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Review of the Role of Structural Geology in Petroleum Exploration

Last Update April 29, 2003

 

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1. Petroleum system

2. Migration

Primary Migration

Secondary Migration

8. Reservoirs

9. Traps

Structural Traps

 

Additional Reading:

Chapter 15- Traps, in Geology for Petroleum Exploration, Drilling, and Production by Norman J. Hyne, 1984, pages 173-197.

This book chapter has a simple discussion of the main types of hydrocabon trap illustrated with diagrams. Download PDF file here.

1. Petroleum system

A Petroleum System links the source rock to a hydrocarbon deposit

These are necessary conditions for a petroleum accumulation:

1. Mature source rock.
2. Permeable reservoir rock.
3. Trap composed of structure, seal and reservoir.
4. Migration path from source to trap.
5. Right timing of all these elements.
6. Preservation

2. Migration

2.1 Primary Migration

Primary migration is the process by which hydrocarbons are expelled from the source rock into an adjacent permeable carrier bed.

Paradox: Most source rocks are black shales which have very low permeabilities. How can the hydrocarbons move through these rocks?

 

 

Oil Phase Migration- Most hydrocarbons probably are expelled from the source rock as liquids. The expulsion of the oil out of the source rock is a dynamic process driven by the oil generation itself. Good source rocks have TOC (total organic content) ranging from 3 to 10%. At high TOC the kerogen is bearing part of the lithostatic load. As the organic matter transforms into oil this load-bearing kerogen turns into liquid. The fluid pressure of the oil within the black shales can become high enough to produce microfractures in the rock. Once the microfractures form, the oil is squeezed out and the source rock collapses.

Microfractures of this type can be seen in most productive source rocks and they are often filled with remnants of oil.

 

 

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2.2 Secondary Migration

Secondary migration is the movement of hydrocarbons along a "carrier bed" from the source area to the trap. Migration mostly takes place as one or more separate hydrocarbons phases (gas or liquid depending on pressure and temperature conditions).

Main Driving force for migration:

• Buoyancy (This force acts vertically and is proportional to the density difference between water and the hydrocarbon so it is stronger for gas than heavier oil)

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3. Traps

Nomenclature of a trap: Closure Spill Point Hydrocarbon column Oil zone, water zone, gas cap Oil-water contact (OWC), gas-oil contact (GOC) Top seal, base seal, fault seal

Distribution of petroleum in a trap: density stratification, tar mats, gas caps.

Tilted Fluid contacts: evidence of water flow.

Seals:

Regional seal (determines migration pathway)

Local seal (seals the trap)

Best seals: gas hydrates, evaporites (salt), organic rich shales, clay rich shales, tight carbonates

 

Important characteristics of seal rocks:

• Low permeability
• Ductility (otherwise they are easily fractured during deformation)

   

   

• Classification of traps:

Structural Traps Fold related Fault related Diapirs Stratigraphic traps Related to unconformities Sedimentological Diagenetic   Combination traps

 

Relationship between plate tectonic setting and structural style:

Tectonic Setting	Stress State	Types of Structures	Examples
Divergent plates	extension	normal faults, roll over anticlines, tilted blocks	North Sea, Red Sea, Basin and Range
Convergent plates	compression	thrust faults, folds, faulted folds	Andes, Zagros Mts (Iran), Canadian Rockies
Transform plate boundaries	strike-slip	strike-slip faults, compressional and extensional flower structures	San Andreas fault, Alpine Fault (New Zealand).

This is important because it allows an explorationist to predict what types of traps to expect in a given sedimentary basin depending on its tectonic setting.

Anticlinal traps:

Example of the Maui Field (New Zealand)

Where is the spill point of this structure?

Is the trap full?

Are there any undrilled prospects in this map?

What role do the faults play in this trap?

	Fault Related traps: The key question is whether or not a fault will be a seal. It partly depends on whether the fault places a permeable or impermeable unit in contact with the reservoir. In some cases the fault itself can be sealing. Faults in extensional settings have a greater chance of being open to migration.
	Normal fault traps: Roll over anticlines related to curved faults Tilted fault blocks
	Thrust fault traps: Parts of a fold and thrust belt. Where are the traps? Faulted anticlines, tilted fault blocks, ramp anticlines, drag folds on the footwall
	Strike slip traps: Positive and negative flower structures related to bends of the fault.

 

What determines whether a fault will be sealing or not?

Know how to identify traps if given a cross section or contour map of an area.

 

Diapirs:

Salt is driven upward by buoyancy of the salt after compaction of the surrounding sediments

There can be many traps above, and surrounding a salt diapir.

In addition to diapirs there are slat-withdrawal structures (turtle backs)

Salt is frequently deposited during the early development of a rift system, so they are often associated with extensional tectonic settings.

Progressive development of salt diapirs	Seismic image of a salt structure. Notice its effect on the sedimentary layers around it.	Types of traps associated with salt diapirs.