Sui Main Limestone Fm
Type Locality and Naming
Ghazij Gr – lower formation in central Pakistan (subsurface). Holotype section: Well Sui-36. Depth interval 1,319-1,787 m. Sui Main Limestone (SML) does not outcrop anywhere in Pakistan; therefore, a subsurface type section is proposed for the formation by Nusrat K. Siddiqui, and has been accepted by the Stratigraphic Committee of Pakistan for consideration. Out of a total Sui Main Limestone thickness of 468 m, 450 m was cored at the type locality. Author: H. R. Tainsh et al., 1959. Reference section: None.
Synonym: Sui Main Limestone
Lithology and Thickness
Limestone. At its type section Sui Main Limestone is composed entirely of limestone with minor dolomite, calcareous shales and marls. The limestone is creamy white to dirty white to off white in color. It is firm to moderately hard, occasionally soft, sub blocky to blocky, micro to cryptocrystalline and chalky in parts. The proposed formation represents a rich bioclastic limestone. It consists mostly of fine grained lime wackstone, packstone and grainstone. Sedimentary structures are almost entirely of burrows, bioturbation and nodular bedding, although vugs occur rarely. Pyrite crystals are found occasionally.
Thickness: 152-486 m. At the type locality it is 468 m, 451 m in Sui Well-1, 443 m at Sui Well-28, 388 m in Uch -1 486 m in Zin-1, 209 m in Kandhkot-1, 152 m in Khairpur-2 wells and
549-577 m in Mazarani gas-field. In the wells drilled on the western limb of Kirthar Foredeep, east of Mazarani, it is 318 m at Jhal-1 and 309 m at Phulji-1. However, Mazarani area represents a transition zone between Laki Fm and the Sui Main Limestone, where Sui Main Limestone's reservoir characteristics have deteriorated. This thickness is not taken into consideration while giving the range of Sui Main Limestone thickness below. North of Zin gas-field the formation is absent below the Pirkoh structure and a thick Shaheed Ghat Fm overlies Ranikot Fm sands of Paleocene age.
Laki Fm is the chronostratigraphic equivalent of Sui Main limestone. Laki Fm limestone outcrops in Kirthar and Pab ranges. It is encountered in wells drilled in southern Pakistan and on the Punjab Platform. However, Laki limestone is a non-reservoir facies of Sui Main limestone.
Relationships and Distribution
Lower contact
Conformably underlain by Dunghan Fm
Upper contact
Conformably overlain (transgressively) by upper portion of Shaheed Ghat Fm (with about 20 m of transition zone within Sui Main Limestone. Only a 20-40 m thick Shaheed Ghat Fm provides the cap rock for the gas reservoirs in the region).
Regional extent
Sui Main Limestone, a prolific gas reservoir, is well developed, but is encountered only in subsurface in central Pakistan, broadly between 29° and 27°N and 67° and 70°30' E. Laterally it pinches out against Laki Fm in the east, northeast and south. In the north it abruptly pinches out north of Zin structure (~29°N), where the entire Ghazij Group is represented by shales. In the west, the Western Boundary Thrust (WBT) in the foothills of Kirthar fold belt probably marks the limit of SML. West of the WBT it was probably not deposited or the great uplift has probably resulted in the erosion of entire Sui Main Limestone sequence, and only Dunghan Fm. outcrops. SML reservoir facies, within the prescribed limits, covers an area of 50,690 sq. km (Siddiqui, 2004).
Sui Main Limestone is overlain by the shales of Shaheed Ghat Fm, and is underlain by the limestone of Dunghan Fm. (Paleocene). Shaheed Ghat Formation acts as a cap rock for all the SML gasfields. As mentioned above, Sui Main Limestone is developed at the expense of Shaheed Ghat shales that resulted in a much smaller shale thickness of only 20 m in Sui and 40 m in Kandhkot gasfields to act as the cap rock. The upper contact of Sui Main Limestone is sharp but the top 20m of Sui Main Limestone represent a transition zone, which is argillaceous with poor reservoir characteristics. On seismic sections, the top of Sui Main Limestone is represented by a conspicuous high amplitude positive reflector and is a good marker horizon in the region. On seismic evidence, the Sui Main Limestone thickens away from the two gravity highs (Kandhkot-Mari-Jaislamer and Khairpur-Jacobabad highs). The lower contact with Dunghan limestone is also sharp and shows a distinct lithological change during drilling, and can also be easily marked on electric logs. In the region where it underlies Sui Main Limestone, the Dunghan limestone represents a non-reservoir facies (Siddiqui,
2004).
GeoJSON
Fossils
Microfossil groups occur in this unit, and algae are much more numerous throughout the proposed formation. The formation consists of a rich bioclastic limestone with diverse and abundant microfaunal assemblages and Assilina, Rotalia trochidaeformis, Lockhartia, Alveolina are abundant. Species of Assilina granulose, Assilina spinosa, Opertorbitolites spp, Nummulites, Miscellanea miscella, recorded towards the base of the Sui Main limestone, support a Lower Eocene (Ypresian) age.
Age
Depositional setting
Being a subsurface entity, the depositional environments are based on core analysis and the electric log response. The deposition of SML represents generally a shallowing and coarsening upward sequence, with several cycles recognized in different levels. There was a regional transgression by the end of SML deposition, and the massive lenticular carbonate buildup was onlapped by the thin Shaheed Ghat Fm shales.
It is believed to have been deposited along a broad open ramp to relatively deep water (~200 m) close to the shelf-break setting.
Additional Information
EMW: Prolific gas producer.