Deformation Of Sedimentary Rocks

Stress, The geothermal gradient, Folding and faulting and how horizontally-directed plate motions cause vertical uplift are the main reasons for rocks deformation. When sedimentary rocks deform, deformation causes some structures to form, many of them are traps for oil accumulation known as structural traps.so we will discuss these structures then we will show which one can form a trap.



Weathering, Erosion, and Unconformities

angular unconformity

Break, transport then rest is the right description of these three geological events. Rocks are broken down and chemically dissolved through weathering .this occurs when rocks are subjected to the atmosphere and ocean bottom. Rivers, wind, waves, gravity (landslides), and glaciers transport and deposit these sediments in a process called erosion. If both weathering and erosion stops for a period of time then starts again forming new layers of sedimentary rocks, the buried erosional or non-depositional surface separating two rock masses or strata of different ages is called unconformity surface.
Unconformity comes in variety of ways. Disconformity occurs between sedimentary rocks whereas Nonconformity occurs between sedimentary and igneous or metamorphic rocks. Angular unconformity is an ancient erosional surface in which the sedimentary rock layers below the unconformity are tilted at an angle to the layers above the unconformity. Blended unconformity is a type of disconformity or nonconformity with no distinct separation plane or contact, sometimes consisting of soils, paleosols, or beds of pebbles derived from the underlying rock. A paraconformity is a type of unconformity in which strata are parallel; there is no apparent erosion and the unconformity surface resembles a simple bedding plane.



Anticlines and Synclines

Anticlines and Synclines

Anticlines and synclines are the up and down folds that usually occur together and are caused by compressional stress. IF they are exposed to weathering and erosion, for anticlines older rocks will appear, on the contrary, younger rocks will appear for synclines. Most anticlines and synclines are not level and are tilted with respect to the surface of the earth. These are called plunging anticlines and plunging synclines.to understand this; imagine it as a quarter of a ball with which you can trap a rat against the wall.



Domes and Basins
Domes and Basins

Dome is an anticlinal uplift that has no distinct trend. Basin is a synclinal depression that has no distinct trend.to understand this; imagine a dome as half of a ball with which you can trap a rat against the ground. Before a dome is eroded down, it forms a hill but if the dome is eroded, it leaves a characteristic bull’s eye pattern on the surface.



Homoclines

It represents sedimentary rocks dipping uniformly in one direction meaning that we have a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.



Fractures

The structures formed by the brittle fracturing of rocks are known variously as joints and faults.
Joints:
A joint is a fracture in the rocks with no movement of one side relative to the other.
joints
Faults:
A fault is a break in the rocks along which one side has moved relative to the other.
You can differentiate between them as follows:

faults

  1. Joints are smaller compared to faults.
  2. Joints have no movement therefore cause none or very little displacement while faults have lateral movement that causes displacement.
  3. Faults are formed because of constant tectonic movement while joints are formed when rocks are stretched to their breaking point.
  4. Faults can cause deadly earthquakes and tsunamis while joints rarely pose any threat to civilization.

So which structure can form a trap?

  • Angular unconformities can form gas and oil traps. One of the sedimentary rock layers tilted at an angle below it must be a reservoir rock that can store gas and oil. The sedimentary rock layer above it must be a cap rock that acts as a seal. The gas and oil form below the unconformity in a source rock. They migrate up into and then through the pore spaces of the reservoir rock until they reach the angular unconformity surface where they are trapped below the caprock. The two largest oil fields in the United States, the East Texas field and the Prudhoe Bay field in Alaska, are both in angular unconformity traps.
  • Anticlines, but not synclines, form high areas in reservoir rocks and can be gas and oil traps. Ghawar Anticline, Saudi Arabia, is the structural trap for the largest conventional oil field in the world.
  • Domes also form gas and oil traps. Oil was first discovered in the Middle East in Bahrain. The traps in Bahrain were domes with a low hill on the surface above each of them.
  • Although homoclines are common, they do not form gas and oil traps.
  • Joints in sedimentary rocks improve the reservoir quality of the rock. They slightly increase the fluid storage capacity of the rock (porosity) and greatly increase the ability of the fluid to flow through the rock (permeability).
  • The fault must be a sealing fault, which means it prevents fluid flow across or along the fault. Any gas and oil migrating up a reservoir rock will be trapped under the sealing fault. The largest oil field on land in England is the Wytch Farm field, located southwest of London on the South Dorset coast, wasformed by a fault.


References:

  • Wikipedia
  • Nontechnical guide to petroleum,exploration, drilling&production –third edition-Norman J. Hyne, Ph.D.
  • Earth structure-an introduction to structural geology and tectonics-second edition-