Hydrocarbon Prospectivity And Risk Analysis Of An Under-Explored Western Kirthar Fold Belt Of Pakistan

Arshad, Khan (eni Pakistan Limited) | Imran, Muhammad (eni Pakistan Limited) | Iqbal, Muhammad (Pakistan Petroleum Limited)



Western Kirthar Fold Belt (WKFB) can be described as an inverted passive margin, belonging to a highly complicated transpressional deformational system. It displays complex structural geometries with variable component of NS oriented, left-lateral strike slip and east verging compressional deformation system. This fold belt is located at the western part of Indo-Pakistan plate, bounded by Kirthar complex in the east and Ghazaband and Ornach Nal faults in the west. It was developed during Oligo-Miocene times as a result of the oblique collision between Indian Plate and Eurasian (Afghan) Block. Sedimentary rocks from Jurassic to Recent ages are exposed in the area. The Western Kirthar fold belt is dominated by the positive inversion of Jurassic rift normal faults, with basement involved in the deformation. Main evidences of the inversion in the area includes low structural shortening (Approx. 10-20%), presence of En-Echelon and doubly plunging anticlines, inherited rift structures such as relay ramps and High angle reverse faults. Some thin skinned deformation has also been reported in the area but at shallow level within Eocene age decollement surface.

Due to the complexity of different tectonic elements, the study area has been divided in to a number of structural zones. Satellite image interpretation of the study area reveals two main surface anticlinal structural trends, i.e. NNW- SSE in Khuzdar Block located in central part of the fold belt and it swings towards west in Khuzdar Knot area and NNE-SSW in Kalat fold belt in the northern part. This major change in structural trend between Khuzdar Block and Kalat fold belt is a result of the counter clockwise movement of the Khuzdar Block, later after the collision of Indian plate with Eurasian Afghan Block. This results in main NNE-SSW structural deformation in the area in response of NNW-SSE principle stress component.

In the Western Kirthar Fold Belt Potential hydrocarbon plays, ranging from Jurassic to Palaeocene age, have been identified. The main reservoirs include Jurassic Spingwar Sandstone and Chiltan Limestone, Cretaceous Pab and Mughalkot Sandstones and Palaeocene Dunghan Limestone. These reservoirs have a potential of several TCF of gas in place. Common risk segment mapping of Spingwar, Chiltan and Dunghan plays indicates eastern part of the study area as a low play risk area, while for Pab and Mughalkot plays the low risk area lies in the southeastern part. Geochemical data on main source rocks, including Sembar and Goru shales of Early Cretaceous age and Anjira shales of Early Jurassic age, indicates most of the area is within the gas generation window at present day conditions.