a case study that I am working on, I noticed that one of the wells change its regime from strike-slip to normal regime passing thought a major unconformity, I understand the lithologic discontinuities can have an impact on stress regime change, but another well pass through the same unconformity but keeps the same tectonic regime.
Kamr: there's not enough context to understand your question. Is this a deepwater sedimentary succession of stratified mudstones above the u/c? Is the u/c an erosional surface or has it been modified by structural movement? What is/are the rock types below the u/c? A seismic reflection profile passing through your walls would be very useful if you can post it!
Chris Woodworth-Lynas
Hello, Sir.
Thank you for considering answering my question.
The reservoir successions are located in the Saharan platform in Algeria, it starts from 3300m (my data) and contains Triassic shales (at the top) separated from Ordovician quartzites (below the u/c) by a major Hercynian unconformity, which is an erosional surface.
Shmin and Shmax are estimated from the poroelastic strain model and calibrated using Lot data. Pore pressure is directly obtained from welltest.
The shales exhibit high stress, which is pronounced (lithology-dependent).
As I said before, another Well path through the u/c but have no change in stress regime and keep a strike-slip regime.
This change in stress regime may be due to surrounding fault, so I asked the question to see if someone else noticed the same change.
Unfortunately, seismic data are not available.
regards.
Hi Kamr: if I understand correctly you are saying that the shear stress measurements from the well logs indicate dominant horizontally directed stress in the Triassic shales, and vertically directed stresses in the Ordovician quartzites below the u/c. Logs from only one well indicate horizontally directed stresses both above and below the u/c.
Do you have downhole caliper data to indicate orientation of borehole ovalization? This might help. Do you have access to actual physical core samples? If you do you could examine them and compare the shale and quartzite micro-structures (visually and microscopically) for indications of strain orientation.
It's too bad you have no seismic reflection profiles to compare large scale fault styles and offset above and below the u/c.
I hope this helps!
Hi, Sir.
Again thank you for your response.
The wells are vertically drilled, and yes, it is a relatively compressive area where you find a dominant strike-slip regime.
The direction of the maximum horizontal stress is determined from borehole breakout seen on image log, which gives an average direction of NW-SE.
For core samples, No they are not accessible, fractures are only seen on image logs, I compared both wells, but i didn't found any particle difference (fracture's azimuth and dip). It is better to note that breakouts (ovalizations) are not present in shales successions.
I agree seismic surveys would have been handy to analyze faults and see if they are the cause of the stress regime change, this case is prevalent in the earth, where we can find a mixte regime (Strike-slip/Normal), and this could be due to surrounding faults. But in my case its only present in one of the Wells passing through the u/c.
I will publish my master Thesis here (unfortunately it will be in french) but I hope to publish it as an article so everyone can see our results.
Regards.
It's always the way: working with an imperfect data set! Good luck with your thesis research and writing!
Indeed, Sir. Data restrictions are always a problem.
Many thanks.
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