Oil and gas (hydrocarbons) are valuable resources hidden in the subsurface of the Earth.
Geologists and geophysicists use a myriad of advanced techniques in order to find commercial accumulations of oil and gas.
Oils from the Norwegian North Sea
The investigation of organic rich rocks (hydrocarbon source rocks) and their geological history is important to understand the petroleum system in a sedimentary basin. The basic elements of a petroleum system consists of a source rock, a porous and permeable reservoir rock and a tight cap rock.
When organic rich rocks (usually shales containing 4-20 weight % total organic matter (TOC)) are buried, they are subjected to increasing temperatures and pressures (typically 30 degrees Celsius/km).
Hydrocarbon source rock, Carboniferous, Spitsbergen
At about 60 degrees Celsius, oil begins to form in the source rock due to the thermogenic breakdown (cracking) of organic matter (kerogen).
The oil window is a temperature dependant interval in the subsurface where oil is generated and expelled from the source rocks. The oil window is often found in the 60-120 degree Celsius interval (aprox. 2-4 km depth), while the corresponding gas window is found in the 100-200+ degree Celsius interval (3-6 km depth).
After expulsion from the source rock, the oil/gas (lighter than water) migrates upwards through permeable rocks (sandstones) or fractures until they are stopped by a tight, non-permeable layer of rock, like a shale. If hydrocarbons get trapped in a subsurface, geological structure, they may be produced from a hydrocarbon accumulation (reservoir) through an oil well. If not trapped, the hydrocarbons may eventually migrate up to the surface, where they can be seen as seeps.
Cross-plot of data from GC-FID and carbon isotope analysis of natural gases from wells in the Norwegian Barents Sea. Original plot from Whiticar (1994)
Source-rock, oil and gas samples from wells and outcrops are analysed in different ways to assess the compostion, quality and thermal maturity, i.e. what type and how much hydrocarbons the source rocks may generate, and how far in this process the source rocks have come. Hydrocarbons are correlated to their respective source rocks by comparing the contents of specific organic molecules (biomarkers) in the hydrocarbons and in extracts of the source rock.
Geophysical, geological and geochemical data are often put together in a digital basin model, which simulates the development of a sedimentary basin through time. Important input is the depth and age of geologic layers, source rock properties and subsurface temperatures. The basin model should be calibrated against measured data, from wells or outcrops.
The results from such analyses are evaluated in the context of the geological and thermal history of the sedimentary basin. By doing this, a basins petroleum system may be defined in time and space. This knowlegde is important when exploring for oil and gas.
Example of cross-plot used for describing the burial of sediments through time. The subsidence curves describe the burial history of the sediments. The figure indicates when they are within the oil/gas window. If organic rich sediments are uplifted, they also cool, and stop generating hydrocarbons until the are reburied.
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