In the Lusitanian Basin (central-western Portugal), the Lower Jurassic carbonate-dominated succession is thought to have significant source rock potential. One of the most important units is the Água de Madeiros Formation (Upper Sinemurian – lowermost Pliensbachian) which is composed of alternating organic-rich marls and limestones including black shale horizons. This paper is based on a study of this formation at its type locality at S. Pedro de Moel in western Portugal. Data includes Total Organic Carbon (TOC) measurements, palynofacies analyses and results of Rock-Eval pyrolysis presented within a high-resolution lithostratigraphic framework.
TOC contents were measured in some 200 samples from the Água de Madeiros Formation covering a stratigraphic interval of 58 m, and vary widely up to a maximum of about 22 wt %. Kerogen assemblages are dominated by marine amorphous organic matter with varying contributions by phytoclasts and palynomorphs. A majority of the 85 samples analyzed by Rock-Eval pyrolysis have S2 values above 10 mg HC/g rock, reaching a maximum of 78 mg HC/g rock. These high S2 values are correlative with maximum values of the Hydrogen Index which averages 355 mg HC/g TOC (maximum of 637 mg HC/g TOC). However in spite of these indicators of source-rock potential, the Água de Madeiros Formation in the study area is thermally immature or very early mature, as indicated by Tmax values below 437 °C and average vitrinite reflectance values of 0.43 % Ro.