Migration
“Migration is a process, whereby hydrocarbons move from source rocks to reservoir rock under traps”
Mainly migration (migration phases) is divided into:
Primary Migration: the release of petroleum compounds from solid organic particles (Kerogen) in source beds and their transport within and through the capillaries and narrow pores of a fine grained source bed has been termed primary migration. -OR-The process of loss of hydrocarbons from the source rock (also expulsion)
Secondary Migration: The oil expelledfrom a source bed passes through wider pores of more permeable porous rock unit. -OR- Migration from source to reservoir rock in trap configuration along a carrier system, including the migration within the reservoir itself.
Distinction is made between primary and secondary migration by facies
Petroleum generated from finely disseminated O.M (source beds); it is the first appearance of petroleum, which is in dispersed form.
This dispersed form will finally leads to the formation of petroleum accumulation by the process of primary and especially secondary migration.
Other migration
includes:
Tertiary Migration: Migration to the surface, either from the reservoir or source rock (dis-migration)
Re-Migration: Migration from one reservior system position through an intervening section into another reservoir position (trap) in the same or different reservoir.
Why do hydrocarbons Migrate?
Fluid migrates along pressure gradient: (pressure driven)
Density contrast between hydrocarbons and water: (buoyancy driven)
Diffusion due to concentration differences: (chemical gradient driven)
How do hydrocarbon migrates?
Hydrocarbons migrate as a separate phase from higher potential to a lower potential via the most efficient way: (topography driven).
Physio-chemical Factors Controlling the Primary Migration
1 Pressure and Temperature
2 Compaction
3 Fluids
1 Pressure and Temperature
2 Compaction
3 Fluids
1. Pressure and Temperature Temperature:
T increases with increase in burial (geothermal gradient)
Average 25oC/Km;
but not linear there may be change due to thermal conductivity difference in rocks e.g., may found 5oC/Km in well of Andros Island in Bahamas and 90oC/Km in walio oil fields of Indonesia.
Few factors which control G.G;
Thermal conductivity difference; e.g, salt good conductor than shale thus energy transmission is easy from greater depth
Regional flow
Subsurface fluid flow
These are inter-connected, increase in temperature; would increase in volume; would decrease in viscosity (fluids); pore spaces wider and flow is easily (Fig.)
T increases with increase in burial (geothermal gradient)
Average 25oC/Km;
but not linear there may be change due to thermal conductivity difference in rocks e.g., may found 5oC/Km in well of Andros Island in Bahamas and 90oC/Km in walio oil fields of Indonesia.
Few factors which control G.G;
Thermal conductivity difference; e.g, salt good conductor than shale thus energy transmission is easy from greater depth
Regional flow
Subsurface fluid flow
These are inter-connected, increase in temperature; would increase in volume; would decrease in viscosity (fluids); pore spaces wider and flow is easily (Fig.)
Pressure:
P increases with increase in depth w.r.t overburden of sediments (gravity load); causes compaction and decrease porosity
Three types of P on fluids; (i) Overburden Pressure, (ii) Hydrostatic Pressure and (iii) Petro-static Pressure (litho-static/Geo-static Pressure)
These are inter-connected; when over-burden pressure increases, the normal hydro-static pressure within sediments increases; causing sediment to over-pressure and thus attain petro-static pressure; which allow fluids to flow
Nature of pressure depends upon; (i) pore-fluids, (ii) Temperature and (iii) Geological setup/tectonic effect
> Average pressure may be 70-250 mg/l/m and reach upto 300mg/l/m
P increases with increase in depth w.r.t overburden of sediments (gravity load); causes compaction and decrease porosity
Three types of P on fluids; (i) Overburden Pressure, (ii) Hydrostatic Pressure and (iii) Petro-static Pressure (litho-static/Geo-static Pressure)
These are inter-connected; when over-burden pressure increases, the normal hydro-static pressure within sediments increases; causing sediment to over-pressure and thus attain petro-static pressure; which allow fluids to flow
Nature of pressure depends upon; (i) pore-fluids, (ii) Temperature and (iii) Geological setup/tectonic effect
> Average pressure may be 70-250 mg/l/m and reach upto 300mg/l/m
2.Compaction
Compaction results in; increase in bulk density and loss of porosity by increase in P.
Compaction results in; increase in bulk density and loss of porosity by increase in P.
Rate of compaction depends upon material properties
Ø Physical
Ø Chemical
Ø Rate at which fluid is expelled; considered an important factor in fluid migration
Compaction related fluid behaves differently in clastic sediments and CaCO3
Ø Physical
Ø Chemical
Ø Rate at which fluid is expelled; considered an important factor in fluid migration
Compaction related fluid behaves differently in clastic sediments and CaCO3
Compaction through Clastics
Higher in upper 100-300m as compared to 1000-3000, then slowly diminished
Shale: in a young sediments due to high pressure; can generate methane and other low molecular weight H/C
If gas produced; increase in volume; increases fluid pressure (nonequilibrium state); thus fluid move from: hot to cold and deeper to shallow
Higher in upper 100-300m as compared to 1000-3000, then slowly diminished
Shale: in a young sediments due to high pressure; can generate methane and other low molecular weight H/C
If gas produced; increase in volume; increases fluid pressure (nonequilibrium state); thus fluid move from: hot to cold and deeper to shallow
Compaction through CaCO3
Chemically reactive, which may increase or decrease porosities/permeabilities
Shallow depths (100-300m) may decrease porosity
Higher depth may increase porosities due to recrystallization and dolomitization
Eventually at depths; fine grained OM; concentrates on grain boundaries
Chemically reactive, which may increase or decrease porosities/permeabilities
Shallow depths (100-300m) may decrease porosity
Higher depth may increase porosities due to recrystallization and dolomitization
Eventually at depths; fine grained OM; concentrates on grain boundaries
3.Fluids
Water abundant fluid may be (a) interstitial: expelled due to compaction, but still some water left (b) structural; due to van der waal forces, temporary, developed on mineral poles
Other fluid may be H/Cs; which present in pore spaces within sediments.
Water abundant fluid may be (a) interstitial: expelled due to compaction, but still some water left (b) structural; due to van der waal forces, temporary, developed on mineral poles
Other fluid may be H/Cs; which present in pore spaces within sediments.
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