QUESTION: AOA, my question is geological in nature. Interrested to know what was /is the geological activety which gave the seismic expression seen on seismic as " chaotic zone or gas cloud or gas chimneys.I have observed this chimneys in area of JACOCABAD -KHAIRPUR HIGH- where wells like Jhatpat, Jacobabad,Hasssan were drilled. gas cloud can be seen extending vertically up from bottom till SML horizon. SML horizon shows hi amplitude zone and in intact and again little activity of gas cloud is seen above SML in GHAJIZ -KIRTHAR HORIZONS. WHY? WHAT WAS THE GEOLOGICAL ACTIVITY? WHY WE SEE SML AS ONE CONTINEOUS HORIZON ? THANK YOU Regards
ANSWER: Dear Farah,
Thanks for your question and my sincere apologies for delayed answer.
I am not able to comment about the seismic data without looking at it. However, there are several reasons when chaotic reflection pattern observed on seismic data.
This is may be due to gas chimney as you described yourself which means that structure witnessed some kind of deformation after the gas accumulation a breach of top seal caused these chimneys. Secondly, there is a possible overspressured zone which means that rocks pressure exceed the normal hydrostatic pressure.
Thirdly, these chaotic zones may represent mass transport deposits complex, commonly known as MTC.
That is depositional pattern of the sudden and very rapid gravity flow of the sediments due to failure of the shelf and subsequent sliding and later deposition at toe of the slope or in the deeper part of the basin. Deep sea turbidites may also be associated with such depositional processes.
You can yourself judge better that me that what causes the chaotic zone to appear on seismic because you know the local geological conditions and any one of the three reasons, as mentioned above, may likely to be the major cause and I can only infer that possibly, deep seated gas below SML accumulation, any any deeper reservoir rocks, likely to breach to the SML level and hindered to flow further upward due to thick presence of ductile Ghazij shales. You need to identify faults along that zone to delineate the contribution of the faults in providing upward gas flow.
If you send me the seismic section I would probably can comment better.
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QUESTION: Thank you , yur answer is much appreciated. I further would like to know if research/paper has been written on such zones in Pakistan if yes; plz give the references.I really would like to send the seismic line but I don't want to breach the confidentiality ! Should I send u with out the teling the location of it ?
I am not sure about how many papers have been written about Pakistan on this subject. I think you need to search on the following site
I am sure you will find many good papers even if not related to your area.
If you have any confidentiality issue with data please don not send me unless you are sure that release of data to any third party is safe. Its pure your own judgment and decision.
Please do not forget to RATE my earlier and present answer.
I thank you for your interest and sending me your questions.