Assignment 1: Introduction to Petroleum Geosciences

HIMALAYAN RANGE FORMATION AND PETROLEUM GENERATION

Name : Mohd Ashraf bin or AzrolID : 12009 PROGRAMME : PE 1st Year 2nd SemesterLECTURER : Assoc. Prof. Askury Abd Kadir

1. BACKGROUND

Himalaya is located in Nepal, a small country in between India, China, and Tibet. Lately, Nepal has designate 10 blocks of her southern lands for petroleum exploration as the area has known sedimentary basin and evidences of gas and oil seeps present. This paper will focus on explaining the phenomenon of oil generation at high altitude such as Himalaya, in terms of plate tectonic and continental drift theory.

2. HISTORICAL FORMATION

The formation of the huge Himalayan range is a result of a continental collision or orogeny along the convergent boundary between the Indo-Australian Plate and the Eurasian Plate. About 75 million years ago, the India land mass drifted towards the Eurasian plate.

Because of the very slow movement, about 65 million ago, the oceanic crust of the ‘Indian’ plate subducted under the Eurasian plate. This created a quite

environment for sediment accumulation. The basin developed here accumulated many forms of organic materials from algae, plankton, marine animals, and terrestrial sources. The accumulation lasted for another 10 million years before the continental plates collided. It may well be stipulated that enough organic matter was collected for mature source rock formation to develop.

Continental vs. continental collision

Since both are continental crusts which are having equivalent density, instead of one subducted the other, both crusts crumpled, piled up on each other forming folded mountain belt and very high mountain range. Because subduction ceases, intense pressure built up and formed extensive

mountain belt for hundreds of kilometres. That is why Himalaya has a very high mountain system in a very large area.

This also explains the absence of volcano eruption in Himalaya lately because the remelting process of the crust is too little (both crusts remain afloat above the magma) to cause a constant magma rise from below. The small portion of the reheated rock cooled within the crust before it can reach the surface. Shallow earthquake might occur because the plate movement but volcanism is very few.

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This diagram suggests the possibility of earthquake along the Himalayan range. The Indian plate is moving into the Eurasian plate at the velocity of 20mm/year. This movement increase the compressive stress along the Himalaya range annually thus increasing the tendency for future earthquakes with huge magnitude. According to the diagram, the average expected earthquake is 8.0 scalars Richter. However, the stress built up provides suitable pressurized condition for petroleum generation.

3. PETROLEUM GENERATION

The petroleum generation at the Himalayan range is possible because of the uplifting of organic-rich sedimentary rock and metamorphic rocks during the collision. All the Palaeozoic and Mesozoic sediments were trapped between the Indo-Australian Plate and the Eurasian

Plate before the collision. When the two plates collided, the sediments were uplifted from below ocean level to a high mountain top. This provides sufficient organic matter for petroleum generation at Himalaya since sedimentary rocks are rich with organic matter. On the other hand, due to the collision as well, huge pressure built up, and the temperature at the collision zone rose significantly. This condition ‘cooks’ the organic material to suitable condition for kerogen formation and later hydrocarbon generation.

However, since the collision is relatively new, the ‘cooking’ process might not be long enough to produce late mature kerogen. Nevertheless, according to a report released by Nepal’s Department of Mines and Geology, there is sufficient ‘cooking’ process from 3000m to 7000m deep.

This is logical since the pressure needed for petroleum rock (source rock) maturity is very intense. If it is in a deep sea environment, the ocean density adds up to the pressure build up in the subsurface layer of sedimentary rock formation. Here in Himalaya, there is no ocean water density to increase the pressure. The only source of such pressure comes from the movement of the plates explained as convergent plate boundary/margin. Therefore, the thermally matured rock can only be found deeper into the of the rock formation.

The collision also caused gigantic fold (syncline and anticline) formation along the range. This offer potential reservoir as folds can trap migrating hydrocarbons provided the capping rock is having very low permeability and porosity.

Fold fractures, joints, and unconformities

At Siwalk fold belt, there are extensive fold and fracture formation which might provide a suitable hydrocarbon accumulation area. Several unconformities also detected along the Himalayan range. This is due to erosion of the uplifted Palaeozoic and Mesozoic sediments by weathering processes.

During the collision of both crust, angular folding occurred, thus angular unconformity is expected to form here. Actually, for continent vs. continent convergent plate boundary, such as the Himalaya and the Alps, angular conformity is most common along the collision zone. In terms of petroleum system, unconformity is a suitable migrating and accumulation medium for hydrocarbon. This might further favour hydrocarbon presence in Himalaya.

Next, we look at the effect of the continental to continental collision in term of rock and minerals composition. As we can see, the middle mountain is rich in sedimentary deposits from Palaeozoic and Mesozoic sedimentation. Therefore, there are sufficient amount of sandstone and limestone presence to become reservoir for the hydrocarbon. Furthermore, the metamorphosed rock due to the collision such as gneiss and schist are suitable to become seal and trap rock because they are highly compacted by elevated pressure during metamorphism.

4. CONCLUSION

It is now known that petroleum generation is possible even at high altitude due to a phenomenon described by plate tectonic and continental drift theory called convergent plate boundary. This geological movement between two crusts (Indo-Australian Plate and the Eurasian Plate) has surprisingly enabled hydrocarbon-friendly condition high above the Himalayan plateau. The development of petroleum in Himalaya is divided into two. The oceanic to continental plate collision allow sedimentation and organic matter accumulation. Occasionally, the collision between both continental crusts created a folds, fractures and unconformities during the process. Therefore, it can be concluded that two subsequent collisions of convergent plate boundaries (oceanic-continental and continental-continental) can result in petroleum generation.