrolysis S1 = the amount of free
hydrocarbons (gas and oil)in the sample (in milligramsof hydrocarbon per gram of
rock).
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S2 = the amount of
hydrocarbons generated
through thermal cracking of
nonvolatile organic matter.
S3 = the amount of CO2 (in
milligrams CO2 per gram of
rock) produced during pyrolysis
of kerogen.
Oxygen bearing volatile
compounds are passed to a
separate detector, which
produces as S3 response.
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Rock Pyrolysis
Espitalie developed a standard procedure for the pyrolysis ofrock samples known as ROCK-EVAL PYROLYSIS.
Method: About 100 mg finely ground rock sample is placed into
a furnace at 250 degree C in an inert atmosphere than raised toa temperature of 550 degree C.
The amount of Hydrocarbon products evolved is recorded by a
Flame Ionization Detector(FID) as a function of time.
Three Peaks are typically, Known as S1, S2 and S3 peaks are
evolved and recorded.
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Example of Rock Eval trace. HC = hydrocarbon IfS1 >1 mg/g, it may be indicative of an oil show.
S1 normally increases with depth. Contamination of samples by drilling fluids and
mud can give an abnormally high value forS1.
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S2 is an indication of the quantity of hydrocarbonsthat the rock has the potential of producing.
The burial and maturation should continue at this
stage.
This parameter normally decreases with burial
depths >1 km.
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S3 = the amount of CO2 (in milligrams CO2 per gram of
rock) produced during pyrolysis of kerogen.
Oxygen bearing volatile compounds are passed to a
separate detector, which produces as S3 response.
S3 is an indication of the amount of oxygen in the kerogen
and is used to calculate the oxygen index.
Contamination of the samples should be suspected ifabnormally high S3 values are obtained.
Example of Rock Eval trace. HC = hydrocarbon
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Hydrogen index/oxygen index plot from Rock Eval pyrolysis
data. TOC
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HI = hydrogen index(HI = [100 x S2]/TOC).
HI is a parameter used to characterizethe origin of organic matter.
Marine organisms and algae, ingeneral, are composed oflipid-
and protein-rich organic matter,where the ratio of H to C is higherthan in the carbohydrate-richconstituents of land plants.
. PC = pyrolyzable carbon (PC = 0.083
x [S1 + S2]).PC corresponds to carbon content of
hydrocarbons volatilized andpyrolyzed during the analysis.
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Hydrogen index/oxygen index plot from Rock Eval pyrolysis
data. TOC
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. OI = oxygen index
(OI = [100 x S3]/TOC).
OI is a parameter that correlates with
the ratio of O to C, which is high for
polysacharride-rich remains of land
plants and inert organic material(residual organic matter)
encountered as background in
marine sediments.
PI = production index
(PI = S1/[S1 + S2]).
PI is used to characterize the evolution
level of the organic matter.
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EXPULSION EFFICIENCY
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PGIandPEE
Mackenzie and Quigley (1988) has classified source rocks into
three end member classes on the basis of initial kerogene
concentration and kerogene type.
These parameters determine the timing and composition of
petroleumexpelled.
PGI : (Petroleum generation Index)
is the fraction of petroleum prone organic matter that has been
transformed into petroleum, and is thus a measure of source
maturity.
PEE: (Petroleum expulsion efficiency)
is the fraction of petroleum fluids formed in the source rock that
have been expelled
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Class I:
Predominantly Labile Kerogen at concentration of10Kg/ton and generationstart at about 100 degree C.
This rapidly saturates the source rock and between 120-150 degree C 60%-90%is expelled as oil with dissolved gas.
The remaining fluid crack to gas at higher temperature and expelled as gas.
Class II: This is linear version of Class I with initial Keregen concentration < 5Kg/ton.
Expulsion is inefficient up to 150 degree C because insufficient oil-richpetroleum generated.
Petroleum is expelled mainly as gas condensate formed by cracking above
150degree C followed by some Dry Gas. Class III:
Source rocks contain mostly Refractory Kerogen. Generation and expulsiontake place only above 150 degree C and petroleum fluid is a relatively dry Gas
PGI and PEE
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The Origin of PetroleumThe Origin of Petroleum
Organic-richOrganic-richSource RockSource Rock
Thermally MaturedThermally MaturedOrganic MatterOrganic Matter OilOil
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GeneratioGenerationn
MigrationMigrationAccumulationAccumulation
andandPreservationPreservation
Petroleum System: Timing is CriticalPetroleum System: Timing is Critical
Processes:
Elements:
SourceSourceRockRock
MigrationMigrationAvenueAvenue
ReservoirReservoirand Sealand Seal
Trap Must Be Available Before/During MigrationTrap Must Be Available Before/During Migration
For accumulations to occur, a trap must exist either before or coincident with the time ofmigration. The petroleum system events chart helps capture these critical aspects of timing.
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Spill PointSpill Point
Seal Rock(Mudstone)Reservoir Rock
(Sandstone)Migration fromKitchen
1) Early Generation
2) Late Generation
Gas displaces all
oil
Gas beginning todisplace oil
Displaced oil
accumulates
Petroleum SystemPetroleum System
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Several specific forms of hydrocarbons-
Dry gas-contains largely methane, specifically
contains less than 0.1 gal/1000ft3 of condensable(at surface T and P) material.
Wet gas-contains ethane propane, butane. Up to
the molecular weight where the fluids are alwayscondensed to liquids
Condensates- Hydrocarbon with a molecular
weight such that they are gas in the subsurfacewhere temperatures are high, but condense toliquid when reach cooler, surface temperatures.
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Liquid hydrocarbons- commonly known as oil, or crude oil, to distinguish it from
refined hydrocarbon products.
Plastic hydrocarbons- asphalt Solid hydrocarbons- coal and kerogen- (kerogen strictly
defined is disseminated organic matter in sediments that isinsoluble in normal petroleum solvents.
Gas hydrates- Solids composed of water molecules surrounding gas
molecules, usually methane, but also H2S, CO2, and otherless common gases.
Several specific forms of hydrocarbons-
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Department of Petroleum Technology, University of Karachi
Hydrocarbon Generation Stages
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Cross section through part of a sedimentary basin in which ahydrocarbon source rock layer has been buried to different depths.
Due to increasing temperatures with increased burial depth, organicmatter within this source rock `cooks', resulting in partialdecomposition and petroleum generation (mature source rock).
With further burial, organic matter decomposes to generate natural gas(over-mature source rock).
Generated petroleum and natural gas are expelled from the sourcerock and migrate upward into porous overlying rock layers.
If appropriate conditions exist, petroleum and natural gas are trappedand accumulate.
If appropriate conditions do not exist, natural gas is eventuallyreleased to the atmosphere and petroleum seeps at the surface to formasphalt (tar) deposits.
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OIL AND GAS MIGRATION
Traditionally (Illing, 1933), the process ofpetroleum migration is divided into two parts:
primary migrationwithin the low-permeabilitysource rocks
secondary migrationin permeable carrier bedsand reservoir rocks.
It is now recognized that fractured source rockscan also act as carrier beds and reservoir rocks somore modern definitions are:
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OIL AND GAS MIGRATION
Primary migrationof oil and gas is movementwithin the fine-grained portion of the maturesource rock.
Secondary migrationis any movement in carrierrocks or reservoir rocks outside the source rockor movement through fractures within the sourcerock.
Tertiary migrationis movement of a previouslyformed oil and gas accumulation.
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Mechanisms of Migration With regard to the mechanisms involved in migration there are seven main
questions to answer.
When did migration take place?
What form were the hydrocarbons in when they migrated?
What moved the hydrocarbons?
If water was involved: where did the water come from?
What caused the water to move?
In which direction did the water move?
Have much water moved?
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PRIMARY MIGRATION
Type III kerogens are the
most likely source.
Migration can also occur in
aqueous solution for the
smallest and most soluble
molecules (methane,
ethane, benzene, toluene).
Migration by diffusion is not
significant.
Primary oil migration within a fine-
grained mature source rock with> 2% total organic carbon (TOC)occurs initially as a bitumen thatdecomposes to oil and gas andmigrates as a hydrocarbon (HC)phase or phases.
The process of HC generation
causes expulsion of petroleum
and is often a more potentialmechanism for migration thanmechanical compaction.
Generation and expulsion of light
oil, and gas can come from low
(< 2%) TOC source rocks without
a bitumen intermediate.
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PET 631 Mi ti
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PET 631 Migration
There are two types of migrationwhen discussing the movement ofpetroleum, primary and secondary.
Primary migration refers to themovement of hydrocarbons fromsource rock into reservoir rock andit is this type that the following
discussion refers to.
Secondary migration refers to thesubsequent movement ofhydrocarbons within reservoir
rock; the oil and gas has left thesource rock and has entered thereservoir rock.
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A problem in close relation
to the later stage of theproject is the expulsion of
hydrocarbons from source
rock (primary migration).
The chemical aspects of
this process has been
extensively studied, but the
physical aspects are poorly
understood.
Primary Migration from shale source Rocks
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Primary Migration
Fig.Generalized view of oilmigration using invasion
percolation concepts
(from Carruthers and Ringrose,
1998).
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Mi i
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Micro-pressuring. Mompers [1978] clearly outlines
the characteristics of a source
rock which are important in thedevelopment of micro-pathwayswith the rock.At some point the pressureincrease causes micro-fracturingin the rock, and the hydrocarbonsmigrate into the micro-fractureswhich lead out of the source rock.
This concept allows thehydrocarbons to migrate in aliquid phase.
This is regarded as the mainmechanism for primary migrationout of the source rock.
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GENERATION MIGRATION AND TRAPPING OF
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Seal
Reservoirrock
Seal
Migration route
Oil/watercontact (OWC)
Hydrocarbon
accumulationin thereservoir rock
Top of maturity
Source rock
Fault(impermeable)
GENERATION, MIGRATION, AND TRAPPING OF
HYDROCARBONS
Seal
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Migration through Fractures
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Mechanics of Secondary Hydrocarbon Migration
The mechanics of secondary hydrocarbon migration andentrapment are well-understood physical processes thatcan be dealt with quantitatively in hydrocarbonexploration.
The main driving force for secondary migration ofhydrocarbons is buoyancy.
If the densities of the hydrocarbon phase and the waterphase are known, then the magnitude of the buoyant forcecan be determined for any hydrocarbon column in thesubsurface.
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Mechanics of Secondary Hydrocarbon Migration
Hydrocarbon and water densities vary significantly.
Subsurface oil densities range from 0.5 to 1.0 g/cc;subsurface water densities range from 1.0 to 1.2 g/cc.
When a hydrodynamic condition exists in the subsurface,the buoyant force of any hydrocarbon column will bedifferent from that in the hydrostatic case.
This effect can be quantified if the potentiometric gradient
and dip of the formation are known.
The main resistant force to secondary hydrocarbonmigration is capillary pressure.
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Mechanics of Secondary Hydrocarbon Migration
The factors determining the magnitude of the resistantforce are the radius of the pore throats of the rock,hydrocarbon-water interfacial tension, and wettability.
For cylindrical pores, the resistant force can be quantified
by the simple relation: , where Pd is the hydrocarbon-water displacement pressure or the resistant force, isinterfacial tension, is the wettability term, and R is radiusof the largest connected pore throats.
Radius of the largest connected pore throats can bemeasured indirectly by mercury capillary techniques usingcores or drill cuttings.
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Mechanics of Secondary Hydrocarbon Migration Subsurface hydrocarbon-water interfacial tensions range
from 5 to 35 dynes/cm for oil-water systems and from 70 to30 dynes/cm for gas-water systems.
Migrating hydrocarbon slugs are thought to encounter
water-wet rocks.
The contact angle of hydrocarbon and water against thesolid rock surface as measured through the water phase, ,
is thus assumed to be 0, and the wettability term, , is
assumed to be 1.
A thorough understanding of these principles can aid both
qualitatively and quantitatively in the exploration and
development of petroleum reserves.
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Dri ing forces for migration
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Driving forces for migration: Secondary migration is the movement
of hydrocarbons along a "carrier bed"from the source area to the trap.
Migration mostly takes place as one ormore separate hydrocarbons phases(gas or liquid depending on pressureand temperature conditions).
There is also minor dissolution in
waterof methane and short chainhydrocarbons.
Buoyancy (This force acts verticallyand is proportional to the densitydifference between water and thehydrocarbon so it is stronger for gas
than heavier oil)
Hydrodynamic flow (water potentialdeflect the direction of oil migration,the effect is usually minor except inover pressured zones (primary migration))
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R i ti f
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Resisting forces:
Capillary pressure (opposesmovement of fluid from coarse-grainto fine- grain rock, also the capillarypressure of the water in the reservoirresists the movement of oil)
One result of hydrodynamic flow is atilted oil-watercontact (OWC) in atrap. OWC is an equipotential
surface, but if the water is flowing theequipotential surfaces are inclined inthe direction of flow, so the OWC willbe tilted too.
During migration the pressure and
temperature conditions of thehydrocarbons can change a lotaffecting the phase behavior of theoil.
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Retardatin of buoyant movement as an oil globule (X) is deformed tofit into a narrow pore throat (Y). The upward buoyant force is partlyor completely opposed by the capillary-entry pressure, the forcerequired to deform the oil globule enough to enter the pore throat. If
the capillary-entry pressure exceeds the buoyant force, secondarymigration will cease until either the capillary-entry pressure isreduced or the buoyant force is increased.
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MECHANISM
Once hydrocarbons are expelled from the source rock ina separate hydrocarbon phase into a secondary-migration conduit, subsequent movement of thehydrocarbons will be driven by buoyancy. Hydrocarbons
are almost all less dense than formation waters, andtherefore are more buoyant. Hydrocarbons are thuscapable of displacing water downward and movingupward themselves. The magnitude of the buoyant forceis proportional both to the density difference between
water and hydrocarbon phase and to the height of the oilstringer. Coalescence of globules of hydrocarbons afterexpulsion from the source rock therefore increases theirability to move upward through water-wet rocks
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Two basic types of traps:
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Two basic types of traps:
Structural traps hold oil and gas because
the earth has been bent and deformed in
some way. The trap may be a simple dome
(or big bump), just a "crease" in the rocks, orit may be a more complex fault trap like the
one shown at the right.
Department of Petroleum Technology, University of Karachi
Stratigraphic traps are depositional in
nature.
This means they are formed in place,
usually by a sandstone ending upenclosed in shale.
The shale keeps the oil and gas from
escaping the trap.
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