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Eirik Grude Flekkøy in ORG Geophysical responds to a recent commentary by Bjørlykke and co-authors regarding hydrocarbon exploration using induced polarization.


530x386 Figur1The slow micro-seepage of hydrocarbons gives rise to mineralogical changes high above a reservoir. Text and illustration: ORG Geophysical

In a recent geoforskning.no article Knut Bjørlykke, Helge Hellevang, Per Aagaard and Jens Jahren discuss the link between pyrite formation and hydrocarbons.

Their article contains some misunderstandings and over-simplifications that require a clarifying response in order to keep the discussion on track.

First, ORG Geophysical is not selling pyrite detection as a hydrocarbon indicator, as the authors claim. The company is selling Induced polarization effects as a hydrocarbon indicator.

The difference is fundamental as pyrite is only one among several explanations that have been proposed for IP-anomalies. This is abundantly clear in the literature, including the geoforskning.no-article that Bjørlykke and company are themselves citing.

The basis for using the IP-technology is the observed fact there is an 80 - 90 % agreement between the locations of IP-anomalies and hydrocarbon reservoirs on the Norwegian continental shelf.

This success rate holds for reservoirs that were discovered after the IP-measurements were performed as well, i.e. for the 22 blind-tests that have been conducted since 2012.

From a practical point of view this empirical correlation justifies the use of the method as a de-risking tool.

However, it is important that there exists ways of explaining the observed correlation.

Among the more interesting explanations, that have nothing to do with pyrite formation, is the bacterial degradation of hydrocarbons above a reservoir.

It has been shown in lab-experiments that the bio-films formed by such processes give rise to significant changes in the IP-response, see for instance Gamal Z. Abdel Aal, 2006.

Other mechanisms that give rise to altered IP-responses include the so-called membrane polarization effects and the formation of minerals that do not contain sulfur, such as magnetite.

It is true that pyrite has a particularly strong IP-response, as has been established in well-controlled experiments. Bjørlykke et al. argues that the amount of pyrite probably is too small to function as a hydrocarbon indicator.

However, it is a misunderstanding that the amount of pyrite itself is the governing factor for IP-measurements.

To the extent that pyrite is relevant at all, most studies indicate that it is the amount of pyrite-fluid surface-area that controls the response — see Flekkøy 2013 for a recent study and review of this.

Sedimentary rocks are complex and composite systems, and it should be no surprise that their response to electrical excitations are similarly complex. Which effect dominates in different geological settings is a highly interesting question, both from a practical and academic point of view.

References

Gamal Z. Abdel Aal, Lee D. Slater, and Estella A. Atekwana Induced-polarization measurements on unconsolidated sediments from a site of active hydrocarbon biodegradation. GEOPHYSICS, 71,  2006.

Eirik G. Flekkøy A physical basis for the Cole-Cole description of electrical conductivity of mineralized porous media GEOPHYSICS 78, 2013.

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