Sui Main Limestone: Regional geology and the analysis of original pressures of a closed-system reservoir in central Pakistan

Abstract

The lower Eocene (Ypresian) Sui Main Limestone (Sui Main) is the most prolific gas reservoir in Pakistan south of latitude 29N. 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.