Abstract
The Mannar Basin is a frontier failed rift basin between India and Sri Lanka. The Sri Lankan part has an area exceeding 42,000 km2. Although the recent two gas discoveries have confirmed the existence of an active petroleum system in the Mannar Basin, a major portion of the basin is still poorly explored. This article summarized the progress of current exploration activities and the hydrocarbon potential of the Mannar Basin. This basin began to evolve since the Upper Jurassic and experienced two rifting events; an early Late Jurassic syn-rift phase associated with East–West Gondwana break up; and a later, earliest Cretaceous syn-rift phase associated with Antarctica separation from greater India around 142 Ma. Rifting was followed by a post-rift phase comprising a thermal sag period and an inversion period. Three potential source rocks intervals have been interpreted at Maastrichtian–Campanian, Albian–Aptian, and Late Jurassic stratigraphic levels. The basin modelling work has confirmed that (1) mature potential source rocks (mainly Type II) exist below the Maastrichtian–Campanian strata and (2) the best potential source rocks (mainly Type II) exist at Albian–Aptian stratigraphic levels. The Late Jurassic source rocks have more potential for gas, while other sources have potential for both oil and gas. According to basin modelling results, Maastrichtian–Campanian and Albian–Aptian source rocks reach the oil window in the present-day depocentre around 45 Ma and 80 Ma, respectively. The Late Jurassic source rocks (mainly Type III) reach the gas window around 112 Ma in the present-day depocentre. Five play levels were defined for the whole stratigraphic section of the Mannar Basin. Tertiary play level is dominated by submarine fans, mounds and rollover anticline like structures. The Upper Cretaceous play is dominated by forced-fold structures, intra-basalt turbidite sands, and sub-volcanic sand-rich systems. The Lower Cretaceous play is dominated by reefs and abrupt margin pinch outs. The Upper Jurassic play is dominated by abrupt margin pinch outs. The Basement play consists of weathered basement rocks. The main challenge of the Mannar Basin is imaging below the flood volcanic layer, which inhibits the penetration of seismic energy and results in low-quality seismic data. Therefore, hydrocarbon potential assessments have become a major challenge below the Upper Cretaceous. The interpretation shows that the basin has a low risk for the source and reservoir, and high risk for seal and traps. New exploration activity would unlock more potential areas for hydrocarbon accumulations. Finally, the findings of this study can help for better understanding of hydrocarbon potential areas and current progress of exploration activities in the Mannar Basin, Sri Lanka.
Highlights
90% of the Sri Lankan landmass is composed of high-grade metamorphic rocks of granulite to amphibolite facies (Cooray 1984)
The late syn-rift phase of the Mannar Basin was initiated in Early Cretaceous times (Fig. 7) after the splitting of East and West Gondwana, Madagascar, Seychelles, and Greater India were separated from Australia and Antarctica (Schlich 1982; Mizukoshi et al 1986)
The tectonostratigraphy reveals that the Mannar Basin is the oldest sedimentary basin in Sri Lanka evolving since Late Jurassic times and contains greater sediment thickness than the Cauvery and Lanka Basins
Summary
90% of the Sri Lankan landmass is composed of high-grade metamorphic rocks of granulite to amphibolite facies (Cooray 1984). Namely Cauvery, Mannar, and Lanka, have been demarcated in Sri Lanka (Fig. 1). The Mannar Basin is located to the southwest and northwest of Sri Lanka, southeast of India and south of the Cauvery Basin (Fig. 1). It forms an intra-cratonic failed rift between India and Sri Lanka, and the Sri Lankan side of the Mannar Basin has an area exceeding 42,000 km. Exploration in the basin is limited (described below), two gas/condensate discoveries were made in 2011 (Fig. 1) These discoveries are located in the north part of the basin, and they prove the existence of a petroleum system. This article reviews the current progress of the hydrocarbon exploration of the Mannar Basin, Sri Lanka
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