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1 Pakistan Petroleum Limited, 3rd Floor P. I. D. C. House, Dr. Ziauddin Ahmed Road, Karachi 75530, Pakistan; n_kamal{at}ppl.com.pk
Nusrat Kamal Siddiqui joined Pakistan Petroleum Limited in February 1981 and is presently designated as senior manager of exploration. He obtained his B.Sc. (hons.) and M.Sc. degrees in 1969 from the Geology Department, Punjab University, Lahore, Pakistan, specializing in petroleum and structural geology. In 1979–1980, he completed a postgraduate diploma course related to photogeology and remote sensing, with emphasis on hydrogeology from International Training Center, Netherlands. Nusrat has over 33 years of diversified experience in engineering geology for working on earth fill dams (1970–1975), hydrogeology with the Ministry of Agriculture, Government of Libya (1975–1979), and hydrocarbon exploration (1980 to present). His interests involve log evaluation, field studies/geological modeling of reservoirs, field geology, remote-sensing applications, and prospect generation. He has published papers on petroleum exploration, field development, and remote-sensing applications to petroleum geology and flood control. He is an active member of AAPG, Pakistan Institute of Petroleum, and a founding member of the Pakistan Association of Petroleum Geoscientists (PAPG, an affiliate of AAPG). He was the chairman of the Technical Program Committee for the PAPG-Society of Petroleum Engineers Annual Technical Conference (ATC) and Oil Show 2002 and the chairman and organizer of ATC and Oil Show 2003, October 3–5, 2003, both held in Islamabad, Pakistan.
The lower Eocene (Ypresian) Sui Main Limestone (Sui Main) is the most prolific gas reservoir in Pakistan south of latitude 29°N. It does not outcrop anywhere in Pakistan. In the Kirthar Range, southern Pakistan, and Punjab platform, the Sui Main's chronostratigraphic equivalent is the Laki Formation, which is a nonreservoir facies. To date, only gas has been encountered in Sui Main, which has recoverable reserves of more than 20 tcf in the 14 discovered fields.
The paper describes the subsurface geological setting of Sui Main in a regional context. Facies distribution, based on the study of cores and electric logs, is discussed to establish the depositional environments. On the basis of surface geology and evidence from wells, the limits of Sui Main, as a reservoir, have been delineated. It is a regional lenticular development of porous limestone that is probably isolated on all sides by shales or poor reservoir facies and structural barriers. A subsurface type section of the Sui Main Limestone is designated.
To support this geological assumption, a comparative study of original reservoir pressures from different fields and formations has been conducted. It indicates that the Sui Main is an isolated reservoir and not in communication with the surface. On this basis, I conclude that the Sui Main is a closed-system reservoir, having a huge common aquifer system with all the known and unknown hydrocarbon fields perched at different hydrostatic levels. Being a closed-system reservoir, the gas pools are expected to experience weak aquifer support during the producing life of the fields. This phenomenon has so far been observed in fields that are in a mature stage of production like Sui field.
The area of the common aquifer, the reservoir-quality rock volume, and the cumulative voidage (pore space) that could be present in Sui Main and could contain hydrocarbons and water have also been computed and delineated.
This article has been cited by other articles:
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  P. M. Compton The geology of the Barmer Basin, Rajasthan, India, and the origins of its major oil reservoir, the Fatehgarh Formation Petroleum Geoscience, May 1, 2009; 15(2): 117 - 130. [Abstract] [Full Text] [PDF]
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