Five oil fields have been discovered in Ceara basin, northeastern Brazil, as a result of 33 wildcats drilled. All of the discoveries are localized in Mundau area in the eastern part of the basin. In this area four major stratigraphic units are recognized in seismic and well data: rift (Aptian), transgressive (Albian/Cenomanian), slope (Upper Cretaceous/Miocene), and shallow platform (Paleocene/Miocene) sequences. The main potential reservoirs are deltaic-lacustrine sandstones of the rift sequence and turbiditic sandstones of the slope sequence. Good source characteristics are exhibited by the shales of rift and transgressive sequences; the slope sequence offers fair to good content of adequate organic matter, but is partly immature.

Three distinct types of oil traps have been found in Ceara basin. (1) Structural traps, with the reservoir, source, and seal belonging to the rift sequence; the pool is confined to horst blocks, formed by the intensive faulting which affects this sequence. (2) Combined traps, where rift sequence reservoirs, dipping landward, are truncated by an unconformity and sealed by overlying transgressive shales; the source may be from both rift and transgressive sequences. (3) Stratigraphic traps, formed by turbiditic sandstones immersed in the shales of the slope sequence, which provide both source and seal.

The fields discovered so far are small, with areas ranging from 2 to 5 sq km. Delineation is made difficult by faulting (traps of types 1 and 2) and by lack of continuity of reservoirs (traps of type 3). Advance seismic techniques, such as 3-D migration and higher resolution surveys, have been used as a help in solving these problems.

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