Petroleum geochemistry

Source: Weatherford

This is a branch of geology that is important in the exploration phase of oil. Nearly all explorations employ geochemical surveys. Geochemists study the surface geochemistry which is usually closely similar in the subsurface properties.

Geochemical surveys also include studying the source rocks to determine, where they form, the type of organic matter, thermal maturity as well as the geochemical processes involved in the transformation of the organic matter i.e biodegradation and thermal cracking.

The organic matter is typically deposited over a long period of time, during which it undergoes degradation. The conditions for organic matter degradation vary i.e they can be anoxic or oxic. Another element is the duration which the organic matter is exposed to the organic matter degrading environment. These conditions ultimately determine the organic content of the products of degradation. Furthermore reservoir conditions such as temperature can determine the type of oil alteration processes (biodegradation or thermal cracking) that would occur and in turn the quality of oil i.e heavy or light.

Organic matter is usually measured in terms of;

• Total organic content (TOC) which is often a percentage (%) of carbon often with a percentage (% ) of Sulphur.
• Solvent extractable organic matter – sometimes referred to as bitumen (measured in mg/g for mudrocks)
•    HC (aromatic and saturate)
•  non HC
• Kerogen (which is the insoluble residue after solvent extraction and demineralisation)

Usually a potential source rock contains > 0.5% TOC, while a very good source rock contains > 2% TOC.

Thermal maturity is measured by four methods. Optical, elemental, experimental and modelling.  Experimental methods include rock eval, while optical methods include vitrinite reflectance.

Through petroleum geochemistry, it is possible to determine the onset of oil generation and the onset of gas generation or whether the reservoir is immature.  Kinetic data is also acquired during the geochemical survey and is used to calculate how much petroleum is generated.  Furthermore, the composition of oil can be obtained via the SARA (Sulphur, Aromatics, Resins and Asphaltenes) analysis.  The composition of oil is key to determining its value. The following characteristics are key;

• API Gravity
•  Sulphur content

The Sulphur content comes from the thiophenic compounds in oil, although some of it may be due to reservoir alteration processes such as biodegradation(80°C) by sulphate reducing bacteria or thermochemical sulphate reduction (100-140°C).