Geology/Geology of Sawan area
I want to know about the geology of Sawan area with its location that may help me to build a background about the area.
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GEOLOGICAL SETTING OF THE SAWAN AREA IN THE MIDDLE INDUS BASIN
The Sawan gas field is located in the north-south–trending Indus Basin's central part which is often known as Middle Indus or Central Indus Basin. The known stratigraphy in this part of the basin is from the Permian to Middle Jurassic, the present-day Indus Basin, Pakistan was located at the outer rim of the on the continental margin of the Indian plate and formed part of the southerly continent of Gondwana.
In dus Basin has subjected to several tectonic events that have significantly affected hydrocarbon habitat either developing traps or providing uplifting and erosion at one part and depositing on the other side of the basin that have positively provide thermal blanked to the source rock.
The most significant tectonic phase was the Late Jurassic to Early Cretaceous rifting from the African continent as part of the larger Gondwana land located in the southern latitude. The next phase was the northward drift of the Indian plate and remained actively in the drifting phase unless it collided with the southern margin of the Eurasian plate where world highest mountains are housed and known as Himalayas and Karakoram. This collision phase provided the compression stresses to the Indian plate which formed the large anticlines and folded and thrust belts during Tertiary. Three post-rifting tectonic events can be distinguished ;a late Cretaceous uplift and inversion, a late Paleocene right-lateral wrenching, and a late Tertiary to Holocene uplift of the Khairpur high. The measured geothermal gradient at the Khairpur high is also is higher than normal gradient of the Indus basin 4.8°C/100 0r 48°C/Km which may be attributed the deeply buried fault system .
In terms of sedimentation and stratigraphic rock record is concerned the Triassic is probably the oldest rock record found in well Jhat pat-1, though the authenticity of age is not certain. Shelf carbonates of Jurassic Chiltan Formation has been confirmed by drilling followed by Early Cretaceous age Sembar and Goru formation have been drilled in many wells and both rock units have been proven as the source and reservoir respectfully. Organic-rich shales within the Sembar Formation are the main hydrocarbon source rock for the lower and middle Indus basins. A time gap of 30 m.y., related to the late Aptian-to early Albian sea level lowstand, is present between the Sembar Formation and the lower Goru Member. The lower part of this member has been informally divided (from bottom to top) into theA, B, and C intervals . These are three westerly prograding, depositionally down-stepping clastic packages. They were sourced from the uplifted, thermally domed Indian shield to the east and deposited within a relatively short period of 7 m.y. The age of the section places it within the Albian to Cenomanian cycle of global sea level rise, and they document third-order fluctuations of the sea level. The sediments of the Upper Cretaceous are comprised of Parh Formation carbonates and clastics of Mughalkot and Pab Formations. The Paleocene is overlain unconformaly with Upper Cretaceous and composed of Ranikot Group including basalt and sandstone. Eocene is composed Sui Limestone/Laki which is overlain by thick shales of Ghazij Formation and the Upper Eocene rocks of Kirthar Formation which has two sub units Habib Rahi limestone (HRL)and Pirkoh limestone. HRL is the principal reservoir in giants Mari gas field not distantly located. The Oligocene-Pleistocene is completely missing in the Kadanwari, Sawan and Miano area. The top most layer covering the Kirthar limestone is the desert sand or recent alluvium which vary in depth from 30 to 100 m.
Medium to coarse-grained sandstones in a shallow-marine setting constitute the main reservoir in the field. The reservoir unit is characterized as deposits of a proximal wave-dominated delta system and barrier-bar complex with a variety of sub-environments. Tidal influence led to back piling of sands and the formation of sub-linear sand ridges. The present-day depositional strike of the sands is roughly north–south, reflecting the orientation of the paleoshoreline with the hinterland located to the east.
The Sawan gas field was discovered in 1998, and commercial production started in 2003. With
an expected ultimate recovery of more than 1 tcf, it is one of Pakistan’s largest gas fields. On production
tests, individual wells flowed more than 100 million standard ft3 (2.8 million standard m3) of gas per day. The gas consists of around 90% CH4, with a CO2 content of less than 10%. The high geothermal gradient in this part of the basin results in reservoir temperatures of more than 175°C.
The above text has been mainly mine with few exceptions but I have also used following references for support and authentication
Berger.A et al Porosity-preserving chlorite cements in shallow-marine volcaniclastic sandstones: Evidence from Cretaceous sandstones of the Sawan gas field, Pakistan AAPG Bulletin, v. 93, no. 5 (May 2009), pp. 595–615
Ahmad, N., and S. Chaudhry, 2002, Kadanwari gas field, Pakistan: A disappointment turns into an attractive development
opportunity: Petroleum Geoscience, v. 8, no. 4, p. 307–316.
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