Petrobras drilling activities for oil exploration in Campos Basin, offshore Rio de Janeiro state, were initiated in 1971. Up to October 1978, 70 exploratory wells have been drilled and an estimated recoverable petroleum volume of 1 billion bbl has been discovered.

The structural style and lithologic characteristics of the stratigraphic column disclosed by seismic and drilling investigations reflect several stages of tectonic basinal evolution, as follows. (1) An Early Cretaceous intracratonic rift-valley stage is recorded by syntectonic terrigenous sediments, commonly underlain by or interbedded with basaltic lava flows; this section contains good oil-source beds but, up to now, only fair reservoirs have been found. (2) A transitional phase, still of Early Cretaceous age, characterized by evaporites, dolomites, and carbonate rocks, marks the transition from continental to marine conditions. (3) An Albian shallow-water marine phase is represented by thick carbonate deposition where oil entrapment is related to porosity variations. (4) A deep-wat r phase characterizes the younger part of the section; the sedimentation of this phase, begun in the Late Cretaceous with the deposition of a transgressive shale section with good source potential, was interrupted during the Paleocene and early Eocene with deposition of a thick turbidite section, which provided excellent reservoirs.

The Lower Cretaceous section exhibits rifting and fault blocks bounded by normal faults. Important halokinesis occurred in the Late Cretaceous and affected the sediments that underlie the depositional hiatus at the base of the Tertiary. As a result of salt extrusion and dissolution, salt scars, collapsed structures, sliding faults, and other types of holakinetic structures are common, and are of paramount importance in reservoir distribution and oil entrapment in the Albian-Eocene section.

The main oil fields in the basin have been found in combination traps, provided by turbidite sandstone lenses or porosity variations in carbonate rocks, mostly associated with collapse faulting. Minor oil fields, structurally controlled by block faulting, have also been found. These modes of oil occurrence seem to be good exploration models for oil prospecting in analogous Atlantic-type basins.

End_of_Article - Last_Page 412------------