Abstract: Distribution and Controls of Lacustrine Source Rocks in the Recôncavo Basin, Brazil

Aragão, M.A.N.F.; Trindade, L.A.F.; Aratújo, C.V.; Silva, O.B.; Scartezini, A.A.; Oswaldo, F.H.; Canário, J.A.; Garcia, A.P. - Petrobras/Cenpes/E&P

Four tectonosequences compose the syn-rift stratigraphic filling of the Recôncavo basin: Syn-Rift I (Necomian) lacustrine, Syn-Rift II (Neocomian) lacustrine and deltaic, Syn-Rift III (Hauterivian/Barremian) deltaic, and Syn-Rift IV (Upper Barremian/Lower Aptian) fluvial strata. The onset of the rifting process (Early Berriasian) is associated with a shift of the Permian-Jurassic Southwestern depocenter to a Neocomian Eastern depocenter, by the distensive stress that has generated the structural framework of the pre-rift sequence and the actual half graben configuration of the basin. During the Syn-Rift I, tectonically controlled lake level rise promoted the deposition of organic-rich sediments, in a deep, anoxic lake (Candeias Fm./Gomo Mb.). This member is easily identified by high concentration of carbonatic layers associated with lacustrine shales, which gradually enhances resistivity profile. The Gomo Mb. was divided into two sequences: (i) a lower sequence, synchronous to Tauá Mb. of the Candeias Fm., comprised between Água Grande Fm. and Electric Marker 60 (maximum flooding surface of the basin); (ii) an upper sequence, which corresponds to the previously defined Gomo Mb.

The Lower Sequence occurs throughout the whole basin. It is within the oil window in most of the basin, and at peak generation conditions in the depositional lows. The overmature zone is reached only in the Camaçari Low (the deepest depocenter of the basin). Average total organic carbon contents (TOC) range from <0.8% to >2.0%. Hydrocarbon source potential (S₂) follows a similar trend, reaching 16kgHC/t rock. Thick shales with TOC higher than 1% are not observed at the deepest areas, but at the adjacents steps. Hydrogen Index (HI) up to 500 mgHC/g TOC characterize an oil-prone Type I kerogen. Therefore, the best pod of generation is in the depocenters, where high maturity conditions have been attained and expulsion is more efficient. The insoluble residue (IR) also increases in direction to the fault border, reaching values higher than 75. At the flexural border of the basin, occurs a narrow zone that is lean in organic matter and doesn?t reach the oil window. It is corresponding to the Tauá Mb. and is composed by splinter lacustrine shale, with low resistivity profile.

At the Upper Sequence the average values of geochemical parameters are comparable to the Lower Sequence, but the areas containing the best source intervals were reduced. At the top of the sequence, the oil window was attained only in the depocenters and peak generation condition is only reached in the deepest depocenter of the basin.

The onset of oil generation window according to vitrinite reflectance varies from 800m to 2400m and peak generation conditions varies from 1500m to deeper than 3000m in the deepest depocenter of the basin. The overmature zone is around 4600m. Apparently, continuous subsidence has occurred on the depocenters, but high Ro values on the surface suggest uplift along flexural border high portion of Mata-Catu Fault and Quiricó Platform.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil