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PETROLEUM SOURCE ROCKS OF CONJUGATE MARGIN BASINS OF BRAZIL AND WEST AFRICA: THE EXAMPLE OF THE BARREIRINHAS BASIN VERSUS THE TANOS BASIN

Texto do resumo

The Barreirinhas Basin, on the Brazilian Equatorial Margin, and the Tanos Basin, located on the West African Transform Margin, represent two correlated basins with, probably, the same source rocks. The Barreirinhas Basin covers approximately 55,000 km² along the state of Maranhão, of which 80% are submerged, with depths reaching up to 3,000 meters. It is separated by the Tutóia High from the Ceará Basin to the east, and the Pará-Maranhão Basin to the west, with inferred structural features. The Travosas Formation (Cenomanian-Present) is represented by distal shales and anoxic depositional environments.
In the Tanos Basin, the source rocks are composed of marine shales from the Cenomanian-Turonian. From the data provided by BDEP/ANP, 122 wells from the Barreirinhas Basin were initially selected, two of which were chosen: one in shallow waters (1-MAS-3A-MA) and another in deep waters (1-BRSA-729-MA), totalizing 38 and 95 samples, respectively. Using parameters from Rock-Eval Pyrolysis and Total Organic Carbon (TOC), Hydrogen Index (HI), Oxygen Index (OI), and Petroleum Generation Potential were calculated.
In shallow waters, the Travosas Formation showed low oil generation potential, below 0.2% of TOC. In deep waters, it exhibited variations from <1 to >4% in TOC, indicating samples with excellent potential for hydrocarbon generation at depths of 3,138 and 3,336 m, with kerogen ranging between type II and III. The grouped data demonstrate that the shallow water well shows little variation in results and low possibility of oil/gas generation, as the oxidated organic matter type and low Tmáx (<440 ºC) demonstrated, while the deep water well expresses specificities at different depths, even showing potential for hydrocarbon generation in deeper layers (between 3,138 and 3,336 m, probably Cenomanian-Turonian).
On the other side of the Atlantic, in the Tanos Basin, Cenomanian-Turonian source rocks, in both shallow and deep waters, exhibit TOC ranging from favorable to excellent (2% to 6.8% TOC). In shallow waters, the source rocks vary from immature to slightly mature, classified as poor to very good source rock, with kerogen type III/IV. At greater depths, the source rocks show immaturity for hydrocarbon generation but are a good source of organic matter and kerogen type II/III.
The asymmetry of data among the wells in the Barreirinhas Basin suggests that facies variations and depositional conditions contribute to the greater hydrocarbon generation potential in the abyssal portion of the Travosas Formation, which seems to be repeated in the African counterpart. The preliminary results indicate that there is room to explore more relevant information about the basin. Therefore, future work in areas such as organic geochemistry and computational modeling is likely to be integrated, thus expanding the understanding of the generative potential of the Barreirinhas Basin.

Palavras Chave

Brazilian Equatorial Margin; Barreirinhas Basin; Travosas Formation; Petroleum System.