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Abstract

The Campos basin is the most prolific basin in the western South Atlantic, accounting for more than 60% of present-day Brazilian oil production. The stratigraphic and structural evolution of the basin reflects the breakup of Pangea and the emplacement of oceanic crust between South American and African continents. Breakup was facilitated by the rifting of an older Precambrian terrane and, in particular, by the normal reactivation of preexisting lines of weakness such as shear zones and thrust faults. The geodynamic model for the basin involves stretching of the continental lithosphere and crustal thinning, with an initial phase of fault-controlled subsidence, followed by a subsequent phase of thermal subsidence.

The oldest sediments drilled in the Campos basin are lower Neocomian elastics deposited on basalts dated about 120-130 Ma. This synrift phase of subsidence is contemporaneous with widespread mafic volcanism, basement-involved normal faulting, and associated half-graben development, with depo-centers commonly being controlled by antithetic faulting. The Aptian is characterized by a sequence of evaporitic rocks that shows a rapid increase in thickness and halokinesis toward deep water. The Albian is characterized by the accumulation of a very thick sequence of marine, shallow-water limestones that grade upward and basinward into marls and shales, reflecting a very rapid increase in paleobathymetry. This section is characterized by abundant halokinetic features and listric detached normal faulting soling out on the low-strength evaporitic horizon. The postrift (thermal) marine Upper Cretaceous to Holocene clastic section shows a more subdued phase of subsidence, although halokinetic-induced listric normal faulting and some compaction faults are observed locally. The basin fill is associated with the superposition of different environments of deposition. The hydrologically open, fresh- to brackish-water lake system in the early Neocomian evolved into a closed saline lake system by the late Neocomian. During the Aptian, the input of elastics was mainly restricted to the western margin of an elongate gulf, which was filled with evaporites. The Albian limestones were deposited on a rapidly subsiding shallow platform and shelf environment. Deep-water facies are identified in the Late Cretaceous to Tertiary, and the present-day bathymetry reflects a regional progradation ofneritic sediments onto deep-water sediments.

The present geothermal gradient in the basin is relatively low (18 to 30 °C/km, with a mean of about 20 to 22°C/km). Geochemical analyses, including oil-oil and oil-source rock correlation, indicate that almost all hydrocarbon accumulations discovered in the Campos basin are sourced from lacustrine calcareous black shales deposited in a late Neocomian lake system with saline to hypersaline waters of alkaline affinities. The excellent hydrocarbon source potential of Thèse rocks is due to the preservation of high-quality organic matter (almost entirely low-sulphur type I kerogen composed of lipid-rich algal and bacterially derived material, with as much as 9°7o TOC) in extremely anoxic conditions.

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