The Bucaco Basin (Portugal): Organic petrology and geochemistry study

Abstract

The Bucaco Basin includes a Carboniferous/Permian sequence that comprises, from bottom to top, the following stratigraphic sequence: Algeriz Formation, Vale da Mo Formation and Monsarros Formation. Algeriz Formation is constituted by deposits of basal breccia with variable thickness and alternating layers of siltstone, mudstone and conglomerate. Vale da Mo Formation corresponds to lacustrine deposits comprising massive red beds in the base, that pass alternatively to silty-mudstones, shales and grey mudstones with organic matter; this formation also includes a thin coal seam. Monsarros Formation includes fluvial conglomeratic deposits prevalent in the base and top, separated by layers of siltstone and mudstone, redder to the top. The Bucaco Basin opens as a pull-apart basin, into the Porto-Coimbra-Tomar shear zone and later is affected by the clockwise (dextral) movement of this N10 degrees W shear zone. The petrographic study of samples from Vale da Mo Formation showed that the organic matter corresponds to a type III kerogen, derived from higher land plants (gas-prone). The thermal maturation level of the strata was determined using random vitrinite reflectance, with values ranging from 0.72% to 0.80% (%Rr). Rock-Eval pyrolysis showed that Vale da Mo Formation is the only one with potential to generate hydrocarbons, an observation consistent with the petrographic characteristics. TOC ranges from 0.08% to 1.52%. Monsarros Formation reported the highest values for S1/TOC index.