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The Irati Formation (Late Permian), in the Parana basin, is a source rock with high oil generating potential.
Different geochemical methods have been applied to this study in relation to geological observations, such as, Rock-Eval pyrolysis, optical observations, gas chromatography of C15+ hydrocarbons, and isotopic analysis.
The results have shown that this formation contains organic matter belonging to type I and type III predominantly. The T.O.C. contents range from 0.5 to 13% according to the quality of the organic matter. Pyrolysis analysis indicates that the area where the Irati has the highest oil-generating source rock is in the north and south of the Parana basin. In these areas petroleum potential can reach 90 kg HC/t of rock. In the central part of the basin the Irati Formation might reach a depth of about 3,200 m (10,498 ft).
The degree of evolution of the organic matter by "burial effect" is generally low (immature), reaching the beginning of the "oil window" in the deepest part of the basin.
In many wells diabase intrusions have more or less completely "cooked" this formation, thus generating oil or gas, and leaving residual organic matter. The phenomenon of migration into the Irati Formation has been observed in many wells. In certain places, oil is accumulated in shales embedded between intrusion levels; in other places oil is accumulated into limestone beds, intercalated in the Irati Formation.
It seems safe to assume that the oil accumulated in the deeper beds resulted from the effect of thermal intrusions and also from the effects of normal burial.
Oil migration occurred after diabase intrusions (Late Cretaceous) during the increasing subsidence of the basin.
In the Parana basin, the Irati Formation may be compared to a "drain" with a lateral oil migration. Vertical migration was hindered by the lack of enough porosity and permeability in the shales above the Irati source rock.
Consequently, migration and accumulation of oil above and below the formation might have resulted from changes in facies of the Irati itself, by faulting, or by fractures due to diabase intrusions.
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