Context: Mumbai high turns 50. This makes one wonder, why no other discovery has earned such repute — despite discoveries being announced by various players.
What are the issues involved?
What ere the unique properties of Mumbai High which increased its prospects?
- Mumbai High is a large anticlinal structural — that is, an arch-like fold in the rock that has its oldest beds at its core.
- Structural traps of hydrocarbon deposits were considered easy oil as these are easily interpreted.
- Such structural features have been discovered in other basins also, but the size has not been comparable to Mumbai High.
- In the deeper parts of the basins, stratigraphic traps are more likely to be present with significant hydrocarbon (oil + gas) deposits. However, these are difficult to identify on classical seismic interpretation as well as difficulty in drilling deeper wells with high temperature-high pressure conditions.
Understanding the petroleum traps:
- Petroleum trap is an underground rock formation that blocks the movement of petroleum and causes it to accumulate in a reservoir that can be exploited.
- The oil is accompanied always by water and often by natural gas; all are confined in a porous and permeable reservoir rock, which is usually composed of sedimentary rock such as sandstones, arkoses, and fissured limestones and dolomites.
- The natural gas, being lightest, occupies the top of the trap and is underlain by the oil and then the water.
- A layer of impermeable rock, called the cap rock, prevents the upward or lateral escape of the petroleum.
- That part of the trap actually occupied by the oil and gas is called the petroleum reservoir.
Many systems have been proposed for the classification of traps; one simple system divides them into structural traps and stratigraphic traps.
- Structural traps are formed by tectonic events, such as folding or faulting of rock units. It forms as a result of changes in the structure of the subsurface. These changes block the upward migration of hydrocarbons and can lead to the formation of a petroleum reservoir.
- The most common type of structural trap is formed by an anticline, a structure with a concave (as viewed from below) roof caused by the local deformation of the reservoir rock and the impermeable cap rock.
- Stratigraphic traps are related to sediment deposition or erosion and is bounded on one or more sides by zones of low permeability.
Note: Structural trap is formed by the tectonic processes AFTER deposition of the reservoir beds involved while stratigraphic traps are formed during the depositions of the reservoir beds.
Sedimentary basins in India:
There are 26 sedimentary basins in India, covering a total area of 3.4 million square kilometer. The area is spread across onland, shallow water up to 400 meter water depth and deepwater farther up to Exclusive Economic Zone (EEZ). Of the total sedimentary area, 49% of total area is located onland, 12% in shallow water and 39% in the deepwater area. There are 16 onland basins, 7 located both onland and offshore and 3 completely offshore.
These basins are divided into three categories based on maturity of hydrocarbon resources as under:
- Category-I (30% of total basinal area): Basins, which have reserves and already producing.
- Category-II (23% of total basinal area): Basins, which have contingent resources pending commercial production
- Category-III (47% of total basinal area) Basins, which have prospective resources awaiting discovery
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