The Tanzanian Coastal Basin is known as a petroliferous area. Hydrocarbons occur at the surface on Pemba Island and the Tanzania mainland and shows of oil and accumulations of gas have been proved in several offshore and coastal island wells. These occurrences display varying geochemical characteristics pointing to the existence of several active petroleum systems in the area.

Surface Occurrences
The Tundaua oil seep on Pemba Island shows isotopic characteristics of derivation from a marine source and a good biomarker correlation with extracts from Campanian shales in the Kimbiji East-1 well. Oil is also known from seismic shot holes on Mafia Island.

At Wingayongo in the Rufiji Basin, 160 km inland from the coast, there is an oil seep associated with bitumen-stained Lower Cretaceous sandstones. It appears that this represents a breached and exhumed paleo-reservoir. The seep displays isotopic and biomarker characteristics indicative of a restricted, carbonate lacustrine source.

Several oil seeps are known on Msimbati Island in the Ruvuma Basin. Two at least show characteristics of true degraded crude oils derived from mixed sources. One component indicates derivation from strongly reducing carbonate environments and another indicates a contribution from Upper Cretaceous or younger sources. There is also a gas seep on the island, but this is believed to be of biogenic origin.

Subsurface Occurrences
The Songo Songo gas field contains petrogenic gas and associated liquids in a Lower Cretaceous sandstone reservoir. The gas is unusually dry (97% methane) and the associated condensate has a restricted boiling range. This liquid is isotopically heaviest hydrocarbon discovered in the area. There is, however, no proven correlation of the condensate with any candidate source rocks.

The Mnazi Bay gas discovery is of biogenic origin. It is reservoired in Oligocene deltaic sandstones and was likely derived from organic matter in the same formation.
Oil and gas shows were reported from Paleogene formations in the Mafia-1 and Pemba-5 wells and gas shows from Ras Machuisi-1, Tan Can-1 and Zanzibar-1.  All the shows are confined to the Paleogene horizons.

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