Think Twice reported on the attractiveness of passive 3D recording in seismic active areas by exploiting the “forgotten” seismic band 0.1 to 8 Hz as demonstrated by Enterprise survey in Northern Greece in 2001. Some years ago the author learned that a Swiss Company had designed a hydrocarbon detection tool based on tomographic signal recording at the surface in the same frequency band, using white tremor noise as a source. Apart from defining structural basin information through passive seismic there may now be the opportunity to recognise the hydrocarbon fill through passive seismic listening as well.
If true this would be in itself a potential breakthroughs for onshore Exploration, and the claimed accuracy would make this method a cheap alternative to time-lapse seismic to detect by-passed hydrocarbons and flood fronts to optimise field development.
The tool has been demonstrated extensively, mainly in the Middle East.
The company postulated two possibilities why hydrocarbon layers might be detected
- Natural tremors cause low frequency waves with a white noise amplitude spectrum which is filtered by the presence of hydrocarbon layers, yielding a distinct peaked spectrum for the surface recorded waves travelling via such hydrocarbon layer
- Waves are generated within the reservoirs yielding distinct peaked spectra in the hydrocarbon part and white noise spectra in the water bearing part
The method consists of a passive recording phase, a processing and interpretation phase.
- The recording of the very low frequent noise and signal (0.5 to 10 Hz) is done using some special designed high sensitive single axial velocity phones
- In already seismic covered areas the hydrocarbon prediction especially for oil can be limited and prospect ranking may be performed using such technique
- This method has been adopted by a different Swiss company and a Norwegian Oil Company has recently invested in this Company (reference Geo Expro January 2006)
- A reservoir monitoring tool replacing expensive 4D time lapse seismic and allowing much more regular recording of e.g. the waterfront injection schemes with a minimum of effort
- An Exploration ranking tool to perform hydrocarbon detection prior to drilling
There is a serious catch to this claim as the pumps used in the production fields have a peak of 4 Hz and together with the pipes may cause a signal response right in the middle of the frequency band claimed to be hydrocarbon related
There is another concern that strong shallow EM anomalies may occur.These are associated with small amounts of hydrocarbons in shallow rocks (leaked from the much deeper underlying HC reservoirs) dissolved in water .
Within these rocks aerobic bacteria can consume the hydrocarbons and use the available oxygen reducing the pore waters to anaerobic .If there is sufficient sulphate ion and organic material sulphate reducing bacteria flourish and sulphate reduces to sulphide and hydrocarbons to bicarbonates .(Sternberg 1994)These tend to reduce the resistivity contrasts with respect to the conductive minerals in the formations .
Even a negligible content of sulphur compounds in sediments (e.g. pyrites or chalcopyrites) causes significant enlargement of residual rock polarisation.
However the shallow anomalies are only an indication of the presence of hydrocarbon seepage but not a guarantee of commercial reserves in the deeper strata. The jury is still out of also some of the published new EM claimed hydrocarbon detection results are in fact contaminated with an effect of these shallow anomalies.
Proper designed field test with 3 component phones, as used in earthquake geophysics to locate minor tremors, will allow to distiguish the shallow from the deep events This provides more insight in the method and claimed hydrocarbon detection method.
We consider the recent explanatian of the ocean wave (1Hz) hitting the shore as a source very speculative indeed.
The phenomena may well exist but the solution resides in the non lineairity of the hydrocarbon filled medium.