Introduction

There are three types of rock types that are studied when considering the geological environment. They are no particular order:

  1. Sedimentary
  2. Igneous
  3. Metamorphic

The circumstantial evidence has suggested that hydrocarbons that are formed and deposited are strongly associated with sedimentary rocks. They are the result of altered organic material derived from microscopic plant and animal life. The organic material is converted to hydrocarbons by the culmination of several key factors and conditions.

          The Conditions

For the organic material to become hydrocarbons the organic material must be exposed and subjected to specific conditions.

In general the organic material must be present in an anaerobic environment, if not the material will simply decompose and be destroyed. Only when the anaerobic bacteria begins to act on the organic material will the environment become a reducing one that is needed to begin the process that leads to the accumulation of hydrocarbons.

Given the anaerobic environment the next two conditions that need to be met they are pressure and temperature. Largely these two conditions will be determined by the depth of deposition. This will determine the overburden which will determine the pressure exerted on the organic material.

It important to note that the final size of the hydrocarbon reserve will be determined by the amount of organic material initially deposited.

             Source Rock

This usually made up of the deposits of fine silts, clays and organic material. These were once ocean margins that would have been good anaerobic environments for the alteration of organic material. Such deposits usually go onto form shale rocks that leads geologists to believe that shale rock is a source of hydrocarbons. The best environments would be black shales originally deposited in a non-oxidizing, quiet marine environment.

             Migration

The hydrocarbons in some altered form migrate from the source rock through other more porous and permeable beds to eventually accumulate in reservoir rock. Migration does not mean that the alteration has stopped. The movement is the result of hydrodynamic pressure and gravity forces. The burial pressure is what forces the water and oil out, the water carries out the hydrocarbons and eventually the hydrocarbons establish an equilibrium with the other fluids in the reservoir.

            Accumulation

This will only occur when there is a trap which stops the migration and promotes the accumulation of hydrocarbons. However this is only possible if the reservoir rock is permeable.

             Trap

Traps are caused by geological processes that create irregularities in the subsurface strata which causes the oil and gas to be retained in a porous formation. The rock that form a barrier or trap are referred to as cap rocks.

This is the geological feature that allows for the migrating hydrocarbons to not continue its upward journey and accumulate. This is what physically keeps or traps the hydrocarbons underground.

   Anti-clinal and Dome traps

The rock layers were originally horizontal then folded upward into and arch or dome. The hydrocarbons migrated upwards later on only to be trapped by an impermeable shale.

The common characteristics is that the oil-water contact surrounds the hydrocarbon accumulation. This structure usually extends through formation of considerable thickness.

   Salt Dome or Salt Plug trap

The intrusion of stratified rock layers by a ductile non-porous salt. The intrusion causes the lower formations to be uplifted and truncated along the sides of the intrusion. The layers above are uplifted. Hydrocarbons accumulate alongside the salt dome if porous, permeable beds and a capable cap-rock exist.

   Fault Trap

This is the result of vertical and horizontal stress. At some point the stress causes the rock to break. The rock faces eventually become offset to each other. When a non-porous rock seals off a porous rock then the hydrocarbons have time to accumulate.

They depend on the effectiveness of the seal at the face of the fault. Fault trap accumulations tend to be elongated and parallel to the fault trend.

   Stratigraphic Traps

Result of differences between or within stratified rock layers. The end result is a change in permeability from one area to another.

Lateral changes that prevent continued migration of hydrocarbons in a potential reservoir lithology. They are directly related to their environment of deposition. One type of trap is called the lenticular trap.

   Lenticular Traps

A porous area surrounded by non-porous area. Pinch out or lateral graded trap-differential deposition when environmental deposition changes up-dip.

  Angular Unconformity

Older strata dips at an angle to newer formations. In most cases the older petroleum bearing rocks are subjected to the forces of younger non-porous formation.

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