Oil and gas that we are drilling today comes from a biogenic origin and it is formed with proper time and temperature. Organic matter is one of the most important parts of hydrocarbon generation. This topic will give you an overview of how organic matter will be transformed to hydrocarbon.
Starting with plants and algae, take carbon (CO2) from the atmosphere and process it to form glucose and this starting process is called photosynthesis. Glucose is transformed into more complex organic compounds. Trees, for example will grow bigger because they use photosynthesis to convert into energy. When animals and trees die, the organic matter is typically oxidized and this will create CO2 and put water back into environment again. However, in some situations when organic matter is buried quickly in areas where there is no oxygen, the organic matter may be preserved. If the organic matters are buried in proper conditions, petroleum may be formed.
When organic matters die and accumulate in sediment areas such as lakes, swamps or the ocean, these will be potential source rocks under some circumstances. If the organic matter is in a high oxygen area, it will be destroyed due to the oxidization process and there will be no chance of hydrocarbon generation. Therefore, it is critical to preserve organic matter from oxygen and there are some situations which will enhance organic matter preservation as listed below;
- High sedimentation rate
- Non-oxygen environment
- Fine grain size of sediment that will prevent oxygen to penetrate to organic matter
Figure 1– Conditions for organic matter sedimentation to become source rocks
These conditions above are found in fine grained limestones and shale. Fine sedimentary rocks with preserved organic matter are common source rocks for petroleum.
Three factors that affect the amount of petroleum generation are as follows;
- Quantity of organic matter
- Nature of organisms preserved in sediments
- Environment (pressure, temperature and time) to cook organic matter
For shale to be a good source rock, it must contain organic matter of at least 0.5% by weight. In some areas, organic content is greater than 50%, it is classified as gas / oil shale and this is referred to as unconventional petroleum resources.
USGS, (2013), ormation of organic-rich sediment layer [ONLINE]. Available at: http://energy.usgs.gov/portals/0/Rooms/geochemistry_research/images/first_stage.gif [Accessed 14 October 15].