Golden Zone
Sunday, 28. October 2007, 16:08:33
No I am not referring to the Golden Triangle. This is about oil and gas. The Golden Zone is the name of a an underground zone where temperatures range between 60° and 120°C. The name refers to a new discovery that 90 per cent of the world's oil and gas reserves are to be found just there.
Earlier it was assumed that the formation of oil and gas was related to temperature. The new discovery is that temperature decides where most of the lighter oil and gas is trapped in the reservoirs.
Hydrocarbons (oil and gas) are formed in source rocks at temperatures higher than 120°C (in the ‘expulsion zone’). They are expulsed and migrate upwards through hydraulically formed fractures to accumulate in reservoir rocks higher up where the temperature is lower than 120°C - this is the accumulation zone or “golden zone”. The upper limit of major entrapment is at 60 °C where hydraulic fractures are generally thought to peter out, simply because the permeability is insufficiently low for hydrocarbons to propagate through the rock.
The accumulation zone or golden zone is a zone of transition between the thermo-chemical compaction regime of the expulsion zone and the overlying mechanical compaction regime of the so-called 'sealing zone’. Here, at temperatures below 60 °C, hydrocarbon volumes are low because the sealing zone is largely beyond the influence of vertical, fracture-controlled re-migration.
Oil in the sealing zone is vulnerable to bacterial degradation (biodegradation). Bacteria consume the lighter oil fractions, leaving behind a viscous mass of heavier oil fractions.
The fact that oil and gas coexist within the same temperature zone is a new discovery and a surprise. Gas is formed at higher temperatures than oil. Consequently it has been a standard rule that there should be more gas than oil the deeper one drilled into the reservoir. The reason why this is not the case is covered by the new theory which predicts that both oil and gas escape through fissures formed at 120°C.
What the researchers stress is that only one parameter, namely temperature, governs all the fundamental processes in the expulsion zone. The fluids (water, oil and gas) are expulsed by thermally controlled processes.
The theory was nicely described 3 years ago in the Norwegian journal GEO (a copy in Norwegian can be downloaded from http://geony.imaker.no/pertra/dag47/ ).
It is now described in English in a paper (Memoir No. 7) from Statoil. The paper can be downloaded by clicking here (
DISTRIBUTION OF HYDROCARBONS IN SEDIMENTARY BASINS
File Format: PDF/Adobe Acrobat - 16 pages, 872 KB -
http://www.statoil.no/statoilcom/technology/svg03268.nsf/Attachments/memoir+7/$FILE/Memoir_7.pdf)
Yesterday, 27 October 2007, Clastic Detritus had an interesting blog post on Global warming and Petroleum geology at http://clasticdetritus.com/2007/10/27/global-warming-and-petroleum-geology/
Earlier it was assumed that the formation of oil and gas was related to temperature. The new discovery is that temperature decides where most of the lighter oil and gas is trapped in the reservoirs.Hydrocarbons (oil and gas) are formed in source rocks at temperatures higher than 120°C (in the ‘expulsion zone’). They are expulsed and migrate upwards through hydraulically formed fractures to accumulate in reservoir rocks higher up where the temperature is lower than 120°C - this is the accumulation zone or “golden zone”. The upper limit of major entrapment is at 60 °C where hydraulic fractures are generally thought to peter out, simply because the permeability is insufficiently low for hydrocarbons to propagate through the rock.
The accumulation zone or golden zone is a zone of transition between the thermo-chemical compaction regime of the expulsion zone and the overlying mechanical compaction regime of the so-called 'sealing zone’. Here, at temperatures below 60 °C, hydrocarbon volumes are low because the sealing zone is largely beyond the influence of vertical, fracture-controlled re-migration.
Oil in the sealing zone is vulnerable to bacterial degradation (biodegradation). Bacteria consume the lighter oil fractions, leaving behind a viscous mass of heavier oil fractions.
The fact that oil and gas coexist within the same temperature zone is a new discovery and a surprise. Gas is formed at higher temperatures than oil. Consequently it has been a standard rule that there should be more gas than oil the deeper one drilled into the reservoir. The reason why this is not the case is covered by the new theory which predicts that both oil and gas escape through fissures formed at 120°C.
What the researchers stress is that only one parameter, namely temperature, governs all the fundamental processes in the expulsion zone. The fluids (water, oil and gas) are expulsed by thermally controlled processes.
The theory was nicely described 3 years ago in the Norwegian journal GEO (a copy in Norwegian can be downloaded from http://geony.imaker.no/pertra/dag47/ ).
It is now described in English in a paper (Memoir No. 7) from Statoil. The paper can be downloaded by clicking here (
DISTRIBUTION OF HYDROCARBONS IN SEDIMENTARY BASINS
File Format: PDF/Adobe Acrobat - 16 pages, 872 KB -
http://www.statoil.no/statoilcom/technology/svg03268.nsf/Attachments/memoir+7/$FILE/Memoir_7.pdf)
Yesterday, 27 October 2007, Clastic Detritus had an interesting blog post on Global warming and Petroleum geology at http://clasticdetritus.com/2007/10/27/global-warming-and-petroleum-geology/
WIDGETIZE