1. General Introduction of Pakistan Offshore: Offshore Pakistan which stretches over 200,000 Sq.kms can be divided into two major units: the Indus Offshore Basin and the Makran Offshore Basin. These two units are separated by the Murray Ridge and the Owen Fracture Zone which form a transition Zone/plate boundary between the two.
The Indus Basin covers the second largest fan in the world after the Bengal fan. The Indus Fan is located in Pakistan and Indian Coast between 160 to 24*. North latitude and 60* to 73* East longitude. It is about 1500 km in length (from the North until it present delta front in the Indian Ocean) and 960 Kms in width (from the India continental shelf to the East to the Owen shear zone to the West). At the thicker part in the basin, the fan is estimated to be about 10 kms, thick. The Indus Fan is cut in the south-eastern corner by a submarine canyon of the River Indus that is 2,900 Kms. long. Presently 14 exploration blocks are being held by E & P companies in offshore area of Pakistan. The details of exploratory wells drilled in Offshore Area are also provided in Table-2 below. Past petroleum exploration efforts in Indus Delta are limited to drilling of only 12 exploration wells. The details of past drilling efforts are indicative of the fact that Indus Basin is highly under explored. Of these wells, only one ie Pakean-1 tested 3.7 MMSFCD in a DST pointing towards presence of movable hydrocarbons in Indus Fan. Moreover Indus basin is analogous to other producing basins of the world in terms of geological setting such as Mississippi Delta, (Gulf of Mexico, USA ), Niger Delta (Nigeria), Mahakam Delta (Indonesia) Mackenzie Delta (Canada) and Gippslan Basin (Australia) etc.
2. Hydrocarbon Potential of Pakistan Offshore
2.1 Reservoir Rocks: With sparse well penetrations, little is known about the reservoir potential. In the upper section (mid Miocene and above) giant channel levee systems are seen on the seismic, which represent the canyon-channel system of the ancient Indus Fan and having degradational to aggradational behavior and seismic facies. The reason why these channels are so large is probably because of the high discharge from the Indus River and because of accommodation space being created by movement along the Murray fracture. High amplitude chaotic reflectors which could be good target for excellent quality sands. High Amplitude Reflection Packages (HARP) can also be seen at the base of the channel levee systems. In the Amazon delta these HARP's have been penetrated and are known to be high net to gross sheet sands. On seismic data, amalgamated channels with good sand development are quite evident. Low relief mounding is observed and the presence of amplitude anomalies points to the presence of channel and sheet sands. Mud diaper cored anticline particularly in the proximity of Murray Ridge are also seen on seismic data with multiple amplitude anomalies pointing towards good reservoir sand development in Indus Delta.
2.2 SEALS: Offshore shelf wells show that the Indus delta is mud rich and the large levee systems seen on seismic confirm this. Extensive quiet zones in Miocene sequence are interpreted to be thick blanket shales which are expected to provide an excellent seal.
2.3 SOURCE: Source rock may be present, but their lateral extent and organic facies is not known. Miocene deltaic sediments are considered to have potential for gas in the offshore. Geochemical modelings by a number of companies suggest that good source rocks could be developed in the Paleocene / Eocene, Gas-prone source rocks are identifiable in the Miocene in shelf wells; whether they are present in the deep offshore is uncertain.
2.4 MATURITY/TIMING: Major burial occurred post -18 million years consistent with Himalayan collision and deltaic loading Paleocene/Early Eocene source rocks is likely to be hydrocarbon generation window. Substantial thickness of Mid-Miocene source rocks is also expected to generate and expel hydrocarbon from 8-10 million years. The timing of trap formation is generally earlier than the maturity of the source rocks. The anticlinal traps show growth as well as channel traps and other stratiographic features were most likely formed immediately after deposition.