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Petroleum Potential: Offshore Sri Lanka

1. Sedimentary Basins of Sri Lanka: An Introduction

2. The Cauvery Basin

3. Mannar Basin


3. Mannar Basin

The Mannar basin is located at approximately 60-90 North latitude by 780-800 East longitude, and lies from south west to north west Sri Lanka, south east of India and south of the Cauvery basin. In terms of size, the SL side of the Mannar basin approaches in an area of approximately 42,000 square kilometers with a sediment accumulation of possibly up to10kms in the deeper water areas of the basin. The first hydrocarbon discovery in the Mannar basin was made in 2011 by Cairn Lanka (Pvt.) Ltd. in their licensed block. However, the major portion of the Mannar basin still remains unexplored.

3.1 Basin Architecture

The N-S trending Mannar Basin is a pericratonic rift basin with several interpretation reports suggesting that the basin evolved in four phases.

The first rifting phase appears to have begun in the late Jurassic, as a result of extension between Africa and Antarctica. The late-syn rift phase, associated with India Antarctica-Australia rifting, presumably began in the early Cretaceous, causing more extension. The third rift transition phase in the basin has been interpreted as being during the Aptian to Albian, due to seafloor spreading between Sri Lanka and Antarctica. The thermal sag phase in the basin is affected by the rifting episode between India and Madagascar separation in the late Cretaceous. It has been interpreted that Mannar basin igneous activities associated with the Marion hotspot probably occurred during this phase. From the late Cretaceous to the Tertiary, the basin remained a passive margin. Based on the 2D seismic data it has been identified that canyons, channel complexes, fans/frontal splays and crevasse splays are the most common depositional environments.

3.2 Stratigraphy

Based on available literature and data, generalized stratigraphy for the Mannar basin can be set out as follows.

Basement

None of the wells drilled in the Mannar basin thus far have penetrated basement. The wells drilled in the Cauvery basin, on the other hand, have done so revealing it to be comprised of Precambrian high-grade metamorphic rock. Also data from outcrops in southern India and Sri Lanka combined with available seismic, gravity and magnetic data provide evidence that the major part of the Mannar basin basement is Precambrian metamorphic.

Syn-rift phase – Upper Gondwana (late Jurassic to early Cretaceous)

There is no direct evidence of a late Jurassic sedimentary rock record in the Mannar basin from drilled wells to date. Jurassic rocks are found locally in fault-bound basins in restricted locations in Sri Lanka itself – the ‘Tabbowa Beds,’ which generally comprise arkosic sandstone, and the ‘Andigama Beds,’ which comprise arkosic sandstone and carbonaceous shales with thin coal beds. But there is no direct evidence of their equivalents in the Mannar basin.

The oldest formations penetrated in the Mannar Basin are Albian to pre-Albian sediments in the shallow water on the Indian side. However, several interpretation reports reveal that possible Permo-Triassic to early Jurassic sections with Karoo-type coals and lacustrine shale equivalents as found in the Somali basin and Madagascar may also be present in the Mannar Basin.

Rift transition phase – Early Cretaceous

There is no direct evidence for this sequence in the Mannar basin. The evidence that exists comes from the wells on the Indian side, and Pesalai 1, 2 and 3, which have penetrated it, with the Pesalai-1 well encountering over 300m of organically rich shale.

Thermal Sag Phase – late Cretaceous

The more recent wells that have penetrated this sequence indicate that it is made up of sandstones with interbedded marine pelagic shales, channel filled sands, turbiditic sands, and claystones, with the deeper part of the basin sandstone layers interbedded with volcanics. The Cairn exploration well Barracuda penetrated below the volcanics, revealing the underlying lithology to be claystone.

Passive Margin – since Palaeocene

Interpretation reports disclose that throughout the Tertiary period the basin was a passive margin, apart from a local late Oligocene-early Miocene inversion.

The Palaeocene section has been penetrated by all the wells drilled in the Mannar Basin. The Pesalai-1 well penetrated 192m of Palaeocene sand-shale intercalated sediment. The Palaeocene sequence in Pearl-1 is predominantly limestone with intercalations of shale and minor sandstone. Cairn discloses that the Palaeocene sequence in the CLPL-Dorado-91H/Z well shows alternations of sandstone and shale layers. They also indicate the presence of turbidities and channels sand within this sequence.

Recent well data reveals that the Eocene sequence consists of highly calcareous shale, marls and calci-clastic deposits, presumably sourced from carbonates over the Sri Lankan shelf. At the same time, the Eocene sequence is pre-dominantly composed of marine pelagic shales.

According to the recent well data, the Oligocene sequence is more argillaceous, when compared with the Eocene. Seismic data also shows channel fills, presumably with sand intervals.

New well data indicates that the Mio-Pliocene sequence consists of unconsolidated clastics. The sequence is arenaceous, with intercalated claystones.

3.3 Hydrocarbon Generation

The presence of an active petroleum system in the Mannar basin is no longer a subject of speculation. The first comprehensive exploration drilling programme was carried out by Cairn Lanka, the details of which can be found in the accompanying History of Exploration section. Two of their four exploration wells resulted in potentially commercial gas/condensate discoveries.

According to the Cairn press release, the Dorado-91H/1z well was drilled to a total depth of 4741m and encountered 24m of hydrocarbon bearing Cretaceous sandstone in three zones between the depths of 4067m and 4206m. The reservoirs are predominantly gas bearing with some additional liquid hydrocarbon potential. The second well Barracuda-1G/1 was drilled to a total depth of 4741m and encountered 24m of hydrocarbon bearing Cretaceous sandstone in three zones between the depths of 4067m and 4206m with in the igneous layer. The reservoirs are predominantly gas-bearing with some additional liquid hydrocarbon potential.

3.4. Source Potential

The latest discoveries in the Mannar basin suggests that, on the basis of sediment character, geochemical analysis and maturity, the lower Cretaceous and probably underlying Jurassic sections appear to contain the best potential hydrocarbon source rocks. More than 300m thick, lower Cretaceous; excellent oil and associated gas source interval penetrated by the well pesalai- 1 may possibly be extended in to the manna basin. Burial history modeling carried out by TGS suggests that inferred late Jurassic to early Cretaceous syn-rift source rocks are in current peak oil generating window throughout the eastern part of the Mannar basin, with the relative abundance of oil verses gas depending on maturation and charge history. Furthermore, seismic data and several other interpretation reports suggest the presence of possible gas-generating Triassic Karoo coal intervals in the basin.

3.5. Reservoir Potential

Recent well data provides very strong evidence that there are good quality reservoir sands with higher porosities in the Cretaceous sequence, the same as in the Cauvery basin. The data further suggests that sands in several intervals in the tertiary section have good reservoir properties. Most of the high reservoir potential sands in Tertiary and Cretaceous could be channel fills, turbidities, frontal splays and crevasse splays. None of the wells thus far in the Mannar basin have penetrated the lower Cretaceous or Jurassic sections. However the documented late Jurassic sediments on the Indian side of the adjacent Cauvery basin are fluvial sedimentary packages of Sivagana beds and these equivalents with higher reservoir potential may be present in the Mannar basin.

3.6. Cap Rock

In the well Dorado-91H/1z, the encountered hydrocarbon bearing reservoir rocks are capped by thick shale rock providing an excellent top seal. In the well Barracuda-1G/1, the hydrocarbon bearing sandstone layers were encountered within the igneous, providing the evidence that these igneous rocks have good cap rock properties. The Pearl-1 well, drilled in the northern part of the Mannar basin shows adequate cap rock intervals, such as clay stones and shales above the potential reservoir rocks.

As an overall, with the aid of new well data, it can be estimated that, as in Cauvery basin, sufficient claystones, shales and igneous rock are present to provide adequate cap rock for all potential reservoirs below the lower Miocene sections.

3.7 Traps

Within the Mannar Basin, a wide variety of trapping mechanisms are evident from seismic sections.

According to the TGS interpretation report, R.D.Shaw points out the well Pearl-1 was located on an antithetic fault complex. The report further suggests that, as in the neighboring Cauvery Basin, the SL01 seismic line shows that horst and tilted fault block structuring is pervasive throughout the Gulf of Mannar basin. Horst and graben development especially in the late Cretaceous has created relief over which subsequent sediments of Paleocene to Eocene are draped creating potential traps. R.D Shaw further recognizes that several periods of turbidite deposition within a large sediment apron that stretches along the lower slope, pinching out towards the Mannar Basin margin, could construct potential traps.

2D seismic data indicates that the eastern margin of the Mannar basin extends to an abrupt margin, and that onlapping sediment sequences to that abrupt margin also could create potential traps.


Total Seismic coverage on the Mannar Basin
New 2D data–5600 line km
Vintage 2D data – 984 line km
3D data –2356 km2

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