Primary Migration under Geological and Geochemical Aspects

Primary Migration under Geological and Geochemical Aspects

Observed facts w.r.t primary migration of petroleum can be related to time, depth or chemical differences or similarities b/w source and related oil.

Therefore petroleum geologists further divide primary migration into early and late migration based on availability of compaction waters.

Early Migration

Takes place at the first 1000-1500m of subsidence, when sediments looses much of their porosity, yielding large volume of water.

Takes place before onset of main phase of petroleum formation.

The minute hydrocarbons, which are produce are directly inherited from organisms or produce form the early diageneis.

T ranges from 50-70oC, this migration can take place sediments not older than 1000years.

Late Migration

Occurs during main phase of petroleum formation and generally takes place at depths between 1500-3000m or 3500m

A special mechanism for primary migration in main phase is dewatering of clay mineral.

Dewatering of clay or clay dehydration is release of bound water during alteration of smectite to illite mainly under influence of temperature (Burst 1959, Weaver, 1959) and pressure. As a result which pressure on pore spaces increases, which expel the petroleum from these pore spaces

The extent of migration may ranging from a few meters to a long distance in kms.

Changes in composition of source rock bitumen versus crude oil

Ø  Hydrocarbon distribution in contact zone between source rock and reservoir rock: Closed to reservoir, the source is depleted in more lighter compounds. In source, where bitumen remaining, contains enrichment of high molecular weight hetero-atomic compounds and depleted in H/Cs.

Ø  Gross chemical composition of crude oils versus source rock bitumen: most oil enriched in saturated H/Cs and depleted in N-S- O compounds

Ø  The phenomenon of oil-source rock correlateion: most of the oil un-changed within source and reservoir, which is good for correlation.

Factors controlling the Secondary Migration

There are three parameters, which controls the secondary migration and subsequent formation of oil and gas pools.

Ø  Buoyancy

Ø  Hydrodynamic fluid flow

Ø  Capillary pressure

Buoyancy

Buoyancy is the force due to differences in the densities i.e., water and petroleum, by which petroleum move upward.

Buoyancy results in the final equilibrium adjustments of gas, oil and water in a pool and in the original concentration of the dispersed particles of petroleum in water.

In hydrostatic condition only buoyancy force is considered for secondary migration.

Hydrodynamic Fluid Flow

There is, however, frequently sub-surface water flow and the effect of hydrodynamic condition on petroleum can not be neglected on secondary migration.

The interaction between buoyancy and flowing water will either enhance or retard the migration. With flowing water the direction as well as the rate of petroleum flow also changed (Fig).

The force of buoyancy acts on trapped petroleum in two ways:

i. When oil and gas reaches to crest of anticlines, it rests there. If the hydrodynamic pressure is much high then the oil will passes to another structural high.

In case of stratigraphic traps the permeability decreases up dip, then oil and gas migrate up dip due to buoyancy then stop by shale. If the water flow downward then it restricts the oil and gas flow or if flow is within the direction buoyancy then oil and gas might enter to small pore spaces.

Capillary displacement Pressure

The force required to move petroleum through a pore opening.

Depends upon:

Radius of the pore opening (R= m)

The interfacial tension b/w petroleum and water (r=dyne cm-1)

Wettability of the rock: the contact angle between petroleum and water interface against the rock (thetha=degrees)

Mathematically

Capillary displacement pressure= 2rcos0/R

Interfacial Tension:

Molecular attraction between fluid boundaries that cause fluid in separate phase

If difference between interfacial tension high, then fluid are immiscible.

Depends upon temperature and decrease when increased.

Wettability:

An angle of contact between solid and liquid

When two fluids occur together, one likely to have more affinity for the solid (rock wall) and wets.

Two possibilities: Oil wet condition: when the rock wall have more contact with oil, large angle. Water wet condition: when the rock wall have more contact with water, small angle

Geological and Geochemical consideration on Secondary Migration

The movement of hydrocarbon is mainly controlled by these three factors, such as buoyancy, capillary pressure and hydrodynamic flow of water.

The difficult to assess the exact point when the hydrocarbons begin to coalesce into pool.

It is very difficult to quantify the relative importance of buoyancy or hydrodynamic forces on secondary migration.

At initial stage, oil are microscopic and more dispersed than the final stage, due to which, in initial stage, oil will not move according to law of buoyancy, rather move by water flow.

So the hydrodynamic water flow is more important in initial stage rather than final stage

As oil grows, they subject to buoyancy, then the upward moving of the oil and gas starts when ever there is low capillary pressure.

In buoyancy, oil and gas always seek highest point in carrier bed or reservoir rock, as they move the oil particle merge into globules, patches and stringers.

Eventually, a point comes where: pore diameter too small, and capillary pressure could not be overcome, thus accumulation starts.

Note: any tectonic or disturbance may change this delicate balance, so tectonic forces are important means of influencing secondary migration

There might be some changes regarding chemistry of oil and gases, which are less understood.

The changes observed to date are difficult to explain because they are not always consistent. The parameter investigation are C and N isotopes, the amount of heavier hydrocarbon distribution.

Along the migration path there is increase of non-polar compounds and a decrease in content of asphaltene, resins, porphyrins and other non-hydrocarbon compounds.

Water washing may cause loss in low boiling, more soluble hydrocarbons along with slight decrease of 13C/12C isotope ratio may cause primarily by the preferential loss of aromatic hydrocarbons.

Note: might be the process of effusion and diffusion, solution and

adsorption phenomena along with temperature and pressure effect.

Termination of Secondary Migration and Accumulation of Oil and Gas

The end of secondary migration and the final stage in the formation of oil and gas pools, is the concentration of oil and gas from disseminated stage into commercial size, in the highest available part of a trap.

Oil or gas may trapped in any rock of suitable porosity and permeability (reservoir rock), regardless of lithology

The cap rock or seal, by virtue of its general decrease in pore diameter, must exert capillary pressures which are great enough to stop the passage of oil and gas particles.The oil or gas may be close to source area or far away, depending upon geometry of basin and conditions.