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Overview of Offshore Exploration in Pakistan

Overview of Offshore Exploration in Pakistan

Offshore exploration in Pakistan began in 1963, with Sun drilling three wells in the Indus Delta’s shallow waters: Dhabo Creek-01, Patiani Creek-01, and Korangi Creek-01. Subsequent attempts included Wintershall’s Indus Marine A-1 and B-1 wells in 1972, and Indus Marine C-1 in 1975. In 1976, Marathon drilled the Jal Pari 1A well, which encountered high pressure and forced a halt in drilling. Later, Husky Energy’s 1978 Karachi South-1 well revealed shaly source rocks in the Mughalkot Formation beneath the Deccan volcanic series, though the reservoir quality was poor. In 1985, OGDCL’s Pakcan-1 discovered gas in the Miocene; however, further efforts, such as Oxy’s 1989 Sadaf-1, yielded no significant results. The 1990s and 2000s saw additional wells by Canterbury, OPC, PPL, and Total Energies, including Pasni-1 and Gwadar-1, yet these too were largely unproductive. The most recent exploration, Kekra-1 by ENI in 2019, also yielded no viable resources. This outcome is likely due to a lack of hydrocarbon charge, attributed to the absence of Early Cretaceous source rocks in the area. No shallow source rocks could be established, despite some occurrences of thermogenic gas.

Overview of Offshore Exploration in Pakistan

Geological Influence of Plate Collision

The collision between the Indian Plate and Eurasian Plate created the Himalayas and initiated significant erosion, transporting sediments southward to form the Indus Fan in the Arabian Sea, one of the world’s largest sedimentary fans. The uplift accelerated erosion, feeding massive sediment into the Arabian Sea and building the Indus Fan. Following the continental collision between the Indian and Eurasian plates in the Oligocene, the influx of clastic sediments buried Paleogene carbonates in the offshore Indus Basin. This process intensified from the mid-Miocene onward, with uplifting and tilting of turbidites east of Murray Ridge. The Upper Oligocene to Recent Indus Fan clastics now form a thick succession of channel-levee systems.

Hydrocarbon Potential and Challenges

Data from offshore wells has yet to show evidence of significant Tertiary-age source rocks, and deeper sections remain largely unassessed. The thick Eocene to Pliocene deposits in the Indus Fan have shifted initial oil-rich zones into the gas window, creating migration barriers due to impermeable basal shale layers. Future drilling could reveal if hydrocarbons bypass these barriers through fault systems intersecting source rock. Findings from deepwater wells, such as Pak G2-1, indicate immature rocks for hydrocarbon generation. If the drilled sections are extrapolated to Paleocene Ranikot Formation, Ro levels around 0.4-0.5% suggesting early maturation for oil generation but lack of data and unconformity between Upper Miocene and Upper Eocene make the accurate predictions of thermal maturity very hard as all readings of present maturity are above unconformity. That’s why the Paleocene hydrocarbon potential might be overstated given limited data, as regional modeling suggests Paleocene source rocks may have become post-mature by the Oligocene, potentially charging Miocene and younger reservoirs via shallower source rocks would have occurred.

In contrast, wells like Karachi South A-1 and Pakcan-1 (Paleocene-Eocene to Mid-Miocene intervals) remain within the hydrocarbon window. Although Pakcan-1 confirms thermogenic gas, data limitations impede a complete source rock assessment across the basin. Ghazij Formation modeling based on Karachi South A-1 suggests it reaches the peak oil window (Figure-01) and about to approach the gas window.

Exploration Outlook and Economic Incentives

In the Lower Indus Basin, the Lower Cretaceous Sembar and Goru formations serve as primary source intervals; however, their offshore extension places them beneath Deccan volcanic rocks, rendering them less viable for the Indus Fan play. Offshore, rapid sedimentation has lowered geothermal gradients, reducing the likelihood of source rock maturation to depths comparable to onshore zones.

Seismic interpretation reveals laminated Deccan basalts within Upper Cretaceous–Paleocene marine-facies strata in the southeastern basin, adjacent to the Somanath Ridge and Saurashtra High (Khurram et al., 2019). Conversely, the northwestern basin, with minimal basalt influence and proximity to the Murray Ridge strike-slip fault zone, may offer potential for oil and gas exploration. With developed fault structures near Murray Ridge, the northwestern region is promising, presenting opportunities to uncover established Cretaceous plays.

To reduce import dependency, the government has introduced favorable terms, including a gas price of US$7-9/MMBtu and contractor profit shares up to 95%, aiming to attract foreign investment. While exploration results have been limited, recent multi-year surveys suggest offshore Pakistan still holds potential.

Offshore Exploration in Pakistan

Reference

Calves G, Schwab AM, Huuse M, Peter DC, Asif I.  2010.  Thermal regime of the northwest Indian rifted margin Comparison with Predictions. Marine and Petroleum Geology, 27, 1133–1147.  doi: 10.1016/j.marpetgeo.2010.02.010.

Chatterjee S, Goswami A, Scotese. 2013. The longest voyage: Tectonic, magmatic, and paleoclimatic evolution of the Indian plate during its northward flight from Gondwana to Asia. Gondwana Research, 23, 238–267. doi: 10.1016/j.gr.2012.07.001.

Jian-ming Gonga, b, c, Jing Liaoa, b, *, Jie Lianga, b, Bao-hua Leia, b, Jian-wen Chena, b, Muhammad Khalid, Syed Waseem Haidere, Ming Meng. Exploration prospects of oil and gas in the Northwestern part of the Offshore Indus Basin, Pakistan

Shahzad K, Betzler C, Ahmed N, Qayyum F, Spezzaferri S, Qadir A. 2018. Growth and demise of a Paleogene isolated carbonate platform of the Offshore Indus Basin, Pakistan: Effects of regional and local controlling factors. International Journal of Earth Sciences, 107, 481–504. doi: 10.1007/s00531-017-1504-7.

Key Concepts of Petroleum Migration by Zhiyong He, ZetaWare, Inc.

Key Concepts of Petroleum Migration by Zhiyong He, ZetaWare, Inc.

Capillary Seals and Migration

Capillary seals are the primary mechanism controlling the movement of hydrocarbons within a sedimentary basin. These seals form barriers within the porous rock layers, preventing the vertical migration of oil and gas. Instead of moving upward toward the surface, hydrocarbons are often forced to migrate laterally along the stratigraphic layers of the basin.

Capillary seals arise due to differences in the rock properties, particularly pore throat sizes. Rocks with smaller pores (like mudstones or shales) generate higher capillary pressures that trap hydrocarbons beneath them. Vertical migration can only occur if the pressure from the buoyancy of oil and gas is strong enough to overcome this capillary force. As a result, lateral migration becomes the more common pathway for hydrocarbons, with the accumulation of oil and gas occurring in traps when they encounter geological structures like folds, faults, and salt domes.

Migration Rate

Hydrocarbon migration is a slow and gradual process, often taking years or even decades for hydrocarbons to move across pore spaces in a reservoir rock. The migration rate is influenced by a combination of capillary pressure, buoyancy, and the properties of the fluids involved, such as the mixture of oil and gas.

Capillary pressure refers to the resistance that hydrocarbons face when moving through small pore spaces in the rock. This resistance increases with smaller pore sizes, requiring higher buoyancy forces to drive the fluids. The rate of migration is also determined by the generation rate of hydrocarbons from the source rock. If the production of oil and gas is slow, migration will occur at a similarly slow pace, with hydrocarbons moving as they are generated.

Long-Distance Migration

In some basins, hydrocarbons can migrate over long distances, often traveling hundreds of kilometers before accumulating in traps. This long-distance migration is typical in low-relief basins, such as forland or continental basins, where geological structures do not impede lateral movement.

The ability of hydrocarbons to migrate over such distances is largely driven by the volume of hydrocarbons generated by the source rock. Rich source rocks that produce significant quantities of oil and gas push these fluids across the basin, allowing them to travel far before encountering a trap. The absence of substantial structural barriers also facilitates extended lateral migration, with hydrocarbons only accumulating when they encounter faults, pinch-outs, or other obstacles.

Capillary Pressure and Traps

Vertical vs. Lateral Migration

Vertical migration is typically more difficult due to the strong capillary pressures associated with impermeable or low-permeability rock layers. These barriers prevent hydrocarbons from moving upward unless the buoyancy forces can overcome the capillary resistance, which often requires a significant column of oil or gas. As a result, vertical migration is less common, and hydrocarbons are more likely to migrate laterally.

Lateral migration is easier because it encounters fewer barriers. Within a stratigraphic layer, such as a sandstone or carbonate, capillary pressures are often lower, making it easier for hydrocarbons to move sideways. This lateral migration continues until hydrocarbons encounter a trap that forces them to accumulate, such as a fault, a pinch-out, or a structural dome.

Capillary pressure and rock types: Different rock types exhibit varying levels of capillary pressure due to differences in pore throat sizes:

Shales or mudstones have small pore throats, creating high capillary pressures and acting as effective seals.

Sands or carbonates typically have larger pore throats, making them more permeable and allowing easier fluid movement.

This variation in capillary pressure influences the arrangement of oil and gas. Gas, being more buoyant, is often found above oil, but capillary pressure differences in the rock can cause hydrocarbons to accumulate in different ways, either vertically or laterally.

Capillary Pressure’s Role in Trap Formation

Capillary pressure is crucial in forming traps where hydrocarbons accumulate. When oil and gas migrate, they accumulate in areas where there is a significant difference in capillary pressure between rock layers. If the capillary pressure in a seal is high, hydrocarbons accumulate below it until the buoyancy pressure exceeds the capillary resistance, leading to the formation of reservoirs.

Traps such as faults, folds, and salt domes are critical for halting lateral migration and forcing hydrocarbons to accumulate. High-capacity traps with strong seals can hold large volumes of hydrocarbons, while weaker traps may only retain smaller quantities. This explains why some fields are larger and more productive than others.

What is happening in the area you’re working, please share your views!!!

Reference

Concepts and explanations regarding capillary seals, migration rates, and long-distance migration have been drawn from the presentation by Dr. Zong on “Capillary Seals and Petroleum Migration”.

Diagrams are also taken from this AAPG paper, “Migration and Charge Risk for Stratigraphic Traps “