Strata exposed near Tabbowa Tank, Tabbowa Basin, western Sri Lanka have yielded the first representatives of the distinctive Permian Glossopteris flora from that country. The assemblage includes gymnosperm foliage attributable to Glossopteris raniganjensis, roots referable to Vertebraria australis, seeds assigned to Samaropsis sp., sphenophyte axes (Paracalamites australis) and foliage (Sphenophyllum emarginatum), and fern foliage (Dichotomopteris lindleyi). This small macroflora is interpreted to be of probable Lopingian (late Permian) age based on comparisons with the fossil floras of Peninsula India. Several Glossopteris leaves in the assemblage bear evidence of terrestrial arthropod interactions including hole feeding, margin feeding, possible lamina skeletonization, piercing-and-sucking damage and oviposition scarring. The newly identified onshore Permian strata necessitate re-evaluation of current models explaining the evolution of the adjacent offshore Mannar Basin. Previously considered to have begun subsiding and accumulating sediment during Jurassic time, we propose that the Mannar Basin may have initiated as part of a pan-Gondwanan extensional phase during late Palaeozoic – Triassic time. We interpret the basal, as yet unsampled, seismically reflective strata of this basin to be probable organic-rich continental strata of Lopingian age, equivalent to those recorded in the Tabbowa Basin, and similar to the Permian coal-bearing successions in the rift basins of eastern India and Antarctica. Such continental fossiliferous strata are particularly significant as potential source rocks for recently identified natural gas resources in the Mannar Basin.

2D Basin modelling was used to evaluate the response of source rock maturation, and of petroleum expulsion, migration, accumulation and preservation to the evolution of the southern part of the inverted Broad Fourteens Basin. Modelling results show that the temperature, maturation and petroleum generation history as well as migration characteristics of both the Jurassic oil systems and the Carboniferous gas systems vary over short distances relative to the differences in burial history of the basin. Model results indicate that no major gas accumulations are preserved in the Slochteren Formation along the cross-section at present-day. Gas accumulations are predicted in sandstone-dominated Triassic units in the southern part of the section. Present-day oil accumulations predicted in the Vlieland Sandstone Formation sealed by the Vlieland Claystone Formation (in P9 and Q1 crestal structures) are in accordance with known oil accumulations. Additional oil accumulations are predicted in the sandstone-dominated Middle Werkendam Member, and in sandstones of the Delfland Subgroup.

The modelling offers an explanation for the different geochemical compositions of the accumulated oils in the P9 and Q1 areas. Modelling implies, that the oils in the Q1 oil field were sourced by remigrated oils expelled over time, from early mature to mature source rocks of the Posidonia Shale Formation. The biodegraded and water-washed nature of the Q1 oil is explained by the concentrated topography-induced groundwater flow through the Vlieland Sandstone Formation during the Late Cretaceous inversion of the basin. The oils accumulated in the P9 area were sourced from an early mature part of the Posidonia Shale Formation and were probably not affected by water washing and biodegradation because of post-inversion charging of the reservoir.