1.1. The Origin of oil and gas - Organic matter

Petroleum was formed from organic matter. The organic mater was deposited in a marine environment and remained buried under anoxic conditions for 100-400 millions years. Over the years, layers of silt, sand and other sediments settled over the buried organic matter. The increase of pressure and temperature slowly transformed the organic matter into hydrocarbons (kerogen, oil, gas). Also, over those millions of years various plate tectonics (continents drift over the underlying mantle) and other geological phenomena resulted in the rearrangement of oceans and continents; thus, we encounter oil in both onshore and offshore locations.

Origin of oil and gas. Petroleum formation

The deposition of organic matter took place as:

  • Marine organisms (zooplankton and algae) that settled to seabed at depths where oxygen concentration is very low to decompose the organisms.
  • Organic mater from nutrient-rich regions such as ancient river deltas. Those large amounts of organic material were covered by subsequent sediments faster that decomposition could take place.

Notes

  • Organic matter was buried before decomposition takes place. The lack of oxygen (anoxic conditions) is an essential factor since it prevents the decomposition of the organic matter which is then transformed to oil.
  • Subsequent layers cause the increase of pressure and temperature and the transformation of the organic matter to hydrocarbons (maturation)

1.2. Geological conditions

1.2.1. Source Rock is the rock where petroleum formed. Source rocks are sedimentary rocks and typically shales (90%). The source rock was form along with deposition of the organic matter; thus, organic matter was abundant at the early age of a source rock.

As mentioned earlier, hydrocarbons are created from organic matter buried in an anoxic marine environment. However, a few more conditions are required for hydrocarbons to accumulate and form a petroleum reservoir.

1.2.2. Maturation is the conversion of organic matter to hydrocarbons. The first stage is the formation of kerogen. As the pressure and temperature is the source rock is further increase, kerogen converts to petroleum. If the temperature is raised above 130C for even a short period of time, crude oil will convert to gas. Initially the composition of the gas will show a high content of C4–C10 components (wet gas
and condensate), but with further increases in temperature the mixture will convert to light hydrocarbons (C1–C3, dry gas).

Thermal maturation table. Kerogen, oil, gas

An average geothermal gradient is about 3C per 100 m of depth.

Oil window: 60–120 °C, 2-4 km
Gas window: 120–180 °C, 4-6 km

Temperature, is the first most important factor for thermal maturation and determines the resultant hydrocarbon type. Time is the second most important factor for thermal maturation. Petroleum geologist use maturation indicators to evaluate potential hydrocarbon accumulations.

1.2.3. Migration takes place after maturation. The hydrocarbons from the impermeable source rock move to the porous reservoir rock.

1.2.4. Reservoir Rock: is a porous rock that contains petroleum. Reservoir Rocks are typically sandstones and carbonates.
Sandstone reservoir are of higher quality because the primary mineral, SiO2 (quartz) is strong and stable (hard to react).
Carbonate reservoir are formed from from coral, shell and other biogenic deposits.

Cap Rock: To locate and explore oil and gas prospects it is important to correctly assess the subsurface geology.

To sum up, for a hydrocarbon reservoir formation it is necessary:
1. A basin were sedimentation took place
2. Deposition of organic matter in anoxic conditions, formation of source rock.
3. Through increase of temperature and pressure source rock must have reached
maturation
4. Migration of the generated hydrocarbons into a porous type of sediment, the reservoir rock.
5. Creation of trap for the migrating hydrocarbons to accumulate.

oil shale: When kerogens are present in high concentrations in shale, and have not been
heated to a sufficient temperature to release their hydrocarbons, they may form deposits.

Further Reading:
Video: https://www.youtube.com/watch?v=_PDOD_FEnNk