The island of Sri Lanka is underlain with Precambrian rocks of Achaean age about 3500 – 1500 million years (MY) old with a few granites of Cambrian age - about 540 MY old.
However, a small strip of the country, covering about one sixth (10500 sq.km) along the northwest coast and the entire Jaffna Peninsula, is underlain by sedimentary rocks of Jurassic, Cretaceous and Cenozoic (Miocene age), spanning a time scale of 250 – 5 MY, with the possibility of accumulation of oil and gas in reservoirs and traps. However, the geological prospectivity has not been properly ascertained by carrying out detailed surveys.
The past investigations include an aeromagnetic survey confining to a strip across the Jaffna Peninsula and a 2D seismic survey covering Jaffna, Mannar and Puttalam areas undertaken by La Compagnie Generale de Geophysique (CGG) of France in 1969.
Further, a gravity survey covering the entire country was carried out by the Geological Survey of New Zealand in 1970 in collaboration with the Geological Survey Department (present GSMB), as a part of New Zealand Aid Programme to Sri Lanka under the Colombo Plan.
The results of the aerial and seismic surveys were deposited at the Petroleum Corporation and those of the gravity survey at the Geological Survey Department (present GSMB).
The gravity survey revealed from 200 observations that the basement plunges fairly sharply northwest closer to Misalai. Gravity increases again towards the coastline between Kankesanturai and Karaitivu and the total effect is of the elongated SW-NE trending basin perhaps tilted or down warped is virtually similar to the Jaffna Peninsula.
Gravity also increases southwest to Punkudutivu but then flattens out. Fitting a two dimensional model to a section from Kudikamam through Chunnakam suggests a maximum depth to basement of 2000 meters in the centre of the basin. Further, with a density constant of 0.25 Mg/m3 the dip (slope) would increase to 15 de3grees and the maximum basement depth of 4000 meters.
However, from drill hole data and seismic evidence, the depth of the basement east of the downturn has been accepted as 300 meters. Accordingly, west of the sedimentary basin is a good target for detailed exploration.
The basement behaviour in the Jaffna area inferred from the gravity anomalies is different from that inferred from the seismic reflection data. The basement, still dipping to the west, according to the reflection data up to the coastline, is not more than 1000 meters deep.
The interpretation of the seismic data by CGG states, “On the western side of the survey, 2 reflection shootings have revealed intermediate velocities, so that we cannot pretend that horizon 2, first selected for energy always corresponds to the top of the basement and may correspond to a formation making the beginning of a cycle of sedimentation in the early Tertiary. In effect at this period a strong subsidence of the continental borders took place.”
Hatherton Ranasinghe and Pattiarachhi (1975), the authors of the ‘Gravity Map of Sri Lanka’ state, “The horizon 2 would lie in conformity either directly on the basement or on sedimentary formations (Gondawana or Jurassic series) according to the stage of erosion which had preceded subsidence.”
Further, Hatherton and Ranasinghe (1971) also suggested, “A block faulted depression or basin filled with Jurassic Gondwana sediments has been down warped to the west. The surface of the Gondwana (Jurassic) or a strong basal reflector in the encroaching marine sediments may provide the reflecting horizon noted in the seismic work.” Superficially, this explanation is attractive to the Mannar area.
It is interesting to note that in September 2007, Japan Oil Gas and Metals Corporation (JOGMEC) reinterpreted the seismic data (600 km) and four well hole data from 1976 to 1982 earlier held by the Ceylon Petroleum Corporation and the potential for seal, reservoir and source rocks are encouraging as indicated in the interpreted pseudo wells.
This reinterpretation indicates that Miocene to Lower Cretaceous sediments from 2500 to 3000 meters are present. The error was that there was no refined seismic data and the holes were located at the crest of the block faults and not on the troughs (depressions). Accordingly, with the recent 3D and 2D seismic data from TGS NOPEC and the three deep hole information offshore held by Cairn Sri Lanka in the Mannar basin indicate a strong correlation with the offshore and onshore sediments which is encouraging and detailed exploration in Jaffna Peninsula and also the area between Poneryn and Karativu and the near shore areas are recommended. Further, near shore areas to the north of the Jaffna Peninsula apart from onshore will also be potential targets.
There are many advantages in exploring on shore oil and gas as (1) costs of drilling offshore are becoming prohibitive and environmental risks are enormous (2) deep water development projects can take a long time to go from discovery to production and generally resulting in major cost and schedule overruns.
Another interesting area that needs attention is the Andigama, Pallama and Tabbowa Gondawana (Jurassic) sedimentary formations in the Northwest part of Sri Lanka. The gravity survey and the earlier seismic surveys have indicated sediments of about 900 – 1200 meters thick are present and a drill hole done by the Geological Survey 1975 went down to 1200 feet without striking the basement.
The rocks in these areas are brown shale with the centre containing ‘kerogen’ (a mixture of organic matter that makes up a portion of sedimentary rock). The underlying black carbonaceous shale contains streaks of coal as well apart from Kerogen in the brown shale.
There are indications that these down faulted sedimentary Block in the Precambrian will continue east along the Deduru Oya and the area needs detailed examination. It must also be mentioned shale is source for oil and gas by the method of ‘fracking’ now perfected in many parts of the world. Further, such rocks also carry coal and a source of coal bed methane gas.
In conclusion, I recommend that E&P IOCs reinterpret the existing seismic and gravity data and to apply new technologies such as gravity grandiomentry, wireless and micro seismic techniques (new seismic techniques to detect shale oil/gas prospects.) It is also possible to identify any rift systems, new graben and horsts (crests and depressions in sedimentary rocks) and intracratonic minibasins in our sedimentary formations.
(Dulip Jayawardena, a retired Economic Officer at United Nations ESCAP, can be reached at email@example.com)