HIGH-RESOLUTION STRATIGRAPHY, PALYNOFACIES AND SOURCE ROCK POTENTIAL OF THE ÁGUA DE MADEIROS FORMATION (LOWER JURASSIC), LUSITANIAN BASIN, PORTUGAL
L. V. Duarte1*, R. L. Silva1, J. G. Mendonça Filho2, N. Poças Ribeiro1 and R.B.A. Chagas2
1 Departamento de Ciências da Terra and IMAR-CMA, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Largo Marquês de Pombal, 3000-272 Coimbra, Portugal.
* corresponding author, email: lduarte@dct.uc.pt
2 Departamento de Geologia, Instituto de Geociências, Centro de Ciências Matemáticas e da Natureza, Universidade Federal do Rio de Janeiro, Cidade Universitária, CEP: 21949-900, Ilha do Fundão, Rio de Janeiro, Brazil.
In the Lusitanian Basin (central-western Portugal), the Lower Jurassic carbonate-dominated succession is thought to have the most 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 ºC and average vitrinite reflectance values of 0.43 % Ro.
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