PETROLEUM DEVELOPMENT GEOLOGY PETRO GEOL

Petroleum System ProcessesAccumulation Source Rock 2480 120° F 350° F Generation Migration Seal Rock Reservoir Rock Reservoir Rock Oil Water Gas Cap Gas Cap Entrapment... HYDROCARBON MIG

a SOURCE ROCKS & MATURATION

b HYDROCARBON MIGRATION

c CAP ROCKS / SEALS

d STRUCTURE / TRAP

e RESERVOIR ROCKS

Petroleum System Processes

•

• Generation - - Burial of source rock to temperature and pressure regime sufficient to convert organic matter into hydrocarbon

•

• Migration Migration - - Movement of hydrocarbon out of the source rock toward and into a trap

•

• Accumulation Accumulation - - A volume of hydrocarbon migrating into

a trap faster than the trap leaks resulting in an

Petroleum System Processes

Accumulation

Source Rock

2480

120° F

350° F

Generation Migration

Seal Rock

Reservoir Rock

Reservoir Rock

Oil Water

Gas Cap

Gas Cap

Entrapment

SOURCE ROCKS

• Hydrocarbon originates from

minute organisms in seas and

lakes When they die, they sink to

the bottom where they form

organic-rich "muds" in fine

sediments (usually become gray–

black shale)

• These "muds" are in a reducing

environment or "kitchen", which

strips oxygen from the sediments

leaving hydrogen and carbon

• The sediments are compacted to

form organic-rich rocks with very

low permeability

• The hydrocarbon can migrate

very slowly to nearby porous

rocks, displacing the original

formation water

The principal zone of oil formation during the thermal generation of

petroleum hydrocarbons

• If the temperature is too low, the organic material cannot transform into hydrocarbon.

• If the temperature is too high, the organic material and hydrocarbons are

destroyed.

HYDROCARBON MIGRATION

• Hydrocarbon migration takes place in two stages:

– Primary migration - from the source rock to a porous rock This is a complex process and not fully understood It is probably limited to a few hundred metres.

– Secondary migration - along the porous rock to the trap This occurs

by buoyancy, capillary pressure and hydrodynamics through a

continuous water-filled pore system It can take place over large

distances.

CAP ROCK

• A reservoir needs a cap rock.

• Impermeable cap rock keeps the fluids trapped in the reservoir.

• It must have zero permeability.

• Some examples are:

– Shales.

– Evaporites such as salt or anhyhdrite.

– Zero-porosity carbonates.

TRAPS

GENERAL

The reservoir form depends on the depositional environment and post depositional events such as foldings and faulting

The criteria for a structure is that it must have:

•Closure, i.e the fluids are unable to escape

•Be large enough to be economical

• Tilted fault-block traps block traps are formed where the upward flow of the petroleum is prevented by impermeability along the fault plane and by an overlying cap or seal: common in the North Sea.

an erosional erosional break

in the stratigraphic stratigraphic

succession is

followed by

impermeable

strata.

SALT DOME TRAP

• Salt Dome traps are caused when "plastic" salt is forced upwards.

• The salt dome pierces through layers and compresses rocks

above This results in the formation of various traps:

• In domes created by formations pushed up by the salt.

• Along the flanks and below the overhang in porous rock abutting

on the impermeable salt itself.

PETROLEUM RESERVOIR ROCKS

PETROLEUM RESERVOIR ROCKS

DEFINITION

•

• A body of porous and permeable rock

containing oil and gas through which fluid may move toward recovery

opening under the pressure existing or that may be applied (Amyx, 1960)

TYPE OF RESERVOIR ROCKS

c Sedimentary:

ÜClastic ; eg Sandstone, Conglomerate

ÜNon Clastic ; eg Limestone, Evavorite.

c Igneous:

ÜPlotunic ; e.g Granite

ÜVolcanic ; eg Basalt

ÜVolcanic Clastic : eg Tuff, Breccia.

c Metamorphic:

Üeg Marble, gneiss, quartzite, slate etc.

Reservoir Rocks

1 Pore spaces able to retain hydrocarbon.

2 Permeability which allows the fluid to move.

DEFINITION OF POROSITY

b

ma b

b

p

V

V V

V V Porosity = φ = = −

POROSITY SANDSTONES

• The porosity of a sandstone depends on the packing arrangement

of its grains.

• The system can be examined using spheres.

In a Rhombohedral packing, the pore space accounts for 26% of the total volume.

In practice, the theoretical value is rarely reached because:

a) the grains are not perfectly round, and b) the grains are not of uniform size.

With a Cubic packing arrangement, the pore space fills 47% of the total volume

POROSITY AND GRAIN SIZE

• A rock can be made up of small grains or large grains but have the same porosity.

• Porosity depends on grain packing, not the grain size.

• Total porosity, φt =

• V ery clean sandstones : φt = φe

• Poorly to moderately well -cemented intergranular materials: φt ≈ φe

• Highly cemented materials and most carbonates:

φe < φt

Volume Bulk

Space Pore

Total

Volume Bulk

Space Pore

cted Interconne

rock such as:

example is the replacement of some of the calcium atoms in

limestone by magnesium to form dolomite.

CARBONATE POROSITY TYPES

Interparticle porosity: Each grain is separated, giving a similar pore space arrangement as sandstone.

Intergranular porosity: Pore space is created inside the individual grains which are interconnected.

Intercrystalline porosity : Produced by spaces between carbonate crystals.

Mouldic porosity: Pores created by the dissolution of shells, etc.

• Carbonate porosity is very heterogeneous It is classified into a number of types:

CARBONATE POROSITY TYPES

Fracture porosity:

• Pore spacing created by the

cracking of the rock fabric.

• Fractures are caused when a rigid rock is strained beyond its elastic limit - it cracks.

• The forces causing it to break are in a constant direction,

hence all the fractures are also aligned.

• Fractures are an important source of permeability in low

porosity carbonate reservoirs.

• Vugs are defined as non-connected pore space.

• They do not contribute to the producible fluid total.

• Vugs are caused by the dissolution of soluble

material such as shell fragments after the rock has been formed

• They usually have irregular shapes.

PERMEABILITY

• The rate of flow of a liquid through a

formation depends on:

– The pressure drop.

– The viscosity of the fluid.

– The permeability.

• The permeability is a measure of the ease at which a fluid can flow through a formation.

• The unit of measurement is the Darcy.

• Reservoir permeability is usually quoted in millidarcies, (md).

DARCY LAW

• K = permeability, in Darcies.

• L = length of the section of rock, in centimetres.

• Q = flow rate in centimetres / sec.

• P1, P2 = pressures in bars.

• A = surface area, in cm2.

• µ = viscocity in centipoise.

PERMEABILITY AND ROCKS

In formations with large grains , the permeability is

high and the flow rate larger.

PERMEABILITY AND ROCKS

• In a rock with small grains the permeability is less

and the flow lower.

• Grain size has no bearing on porosity, but has a large effect on permeability.

K K

1

≤

H V

CLASTIC RESERVOIRS

• Sandstone usually has

regular grains; and is

• Fractures may be

present.

CARBONATE

RESERVOIRS

• Carbonates normally have

a very irregular structure

• Porosity: Determined by

the type of shells, etc and

by depositional and post-