Offshore back on agenda after Mannar basin rethink
  from: Asian Oil & Gas
  by: Dr. Ray Shaw
  Friday, April 26, 2002

With newly acquired seismic data serving to dispel earlier suggestions that the Gulf of Mannar basin was devoid of rift basin architecture, the Sri Lankan government is gearing up for petroleum exploration of its offshore areas for the first time in almost 20 years. Dr Ray Shaw* summarises the current state of play.

A recent report sponsored by funding from the Asian Development Bank recommended new fiscal and legal frameworks be implemented as part of a major overhaul of Sri Lanka’s promotion of its upstream oil and gas potential.

The report, prepared by the School of Petroleum Engineering within the University of New South Wales (UNSW) based in Sydney, Australia, sets out a draft Petroleum Resources Act as well as the conclusions of a technical review of the petroleum prospectivity of the offshore areas.

Prepared in 2001, the draft Petroleum Resources Act was being considered by a cabinet committee at the time Sri Lankan president Chandrika Kumaratunga announced the proroguing of Parliament and new elections. Following December’s election of a new government under prime minister Ranil Wickremesingsinghe and the appointment of Karu Jayasooriya as minister for power and energy, it is expected that reconsideration and passage of the bill will be high on the agenda during the first part of this year.

Once adopted, a formal announcement will be made outlining details of the bidding round process. This is likely to take place during a workshop to be held in Colombo at which senior government and UNSW representatives will outline in detail key legal, fiscal and administrative aspects of the new Act and technical conclusions of the report.

Earlier exploration
From an exploration perspective Sri Lanka has been virtually ignored by the international upstream oil and gas community for nearly 20 years. Prior to Petro-Canada pulling out in 1984 there had been a number of forays, between 1966 and 1984, by both Western and Soviet entities, principally focused on exploration within the shallow offshore areas of the Cauvery basin located off the Jaffna Peninsula.

During this period some 18,000km of seismic data were recorded and seven wells drilled, of which four were structural tests. All were plugged and abandoned without encountering commercial hydrocarbons, although the three stratigraphic wells drilled by the Soviets on Mannar Island (Pesalai 1, 2 and 3), were reported to have encountered wet gas shows at several levels. In the Palk Strait region of the Cauvery basin, Marathon drilled the Palk Bay-1 and Delft-1 wells and Cities Service drilled Pedro-1. Cities Service also drilled Pearl-1 on the eastern flank of the Gulf of Mannar basin located to the south of the Adam’s Bridge high which separates the two basins.

Principal targets were sought by Marathon and Cities within Cretaceous sands across horsts and tilted fault blocks in the Palk Strait region. Interest here was kindled by a flow on test of 1488b/d from Cretaceous sands in the PH-9-1 well located just 30km to the north of the Pedro-1 structure, within Indian waters of the Cauvery basin.

It was this interest in pursuing PH 9-1 lookalikes in the nearby shallow waters of the Sri Lankan Cauvery basin that focused attention away from the comparatively much deeper water setting of the Gulf of Mannar basin. Only Cities Pearl 1 well tested the Gulf of Mannar basin and that well was located high up on the shelf, again in shallow waters. Although penetrating a thick sequence of Cretaceous sands, Pearl-1 failed to encounter any shows. Terminating prematurely in Late Cretaceous basalts, similar in age to basalts in Asamera’s Mannar 1,1A well drilled on the northwestern Indian counterpart side of the basin, Pearl-1 led to speculation that this basin may be underlain by oceanic crust and so lack any of the syn-rift sequences thought to host the likely source rocks of the Cauvery basin to its north.

Alternative viewpoint
Contradicting this view, the UNSW report concluded that the area holding the best potential in the offshore Sri Lankan region is the Gulf of Mannar basin. The Bay of Bengal margin side of the island, first thought to have good potential, appears to have been transform dominated and much of the syn-rift sequences may have been detached during the inception of seafloor spreading along this margin. Following interest by a number of seismic acquisition companies, TGS-Nopec was awarded a licence by the Ceylon Petroleum Corporation (the national oil and gas enterprise of Sri Lanka) to acquire the first reconnaissance seismic coverage across the deeper water portions of the Gulf of Mannar basin.

During June and July 2001, some 1100km of 2D seismic were recorded by TGS-Nopec and subsequently processed by WesternGeco. Their preliminary results, presented in Singapore at a Southeast Asia Petroleum Exploration Society (Seapex) conference last December, confirmed the UNSW view and have now encouraged moves by TGS-Nopec to acquire a much more elaborate survey of some 7000-8000km across the Gulf of Mannar basin during 2002.

The previously-undescribed Gulf of Mannar basin developed during at least two periods of rifting and associated continental breakup, as part of the multiphase fragmentation of Gondwanna during the Mesozoic.

The first phase began as a precursor to the commencement of seafloor spreading in what is now the oceanic Bay of Bengal. This was followed by a second phase of rifting associated with the detachment of Madagascar from the western side of the developing Indian sub-continent, culminating in a second breakup unconformity at the top of the Late Cretaceous.

Subsequently the Sri Lankan margins entered a phase of subsidence, driven by thermal contraction, which continued until uplift and a major period of regressive sedimentation began in the late Miocene.

TGS-Nopec data has confirmed up to at least six seconds of section lies above basement in the Gulf of Mannar basin. Sediments comprise four packages. The oldest package, Megasequence 1, was deposited during the initial syn-rift phase of basin development, prior to the commencement of seafloor spreading west of Sri Lanka within the Bay of Bengal. Megasequence 2 sediments were deposited during the rift and sag phase, after the commencement of seafloor spreading in the Bay of Bengal but before the onset of spreading about the West Indian Ridge. The boundary between Megasequences 1 and 2 coincides with an Albian unconformity, whereas a Late Cretaceous to Paleocene unconformity separates Megasequences 2 and 3. Megasequence 3 was deposited during a Tertiary sag phase of basin development, which terminated in the late Miocene following compression. Subsequent basin-wide regression resulted in deposition of Megasequence 4.

Petroleum geology
There are three potential source intervals. The first are associated with the Upper Gondwanna Group sediments which occur in outcrop both on the Indian and Sri Lankan landmasses. These are likely to be mainly terrestrial and associated with initial syn-rift development. Another source sequence is associated with the basal transgression over the Albian unconformity, and the third with the open marine Cretaceous sediments. On the basis of synthetic burial history modeling discrete hydrocarbon generation phases are envisaged both in the Late Cretaceous-Early Tertiary and again in the Late Tertiary.

Good clastic reservoir intervals are predicted in Megasequence 2, based on the results of the Pearl-1 well. Megasequence 3 contains good potential clastic reservoirs towards the base, immediately overlying the breakup unconformity and carbonate reservoir potential is also present in the Paleocene to middle Miocene section.

There are a wide variety of potential trapping mechanisms including horsts and tilted fault blocks and associated compactional drape. Regionally, compressionally-induced traps together with turbidite-related and carbonate stratigraphic traps are recognized, there being a major fairway along the basin-floor and lower slope of the Sri Lankan margin comprising at least three discrete stratigraphic intervals of stacked and interbedded turbidite and channel deposits, some up to 1.0 seconds thick. Several structures have associated flat spots, phase changes and amplitude bursts on the seismic sections, consistent with direct hydrocarbon indicators. These occur across a wide range of stratigraphic levels.

Conclusions
A review of pre-existing exploration data together with recently acquired new seismic data clearly dispels earlier interpretations that the Gulf of Mannar basin is devoid of rift basin architecture. It is now established as a basin containing thick syn-rift and post-breakup sedimentary infill. The Gulf of Mannar basin represents a new deepwater frontier region which has the indicia for hosting significant hydrocarbon accumulations.

* Dr Ray Shaw, team leader of the ADB-Sri Lanka Project, is with the University of New South Wales’ School of Petroleum Engineering. AOG readers requiring further details should contact either:

DTP Liyanaarachchi, exploration manager, Ceylon Petroleum Corporation, 5th Floor, Rotunda Tower, 109 Galle Road, Colombo 03; e-mail: cpcoilex@ceypetco.com.lk; tel: 94-1-437977. or Peter Baillie of TGS-Nopec, Level 5 MLC House, 1100 Hay Street, West Perth, WA 6005, Australia; e-mail: peterb@tgsnopec.com.au; tel: 618-94800000.