Balochistan Basin

The hydrocarbon prospects of Balochistan basin of Pakistan situated in the west of the country between longitude61° to 66° 30' E and latitude 23° to 30° N are assessed on the basis of a new concept related to the structural and tectonic behaviour of the basin. So far this basin is known as pafore-arc subduction basin. According to the interpretation of the geological, geophysical and other Irelevant data by Raza et al. (1991), the Balochistan is an extra continental subduction basin, which has been subjected to plate convergence to such an extent that the arc has been destroyed by subduction and transform movements, reducing it to a non-arc basin. Thus, oil and gas pools associated with normal faults, positive structures, structural anticlines and stratigraphic traps may form the targets for hydrocarbon exploration in the basin.Projection of the surface geological data in the subsurface in combination with seismic,
aeromagnetic and gravity information fits well in our model of non-arc subduction basin containing wrench zone modification of the originally fore-arc basin. Non-arc subduction basins have the reputation of containing giant field and their yield per sediment volume is the highest among all the ’categoriesofsedimentary basins. Kharaq depression with pools associated with normal fault blocks andPanjgur wrench zone with pools brought up due to the flower pattern of faulting are the two most  prospective areas of the basin where exploration is recommended first. According to the model
presented above by Raza et al. (1991), the maturation, migration and trapping factors arc highly
favourable and in conclusion, they expect that systematic exploration may result in the discovery of
substantialamount of hydrocarbons.
Subduction Zones (Tectonic Feature - F): The feature is situated between the northern
magmatic arc region and the Arabian Sea. Its structural trends start from the Chaman-Bela-Omach
transform fault zone in the east and extend westward beyond the border of Pakistan. The feature
exhibits progressively southward shifting tectonism and sedimentation. Three parallel petroleum
zones basedon theevolutionofaccretionary complex are recognizedfrom north to south.
Mashkhel Depression (Petroleum Zone FJ: Renamed as Kharan fore-arc depression by
Raza ct al. (1991), the Mashkhel depression represents a flat area of Hamuni-Mashkhcl and Kharan
desert, covered by Quaternary sands. Aeromagnetic surveys have confirmed the presence of
depressions containing huge thickness of sedimentary rocks of possible Paleogene age. Seismic
survey is required todelineatesubsurfacestructures.
Panjgur Accretionary Prism (Petroleum Zone F2): It is a prism formed by thrusting up of
accretionary sediments in front of the fore-arc region. The name Panjgur wrench zone has been
proposed for this region by Razaet al. (1991).According to their interpellation the zone is developed
as a result of wrench faulting caused by the pressure of the subduction from the south and deforming
northern region, in combination with transform movement both in the east and west of the basin
squeezing up the sediments in the form of positive flower structures.Eocene Limestone and Miocene
sandstone may form potential targets.
Coastal Depression (Petroleum Zone F3): The zone represents the coastal Makran region,
which includes part of Makran subduction complex of Raza et al. (1991). The tectonic style in the
zone is characterized by huge synclines flanked by small anticlines. Reverse faults are less common
in this zone as compared to the accretionary prism. A large number of mud volcanoes emit gaseous
discharge along the coastline expressing the ongoing subduction. Additional stratigraphic traps
towards the southern flanks of the synclines are produced due to facies changes in Pliocene shelf
sandstone. Kapper dome striking north-south and dipping into the sea represents another departure
from the regional trend and is speculated to be effected by some mud diaperism. Miocene-Pliocene
shelfsandstonesare the target plays in addition to Miocene turbidites.
Offshore (Tectonic Feature - G):A vast offshore area of Pakistan measuring approximately

• 240000 km square is situated in the south of the country between longitude 61° 45' and 68° 10'.

Tectonically, it is divisible into (1) Offshore Indus with a continental crust and (2) offshore Makran
having an oceanic crust.The Murray ridge and Owen fracture zone form a transition plate boundary
between thetwo.
Indus Platform (Petroleum Zone G,): It is a platform area, which is the prolongation of the
onshore Sindh Monocline. The zone is cut in the southeastern comer by a submarine canyon of the
Indus River. The region is dominated by intricate drainage pattern of many mouths of the Indus
River. The stratigraphy in the areas is similar to the onshore Sindh Monocline. The structures are  oriented in northwest-southeast direction. Cretaceous, Eocene and Oligo-Miocene are objective
plays in the zone.
Indus Depression (Petroleum Zone GJ: The zone represents a deep depression, which
appears to be direct prolongation of the onshore Karachi Depression. The area is severely faulted by
sinuous and gravity growth faults.The southwestern margin of this depression is bounded by a gentle
uplift running parallel to the axis of the deep Eocene and Oligo-Miocene plays are the objectives of investigation.
Makran (Petroleum Zone G3):The zonecovers the region in between the coastal depression
zone and east-west oriented trench, where theArabian plate is being subducted beneath the Eurasian
plate. This zone has been renamed as makran subduction complex by Raza et al. (1991). Accreted sediment pile at the prism front is succeeded by slope mantling mud facies and southward prograding
shelf wedge. The intensity of tectonic deformation is relatively mild. The resulting simple fold closures in abyssal plain and slope sediments as well as stratigraphic traps due to facies changes may be targets for exploration.