Growth Faults and Hydrocarbons

Growth faults are faults that were active at the same time as the sediments were being deposited. Many show a Listric geometry with the fault soling out into shale horizons. They are common in areas with thick delta sequences. Growth faults can be recognized because sediments thicken into the hanging wall of a growth fault and the throw of the fault increases with depth. All the individual reservoir units may thicken up across a mapped growth fault. Alternatively, growth can be taken up by additional layers filling the accommodation space in the hanging wall.

• The given figure shows a growth fault that has high up-dip angle and flattened at its base into zone of detachment or decollement.

• Sedimentary layers have different geometry and thickness across fault. Footwall-landward has undisturbed sedimentary strata that dip gently toward the basin while the hanging wall – on the basin side of the fault plane has folded and faulted sedimentary strata.

• These layers perch on a low density evaporite or over-pressured shale bed that easily flows away from higher pressure into lower pressure zones.
• They are helpful in fossil fuel explorations as they from structural traps for oil.

Growth fault dynamics

• Both the new-created space and thickness of the new-deposited sedimentary layers are greater above the sinking zones than above the growth ridges.
• These variations result in an increasing of differential load intensities-unequal distribution of sediments load-across the shelf with time as more the evaporite layer are added.
• Therefore, the rate of pressure increases on evaporite in the sinking zone than the growth ridge.
• So, the flow rate within the evaporite layer is progressively increasing as deferential load intensities.
• The growth ridge end up with salt diapir when the sinking zone sequences weld to the base of the evaporite layer.
• The downthrown block moves basin-ward and the displaced sedimentary layer of the downthrown block bends close to the fault plane forming rollover anticline, syncline and antithetic faults.

Driving force

The main driving forces of the growth faults are the deferential sediments load and the low density layers – evaporites or over-pressured shale that are formed during or right after the rifting process. These margins receive millions of tons of sediments every year which are concentrated on the continental shelf below sea level and above areas where the water velocity is no longer supporting the particles weight.

• The zone is called depositional center (depocenter for short) and has sediments load.

Earthquakes arise and result from the release of the force along the growth fault plane. The depocenter’s exact location continuously changes because eustatic and relative sea level are continuously changing as well. As a result, many different growth faults are created as sediment loads shift basinward and landward.

Importance in Petroleum Industry

Growth faults have significance in stratigraphy, structural geology and petroleum industry.

1. They account for relative and eustatic sea level changes and accommodation space left for new sediments.
2. Growth faults are connected directly to the subsidence in the coastal and continental shelf areas.
3. They explain lateral thickness variation of sedimentary sequences across these faults.
4. The updip area on the downthrown block is the main target of oil and gas exploration because it has synthetic and anthithetic faults and rollover anticlines. These are considered as structural traps.
5. The offset of sand and shale beds occurring along fault planes bring their contacts that blocks oil and gas lateral movements and enhances vertical movement.
6. At shallow level these are considered as pathways for groundwater and at deeper level for petroleum migration up to their final destinations.