Types of faults and associated hydrocarbon traps

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Faults

Faults are discontinuities of the Earth’s crust between two blocks. Depending on the relative movement between these two blocks, we can classify faults in three different main types: normal faults, reverse faults and strike-slip faults fault.

Figure 1: Hydrocarbon reservoirs associated to structural traps. In both cases, low porosity and low permeability rocks act as a seal over the reservoir rocks which have higher porosity and permeability. In both cases the fault behaves as a sealing fault allowing oil and gas storage. Source: geologyin
Figure 2: Examples of Niger Delta oil field structures and associated trap types. Hydrocarbon traps are represented in black and fault movement is represented with arrows both in the hanging wall and footwall. Source : (Tuttle et al.,1999)[1].

In petroleum geology , faults play an important role in hydrocarbon exploration forming a structural trap leading hydrocarbon accumulation in the subsurface. A structural trap forms as a consequence of changes in the structure of the subsurface related to tectonic, diapiric, gravitational and compactional processes. Structural traps are the most important type of trap as they represent the majority of the world´s discovered petroleum plays.

Types of faults and hydrocarbon traps

Oil traps associated to faults may only be formed when faulting can juxtapose impermeable (e.g. shales) rocks over more permeable ones (e.g. sandstones) and the fault has to behave as a sealing fault (Figure 1).

Normal faults

A normal fault is a dip-slip fault in which the hanging wall moves downward relative to the footwall. Most of the times, normal faults are characterized by presenting high-angle fault planes in response to an extensional tectonic regime. It is possible to differentiate two types of normal faults depending on the geological setting: lyric faults, in which the dip of the fault decreases with depth, with the development of a roll-over anticline on the hanging wall; and high-angle normal faults, with more straight fault planes along the entire length of the fault.

Hydrocarbon traps associated to normal faults are more common on the footwall of the fault related to the fault closure. Also in the footwall, hydrocarbon traps are associated to the presence of unconformities, and active drapes right next to the fault plane. On the hanging wall, the most common traps are downthrown closures against the fault plane, and the most important one, hydrocarbon traps associated to roll-over structures developed due to the movement of a lystric fault. If the fault is no longer active, there are two more common traps associated to these type of faults: traps along fault scarp, and passive drapes associated to the wide fold that generated over the fault termination.

One case study (Figure 2) that illustrates the presence of hydrocarbon traps associated to normal faulting is the Tertiary Niger Delta Petroleum System in Nigeria, Cameroon and Equatorial Guinea in Africa [1]. Hydrocarbon traps are related to growth normal faults commonly present in deltaic systems. Traps on the hanging wall are mostly associated to the presence of rollover structures (subtle anticlines) developed as a response to the fault movement. On the other hand, traps located on the footwall are mostly related to fault closures.

Reverse faults

A reverse fault is a dip-slip fault in which the hanging wall moves upward relative to the footwall. They commonly present low-angle fault planes and they develop in response to a compressional tectonic regime.

Figure 3: Oil and gas fields related to the Zagros suture zone. Oil fields are represented in black and gas fields in purple color. Major geologic structures are shown in black. Source: (Bordenave and Hegre, 2019)[2].

In compressional settings, hydrocarbon traps are mostly related to the folds associated to thrusts that form in response to the shortening. There are different types of folds based on the geometry of the fault: fault-propagation folds and fault- bend folds Structural fold. Traps are located both in the hanging wall and footwall, right next to the fault associated with the anticlines and synclines of the beds that produce to accommodate the deformation Fault related folds.

One case study (Figure 3) related to hydrocarbon traps in compressional settings is the distribution of oil and gas fields in the Zagros Fold Belt of Iran[2] . Most of the major oil fields in the are located in the Dezful Embayment (yellow area), a depressed area limited by various mountain fronts at the northwest, southeast and northeast side related to the major thrust fault. Oil traps are located at the top of the anticline structures formed as a response to the compressional setting present in the area.

Strike-slip faults

A strike slip fault is characterized by near-vertical fault planes in which one block moves slide past each other (either left or right) with very little vertical motion in response to a transpressional or transtensional tectonic regime.

Figure 4: Cross section along the Antelope Hills oil field related to the San Andreas Fault. Multiple folding events have occurred in the area associated to the transpressive tectonic regime. Oil traps are represented in black and the movement of the faults are represented with arrows. Source: (Harding, 1976)[3].

The most prolific hydrocarbon traps related to strike-slip faults are those associated with en echelon folds in major wrench faults [4] . The faults act as a lateral seal as in the previous cases, while the hydrocarbons are trap in the anticline structures right next to the faults.

One case study (Figure 4) related to the development of petroleum traps in a strike-slip regime is the San Andreas plate margin in California, USA [5][3]. Hydrocarbon traps are not directly related to the major strike-slip faults, but they are associated to the secondary compressive structures that are developed as a consequence to the movement of the fault. Different anticline structures (en echelon folds) act as traps being unconformities between them seals preventing oil migration.

Growth faults

Another special type of fault to consider in hydrocarbon exploration are growth faults. Those kinds of faults form typically in sediments with a high depositional rate (e.g. deltaic systems) when faulting occurs at the same time of the deposition. As a consequence, the thickness of the sediments at both sides of the fault are different, being higher on the downthrow block.

Final considerations

The geometry of the faults as well as the timing are important factors to take into account in order to determine whether faults will act as a sealing fault forming hydrocarbon traps or they leak facilitating hydrocarbon migration.

Inactive faults formed prior to the basin sedimentation do not affect the possibility of hydrocarbons to be trapped because the only affect the underlying sediments.

Growth faults, as they active during sedimentation, act as a major hydrocarbon trap.

Faults that form late in the sedimentation process, depending on the timing and the geometry, they may or may not be effective as traps.

Reactivated faults, inactive faults that experienced new movements are most likely to destroy an already formed hydrocarbon accumulation.

Finally, in many petroleum fields, structural traps are the responsible for hydrocarbon accumulation, but not only because the presence of a sealed fault, but also because of the development of secondary structures associated to the movement of those faults, such as folds (anticlines), salt domes and/or stratigraphic pinch out combined with tectonic tilting.

This page is currently being authored by a student at the University of Oklahoma. This page will be completed by December 1 2019.

References

  1. 1.0 1.1 Tuttle, M.L.W., Charpentier, R.R. and Brownfield, M.E. 1999. The Niger Delta Petroleum System: Niger Delta Provinde, Nigeria, Cameroon, and Equatorial Guinea, Africa. Open-File report 99-50-H. USGS.
  2. 2.0 2.1 Bordenave, L.M. and Hegre, J.A. (2019). Current distribution of oil and gas fields in the Zagros Fold Belt of Iran and contiguous offshore as the result of the petroleum systems. Geological Society London Special Publications 330(1):291-353.
  3. 3.0 3.1 Harding, T. P. (1976). Tectonic significance and hydrocarbon trapping consequences of sequential folding synchronous with San Andreas faulting, San Joaquin Valley, California. AAPG Bulletin, v. 60, No. 3, pp. 356-378.
  4. Harding, T.P. and Lowell, J.D. (1979). Structural styles, their plate-tectonics habitats, and hydrocarbon traps in petroleum provinces. The American Association of Petroleum Geologists Bulletin, V. 63, No 7, P. 1016-1058.
  5. Davis, T.L. and Namson, J.S. (2017). Field excursion: Petroleum traps and structures along the San Andreas convergent strike-slip plate boundary, California. AAPG Bulletin, V. 101, NO. 4, PP, 607-615.