PetroIeum:

Oil and gas is usually derived from marine sedimentary rocks. There is no dearth of marinesedimentary rocks of Palaeozoic, Mesozoic andTertiary age in Pakistan. Basin set up is ideal for petroleum to have been formed. Shelf facies are dominant. The shelf gives way to deeper troughs and has been divided into subsidiary basins by fundamental highs. Tectonism in the shelf areas has been modest. Indeed up to Tertiary times, all the earth movements were of non-orogenic type. Main orogenic movements had been post marine sedimentation leading to formation of favourable traps. The source-reservoir-cap rock combinations are present. The region is nearby to the oil producing areas of Persian Gulf. In spite of all these favourable factors, the scale of petroleum discoveries to-date has not been a substantial one. The reason, above all, may be that more drilling has to be done. The salient features of petroleum occurrence in the region are described hereunder (Razactal., 1977)
The Indus Basin, where most of the exploration activity has taken place is more prospective than other areas. To date many small oil fields have been discovered in the Upper Indus Basin (Kohat-Potwar Province) and numerous gas fields in the Lower Indus Basin (Sulaiman and Kirthar Provinces). In Balochistan Basin, lesser efforts on oil exploration have been made so far and no note worthy discovery has been reported. Most petroleum prospects of the Indus Basin are in theTertiary and Mesozoic rocks. They form traps in drillable depth and possess the combinations of source reservoir-cap rock.All petroleum fields have been discovered in structural traps. Exploration has not yet reached a stage, where search for stratigraphic traps caneffectively be made.

                      The oil occurrences which are confined to small fields in Pakistan are estimated to have a reserve of 777.518 million barrels and cumulative production up to 2003 is 488.845, leaving behind 288.673 million barrels as recoverable balance. Total production from Punjab oil fields, mostly Potwar Plateau is 8112620 barrels and from Sindh 15,345,224, barrels by the year 2003 (Pakistan

Energy Year Book-2003). Oil fields of Punjab are older than the Sindh field. The fields in the Potwar Plateau are very well studied as compared to other oil fields. The largest field- are Pindori and Adhi, where 2,071,398, and 116,311 barrels in 2002-03 were produced respectively. Total oil production
from all the 24 fields of Punjab has been more than 8 million tones. Potwar oil fields are all anticlinal or domel structures situated on two parallel E-W lines of folding on the northern and southern limbs of Soan syncline, which is bounded in the south by the Salt Range and in the north by Kala Chitta Range. The northern group of oil fields includes Khaur Dhulian, Tut and Meyal, and the southern group comprises Joyamair, Balkassar and Karsal. The quality of the oil is highly variable with densities ranging between 16°API to 50°API. Along with the oil, a substantial amount of gas is also produced. The production is obtained from those reservoirs which are emplaced in Datta Formation (Early Jurassic), Lockhart Limestone (Middle Paleocene), Sakesar Limestone (middle Early Eocene) Chorgali Formation (late Early Eocene) and Murree Formation (Early Miocene). The Jurassic and Eocene reservoirs are the most important producers. Jurassic and Miocene production is mainly due to granular porosity whereas the Paleocene and Eocene production from the limestone is due to fracture porosity. Sealing cap has invariably been provided by the shales. The source of oil in Datta Formation is considered as Jurassic shales; the Lockhart, Sakesar and Chorgali formation's oil is thought to be indigenous whereas that in the Murree Formation is assumed to have migrated vertically from underlying Eocene rocks (Rahman, 1963). The possibility, however, exists of other
sources too,Mianwali Formation (Early Triassic), Patala Formation (Late Paleocene) and Nammal Formation of Early Eocene(Razaet al.,1977).

                     In Sind, there are more oil fields than the Punjab and all of them are small, except Kunnar,

Pasakhi,Thora, Buzdar, Dabi,Ghungro, Mazari, Sonro, Tangriand Zaur. Total oilfields areas many as 72 and their total production of oil during 2002-2003 are stated to be more than 15 million barrels. Most of the oil comes from Cretaceous rocks(Pakistan Energy Year Book-2003).

                    Most of the gas fields of Pakistan are situated in the Lower Indus Basin especially Sindh.

The majority occur in the central part, in an area surrounding the Jacobabad High, here after, informally referred to as the Sui Gas Province. There are eight gas fields in the province, namely Zin, Sui, Uch, Jacobabad Khandkot, Mari, Mazarani and Khairpur. Besides that, there are three small fields in the southern part near Karachi at Sari Sang, Hundi and Kothar. The recoverable reserves of all the fields 'arc estimated to be more than 42.7 TCF. Sui and Mari fields are on production since long. Sui gas is used as a fuel throughout the country whereas Mari gas is utilized in fertilizer manufacturing, Sari Sang and Hundi fields have also been commissioned for production. Sui is the largest field having original reserve over 10.78 TCF, followed by Mari (6.8 TCF), Qadiipur (5.1 TCF), Uch (3.1 TCF), Zamzama (2 TCF) and Khandkot (1.3 TCF) Pirkoh (2.1 TCF). Total reserve including all the small fields of Pakistan are more than 42.7 TCF, with large cumulative production, thebalance recoverable reserveason30th June, 2003 isstated to be more than 15.9TCF.

                   The gas in the Sui Gas Province is a mixture of hydrocarbon gases, with carbon dioxide, nitrogen, traces of helium and sulphur. The proportion of different components in the gas is highly

variable. Noteworthy are the high proportions of carbon dioxide at Khairpur (70%), of nitrogen at Jacobabad (38%), Uch (25%) and at Mari (20%), and of helium at Uch (0.5%). Percentage of methane varies from 90% at Sui, 79% at Khandkot, and 73% at Mari, to 47% at Zin, 27% at Uch, and 25% at Jacobabad. The variation in composition demands special attention as to a certain the reasons of difference and to find out if at all any oil could have been formed with these gases. Presence of oil seepages in nearby areas, traces of oil and bitumen in the limestone cores and small scale production of condensate along with the gases (about 2.3 litres per thousand cubic metres) are factors, which merit consideration(Raza etal.,1977).

                  Reservoir in the Sui Gas Province in all cases (except Mari) is the Sui Main Limestone of Early Eocene age; at Mari field the reservoir is Habib Rahi Limestone member of Middle Eocene age. All traps have been provided by anticlines or domes. The gas column rests directly on water. The Sui Main Limestone is composed largely of foraminifers and is considered to have originated in a reef shoal. Sealing cap in all the reservoirs has been provided by shales. The variation in the composition of gases contained in otherwise closely situated traps have led to confusion about the possible sources. Tainsh et al.(1959) have suggested the Lower Eocene and Cretaceous shales as the possible sources of much of the methane and some carbon dioxide, whereas Paleocene sandstone and Upper Cretaceous and Jurassic limestones are considered to have produced carbon dioxide and some methane.They also consider that much nitrogen and carbon dioxide may have migrated from contact  with igneous rocks in or below Jurassic.