About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

Southeast Asia Petroleum Exploration Society (SEAPEX)

Abstract


Offshore South East Asia Conference, 1978
Pages 136-148

Petroleum Geology of the Northwest Offshore Area of Sri Lanka

Thomas Cantwell, Thomas E. Brown, Don G. Mathews

Abstract

The northwest offshore area of Sri Lanka is part of the Cauvery Basin (see Figure 1 and 2). The Cauvery Basin, located south of 12°N latitude, covers an area of abort 32,000 square kilometers onshore and offshore the west coast of Sri Lanka and the southern part of the Coromandal coast, southeastern India. The western edge of the basin is formed by the Archaean Peninsular shield and to the east, excepting the separated shield segment comprising the island of Sri Lanka, extends into the continental shelf.

Early surface geological studies revealed disconnected outcrops of continental Jurassic sediments (Upper Gondwana) and shallow water marine Cretaceous (clastic as well as carbonates) strata along the basin’s western margin. In addition, scattered outcrops of continental Miocene sediments were also mapped. Over much of the Indian part of the basin, surface exposures are masked by alluvium deposited in the flood plains of the Cauvery River. The geology of Sri Lanka onshore has been treated by Cooray (1976).

In the early 1950’s a geophysical program was initiated by the Geological Survey of India and resulted in the collection of about 5300 gravity-magnetic observations covering an area of about 12,700 sq. km. The results of the early work indicated that the onshore Cauvery Basin can be subdivided into four distinct sub-basins with their separation being defined by large buried basement ridges. Onshore seismic work, begun in 1960, confirmed the existence of the previously defined basement ridges and resulted in the location of the first deep well (i.e. Karaikal #1) to the region. Subsequent offshore seismic work, collected by the Soviets on behalf of ONGC in the early 1960’s, indicated a continuation of basement ridges into the offshore, with apparent draping of Tertiary strata over these highs.

In Sri Lanka waters, offshore seismic work was carried out by the Soviets on behalf of Ceylon Petroleum Corporation in 1973-74. Basement structures were indicated, and estimates of sediment thickness based on these early data in both Palk Bay and the Gulf of Mannar ranged from 5,000 to 15,000 feet.

Contemporaneous with their marine seismic program, the Soviets drilled a stratigraphic well on Mannar Island, followed by two more. These wells were designated Pesalai 1, 2 and 3 (see Figures 2 and 3). All three wells, although subsequently abandoned, did yield significant and encouraging results. The presence of hydrocarbons was said to be indicated in the Lower Tertiary, Upper Cretaceous and Lower Cretaceous intervals. In addition, good porosity sand and carbonate potential reservoir units were present throughout the section. Source rock analysis of the well cuttings of Pesalai #1 performed by Geochem Laboratories, Inc. of Houston indicated good source capabilities for both oil and “wet” gas from the Lower Cretaceous. Cuttings from the Upper Cretaceous section suggest a poor to good dry methane gas source. Thermal alteration measurements indicate that deeper burial with a consequent higher thermal maturity could result in the conversion of these zones into oil or gas sources.

In 1974, a 3200 square mile exploration license in the Palk Bay and the Guff of Mannar was awarded to Ceyoil Corporation. A subsequent farmout to Marathon Petroleum resulted in the collection of 1700 miles of 2400% CDP seismic and accompanying gravity and magnetics. In 1976, two exploratory wildcat wells were drilled; Palk Bay #1 and Delft #1 (See Figures 1, 4, and 5); both within the Palk Bay region. Both wells were plugged and abandoned as dry holes.

In 1977, a group led by Asamera and including ONGC drilled the Mannar 1-1A in the Indian side of the Golf of Mannar. This well was plugged and abandoned as a dry hole.

This report is based upon the information obtained from the open literature and the investigations carried out in and around the 3200 square mile exploration license referred to above, including geophysical and well data.


Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Watermarked PDF Document: $14
Open PDF Document: $24