4. Migration from Source to Reservoir

in oilfield waters with depth from the US Gulf Coast

with SAR. Left: Santa Monica Bay, California. Above: Caspian Sea,

Azerbaijan

the source rock.
This is in general a fine-grained rock

that has a low permeability,
for reasons outlined earlier in this course. During burial, this rock
gets compacted and its interstitial fluid become overpressured
with respect to surrounding rocks that have higher permeabilities
and from which fluids can migrate with greater ease upwards.

Therefore, a fluid pressure gradient develops between the source
rock and the surrounding, more permeable rocks. This causes the
fluids - the water and the hydrocarbons - to migrate along the
pressure gradient, usually upwards, although a downward
migration is possible. This process is called primary migration,
and it generally takes place across the stratification. Why?

regions. The Dickinson curve is from undercompacted
shales in the

Gulf Coast.
The Athy curve from
Paleozoic shales in
compaction equilibrium.

oil generation and
migration together, we end up in a

substantial dilemma:

1. Oil is generated at temperatures of 60° to 120 °C, which are
typically found at depths of 2 to 4 km.

2. Source rocks at these depths are so compacted that their
permeability is too low for efficient primary migration.

This dilemma is not solved yet, but many ideas have been
proposed to circumvent it.
At the core of the problem is the question of how hydrocarbons
migrate.

gaps between the
age of the source and reservoir rocks,

and the formation of the trap (before which migration cannot occur). Most traps are, in fact, relatively young. This suggests that migration and accumulation may proceed in two or more different stages:

An early migration in which permeabilities are relatively high and gas may be available to dissolve hydrocarbons, but where the
hydrocarbon generation is inefficient,

and

a later stage at which oil generation is efficient but permeabilities are low.

porous and
permeable layer to its final accumulation is called

secondary
migration. It is much less controversial than primary migration,
and it is almost entirely governed by buoyancy forces.

primary migration is slow and proceeds over short distances, secondary

migration is faster and can proceed over very long distances (more than one-hundred kilometers).

There are several theories for primary migration, among which
diffusion, oil-phase migration, micro-fracturing and migration in
solution.

Secondary migration is better understood and leads to the
accumulation of hydrocarbon in traps where a seal prevents them
from further migration.

shale layer 1 km
thick and 10 × 10 km

in area, as it is buried from 1 km to 3 km
depth, assuming its compaction is in equilibrium.

Assume now that this is a source rock with 600 ppm weight
percent mature hydrocarbon, of which 15% get expulsed in
primary migration.

How much oil is generated?

What is the average concentration of oil, in ppm, in the water
that escapes from the shale?