Geology/Structural and stratigraphic traps from OIl FIelds in PAkistan
Aoa. Sir, can you please quote the different type of structural and stratigraphic traps from Upper, MIddle and Lower Indus Basin in Pakistan ?
Have we obtained any oil production from SALT DOME type traps in Pakistan ? I have one more question in mind but I want to clear my mind about this first.
Thanks for sending me your question.
Sincere apology for the delay.
Answering your question need a lot of time that allows you to appreciate the exact nature of structures and those controlling factors. Oil and gas bearing traps are mostly in the structural traps but there are few stratigraphic and combination traps (structural-stratigraphic) instances exist in Pakistan.
Pakistani basins has witnessed wide range of tectonic activities which has influenced to form structures related to divergent, collision and wrenching and the last is the combination of any two tectonic regime which have formed the structures adding more complexities in shape and geometry.
I can explain you briefly.
The oldest rift phase probably in the Punjab Platform area during Paleozoic time. these rift related structures have been greatly modified due to later tectonic phases which have inverted the basin couple of time in recorded geological history.
Sargodha High probably is the remnant of the paleo highs series which is exposed on the surface composed of precambrian crystalline rocks but other buried hills in Punjab Platform area have also been identified on gravity and seismic data. These buried hills have also been explored in the past but without any success. Karampur-1 (Shell attempted to drill it in 1958 and found heavy oil) and Fort Abbas-1 (1994 by OGDCL)are similar structures where reservoir objectives were Cambrian and Precambrian.
Mesozoic rifting in Godwanaland has separated Indian plate from African Plate and later the northward drift initiated to the time when the northern part of the Indian plate collided with the southern margin of Eurassian plate and closing the Tethys sea.
The process of the rifting phase believed to be started in late Triassic and continued till the Upper Cretaceous. During this rifting there were two prominent areas which have been affected; one which was the margin of the Indian Plate which have successfully separated from the Africa after breaking of the Gondwana. This margin is close to the present day western part of Indian plate in Pakistan probably very close to the Chaman fault in Baluchistan area (but not Chaman fault itself)
Although, this area is mountainous and is challenging to find out remnant of rift related structures as it has been widely obscured by later stage compressional tectonic. The interpretation of the seismic data acquired in the Kirthar Range and Sulaiman area or along the axial belt have revealed high angle reverse faults which generally believed to be rift related faults.
During rifting there were interior margin of the Indian Plate though have tried to break but could not possibly be successful to be completely separated which is loosely termed as failed Rift. These rift structures are normal faults with tilt and have been tremendous traps in Southern Sindh part of Lower Indus Basin covering Hyderabad, Sanghar, Badin, Mirpur Khas districts areas. The reservoirs are Lower Goru proximal to deltaic clastics deposited during the ongoing rift phase and usually termed as syn-rift deposits. Since these rifts are preserved in its original geometry and shapes are loosely termed as fossil rift.
Khairpur and surrounding area have been affected by rifting which have formed large horst and grabens. Khairpur-Sukkur High, Mari Dome and Pano Aqil graben are part of the rift related mega structure. But apart of large horst and graben there are several smaller structures following similar structural kinematics as usually in rifting, i.e, horst and graben style normal faults.
But this area has also witnessed the minor to severe jolts of the Tertairy compressional phase and modified the earlier formed rift structures which is termed as inversion of the basin. Although inversion has modified moderately to slightly as these area were bit far away from the main collisional zones and the resulting modification did not completely hide the original normal faults but moved laterally which may be termed as strike slip movement or wrenching or often termed as transpressional.
North Potwar Deformed Zone (NPDZ) where large oil and gas bearing structures have been found in Dhurnal, Dhulian, Meyal etc are large thrust fault bounded anticlines which means that original buried strata have been severely affected by the Himalayan orogeny phase which were active during Oligocene to recent time. These compressional structures are large four way dip closed anticlines and either one flank or in some instances both flanks have been bounded high angle reverse fault. Dhulian and Khaur are such large surface anticlines.
In Kohat Plateu, the western part of Potwar Basin has two additional features that have significantly deformed the strata in more complex manner than in the Potwar Plataeu area. Firstly, as this area has been affected by major strike slip fault (N-S Kala Bagh fault)in more recent geological history (Oligocene or recent). Secondly, the presence of plastic rock in Tertiary section including thick shale and salt and gypsum. Plastic rocks have been subjected to compressional and strike slip faulting deformed internally while the overlying strata was more resistant to flow or you can say that these softer rocks buckle in between the resistant rocks. The main traps of oil and gas in this region are mostly compressional anticlines with more complexities than any other area in Pakistan. On seismic data these traps are massive challenge to find.
In Pakistan so far no true Salt dome have been found. I believe Balkassr oilfiled in Potwar Plateau area is formed partly due to the Precambrian salt interaction but yet it can not be termed as salt dome.
Miano and Sawan fields are combination trap, means structural and stratigraphic component are both exist side by side.
Hope this answer helps.
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