The 5 Steps to Heaven, which are source, migration, reservoir, seal and trap, is one of the most important concepts of petroleum geology. This tells us how petroleum was formed, migrated and trapped in a reservoir. Please note that the 5 Steps to Heaven is valid for conventional petroleum resources.
Typically, organic matter from animals and plants is oxidized and turned an organic matter into carbon dioxide and water. However, if organic matter is buried rapidly, it may be preserved and can be turned into petroleum. These following conditions enhance organic matter preservation.
- High sedimentation rate
- Fine grain size so oxygen will not be able to penetrate and oxidize the organic matter
- Anoxic bottom water
These conditions are mostly found in shale and fine limestone, which are common source rocks.
Types of organic matter can be classified into three types.
Type 1 – Fresh Water Lake
- Fine algae
- H:C ratio about 1.6 – 1.8
- Tends to be more oil with a low amount of gas
Type 2 –Marine Environment
- Single cell plankton, algae and bacteria
- H:C ratio about 1.4
- Tends to be a mixture between oil and gas
Type 3 – Swamp
- Land vegetation , spores, pollen and plant fragments
- H:C < 1.0
- Tends to be mostly gas or coal
Figure 1 – Type of Organic Matter
Then organic matter will be chemically processed to transform it into source rocks and this process is called the “cooking process.” Three phases of maturation of organic matter are diagenesis, catagenesis and metagenesis.
Figure 2 – Organic Maturity
- Formation of kerogen type 1,2,3 depending on the type of organic matter
- Begin during initial deposition at shallow surface
- Non-biogenic reaction and biogenic decay aided by bacteria turns organic matter to methane, CO2, H2O and Kerogen
- Depth < 1,000 m
- Temperature < 60 C
- Maturation of kerogen
- Temperature increases with depth.
- Around 60 C oil starts to form from kerogen as the molecules are cracked.
- Oil generation is between 60 – 160 C is called the “oil window”
- Higher temperature cracks liquid hydrocarbon molecule.
- Gas generation is between 160 – 225 C.
- Above 225 C only carbon remains in the form of graphite. No hydrocarbon is formed beyond this temperature.
Hydrocarbon migration is the second step and it is a movement from petroleum fluid from source rock into reservoir rocks. Two processes of petroleum migrations are primary and secondary migration.
- This is happened first when hydrocarbon migrate from source rock goes into reservoir rocks.
- Primary migration is driven by pore pressure and can be both upwards and downwards direction.
- This is the movement of hydrocarbon through the carrier and reservoir rock.
- Buoyancy is a driven force for the secondary migration.
Figure 3 – Primary and Secondary Migration
A reservoir rock is a place where hydrocarbon migrates and is held underground. Reservoir rocks are sandstone, limestone, chalk, dolomite, etc. Reservoir size and shape depends on depositional environment.
Major effects on reservoir properties are as follows;
Porosity = % of pore space in a reservoir rock
Permeability = ability of rock to allow reservoir fluid to flow through
- 1 – 10 md = fair sand
- 10 – 100 md = good sand
- 100 – 1000 md = very good
- 1000 md up = excellent
Net to gross ration – ratio between effective reservoir to entire reservoir interval
Seal is an impermeable rock, which prevents hydrocarbon from passing through. Therefore, further migration of oil and gas is stopped. Typically, they are fine grain sediments such as shale and evaporite (salt). Additionally, deformed shale in a fault zone can be a seal.
Figure 4 – Salt dome as a trap
Traps are impermeable structures where hydrocarbon accumulates underneath. Two types of traps are as follows;
Structural traps are formed as a result of changes in the structure of subsurface. They may be caused by pure tectonic movement (fault and fold) or salt movement. Examples of structural traps are anticline, fault and salt dome trap.
Stratigraphic traps occur where the reservoir itself is cut off up dip and no other structural control is needed. Changes in lithology may be caused by variations in original deposition or due to processing after deposition. Examples of stratigraphy traps are pinch out traps, reef traps and lens traps.
Figure 5 – Structural and Stratigraphic Traps
USGS, (2013), ormation of organic-rich sediment layer [ONLINE]. Available at: http://energy.usgs.gov/portals/0/Rooms/geochemistry_research/images/first_stage.gif [Accessed 14 October 15].