Origins of Oil and Gas
Through millions of years, decayed plants and bacteria has been recycled into useful energy in the form of hydrocarbon compounds (hydrogen and carbon atoms linked together). Today, these rich compounds are being subtracted from different types of ground foundations and the entire industrial world is using it as its main source of energy.
Looking in detail about the origins of this valuable hydrocarbon compounds, it can be said that three conditions must occur for their existence:
1- Sedimentary basin must be created.
2- The sediments stored in those basins must contain a high level of organic material to create what is called source rock.
3- Elevated temperature and pressure must be present through the pass of the years, making possible the transformation of these organic materials into oil and gas.
Given the birth of this slow transformation of materials, a following process takes place called migration. This process is known as the movement of the hydrocarbons from the source rock through the cracks, faults and fissures into porous and permeable reservoir rock. Ending this process, this reservoir rock within their structure and by geologic activity will stop the hydrocarbons’ path. These traps will facilitate the oil and gas accumulation making possible their exploitation.
Now, before entering into the details of the types of reservoir rocks and traps, it is important to mention that another sources of oil and gas comes from kerogen which is the organic material accumulated on the bottom of oceans (subsea burial). These kerogen molecules eventually crack, breaking up into shorter and lighter molecules composed almost solely of carbon and hydrogen atoms. Depending on how liquid or gaseous this mixture is, it will turn into either petroleum or natural gas.
All above give a general idea of how oil and gas is created and how their pathways are; but there are some aspects that need to be explained like the types of reservoir rocks and types of traps. Generally, reservoir rocks with good porosity and permeability is classified as:
Clastic system: Clastic sediments are formed from fragments of various rocks that were transported and redeposit to create new formations. As an example of this clastic rock type there are sandstones, siltstones, and shales.
Carbonate system: This type of rock is formed by a chemical reaction between calcium and carbonate ions in shallow seas. Limestone and dolostone are considered the main examples of carbonate rocks.
Looking at the different type of trap formations, it can be said that several kinds of traps exist because the difference on location and the tectonic action presented during the development of the trap. If a smooth arch like is fold, this is considered as a structural anticline trap. Now from a displacement of rock layers that moved horizontally is classified as strike-slip fault. But if these rock layers moved vertical and down, then a normal fault is formed. If they moved vertical and upward then a trust type fault is formed.
Another type of trap different than the structural types is the straight trap which is the one created when a seal or barrier is formed above and around and oil or gas bearing formation by sedimentary deposition of impermeable rock. A stratigraphic trap accumulates oil due to changes of rock character rather than faulting or folding of the rock. Continuing with types of traps, a combination between the structural trap and the stratigraphic trap can be created. This new trap can is named as combination trap. One example can be a salt dome. Alongside hydrocarbon compounds, it is essential to review composition and classification of oil and gas. Oil in a liquid form is referred to as crude oil. The composition of crude oil and gas is mostly made out of hydrocarbons molecules. Like methane, the primary component of natural gas has a carbon atom bonded to four of hydrogen atoms (CH4). Crude oil contains about 83% to 87%