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Petroleum Potential: Offshore Sri Lanka

1. Sedimentary Basins of Sri Lanka: An Introduction

2. The Cauvery Basin

3. Mannar Basin


2. The Cauvery Basin

The Sri Lankan(SL)side of the Cauvery basin is located at approximately 90-110 North latitude and 790-810 East longitude, lying between north to north west Sri Lanka and south east India, and includes adjacent on land Cenozoic sedimentary cover. In terms of size, the SL side of the northern Cauvery basin approaches in an area of approximately 20,000 square kilometers, with a sediment accumulation at the deeper parts of the sub basins exceeding 4.5kms, thus providing an adequate sedimentary thickness for basin prospectivity.

2.1. Basin Architecture

The basin appears to have been formed in the late Jurassic to early Cretaceous period, through the rotation of Sri Lanka in a counter-clockwise direction, away from the Indian sub-continent. Further examination reveals that the Cauvery basin is a composite tectonic province, divided into a series of sub basins by a set of northeast southwest trending Precambrian basement ridges. The sub-basins formed between these ridges may have different geological and tectonic settings. Also these sub basins terminate to the northeast against Cretaceous sediments or the basement, and presumably, terminate to the southwest in the Mannar basin.

2.2. Stratigraphy

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

Basement

The older exploration wells Pesalai 1,2,3, Pedro1, Palk Bay1 and Delft 1, drilled on structural highs, penetrated the crystalline garnet-biotite gneiss basement rock in the Cauvery basin. Several papersalso refer to the possible presence of other metamorphic rock, and volcanics as well. The stratigraphic successions above the Precambrian basement consist of terrigenous clastics and carbonates, which range in age from the Jurassic through Tertiary to recent, with the presence of several unconformities.

Late Jurassic to Early Cretaceous

]Late Jurassic to early Cretaceous outcrops of coarse-gritty and pebbly sandstones have been observed on the Indian part of the Cauvery basin. On the Sri Lankan side, however, dark grey to black shale with intercalations of fine-grained sandstone and limestone are present in the early Cretaceous sections. No Jurassic sections have been penetrated by the wells drilled thus far.

Upper Cretaceous

The segment that sits higher on the upper Cretaceous section comprises predominantly dark grey, grey, and greenish grey shale with minor stringers of limestone and thin porous grey sandstone. The basal segment is predominantly interbedded sandstone and shale.

Paleogene

The Oligocene, Eocene and Paleocene are composed of grey shales, porous sands, and bioclastic and micritic limestone.

Neogene

The lower Miocene section is predominantly made up of massive porous limestones, becoming marly with grey shale and clayey sandstones towards the base. The mid-Miocene section, however, is absent in some wells. The Upper Miocene to Pliocene sections mainly consist of sandstone with clay, limestone, and marl.

2.3. Hydrocarbon Generation

It is apparent that there has been hydrocarbon generation in the Cauvery Basin. The PH9-1 well, drilled in the Ramnad – Palk bay sub basin, approximately 20km north of the Indian-Sri Lankan maritime boundary, tested 1488BOPD of 560 API crude with 570 Mcf/d gas from 6m pay. This flow came from a sand layer in the Cretaceous. On the Indian side, oil shows have been found further north in Cretaceous and weathered basement rocks. On the Sri Lankan side of the basin, Russians encountered heavily gas cut salt water in the Pesalai-1 well on the Mannar Island, from basal Cretaceous sands between 2532m and 2566m. The rest of the wells drilled on the SL side of the Cauvery basin were plugged and abandoned as dry holes. However, one of the reasons for this result could be that all these wells were drilled on structural highs, and the surrounding source rocks may not have been adequately buried.

2.4. Source Potential

Geochemical studies done so far indicate that the Cretaceous sediments have significant source potential because they have been situated in a hydrocarbon-generating window between the late Cretaceous and early Tertiary.

2.5. Reservoir Potential

The Cretaceous sands of the Cauvery basin have excellent reservoir potential. The wells penetrating the basement of the ridges have shown considerably thick weathered zones, which along with the Tertiary limestone and sand packages show good reservoir potential.

2.6. Cap Rock

Sufficient claystone and shale are present to provide adequate cap rock for all potential reservoirs below the lower Miocene sections. However, thus far, no evidence exists for the existence of cap rock intervals for the lower Miocene Jaffna limestone.

2.7. Traps

Seismic data indicates the possibility of both structural and stratigraphic traps. Basement fault blocks are the main structural features. Simple anticlinal traps are usually caused by drape and differential compaction. Onlapping sediments to the basement highs could be potential stratigraphic traps.

2.8. Drilled wells


The total seismic coverage for the Cauvery Basin blocks is 5098.4 line-kms of vintage 2D seismic.

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