The Mesozoic-Cenozoic organic facies in the Lower Tagus sub-basin (Lusitanian Basin, Portugal): Palynofacies and organic geochemistry approaches

• Type: Article in International Scientific Journal
• Date: 2014

Authors

Paula Alexandra Goncalves (Author) FCUP; Joao Graciano Mendonca Filho (Author) Other; Tais Freitas da Silva (Author) Other; Joalice Oliveira Mendonca (Author) Other; Deolinda Flores (Author) FCUP

Journal

Title: Marine and Petroleum Geology

Vol. 52

Pages: 42-56

ISSN: 0264-8172

Publisher: Elsevier

Indexing

ISI Web of Knowledge - 14 Citations

Scopus - 15 Citations

Scientific classification

FOS: Natural sciences > Earth and related Environmental sciences

Other information

Authenticus ID: P-009-5FE

DOI: 10.1016/j.marpetgeo.2014.01.015

Abstract (EN): Studies of the Mesozoic and Cenozoic sequence crossed by the Barreiro-4 borehole provide an improved understanding of the organic matter deposited in the Lower Tagus sub-basin (Lusitanian Basin, Portugal) and the implications for the potential source rock and depositional environment. This study focused on 43 samples (Middle Jurassic to Neogene) that were subjected to palynofacies and organic geochemistry analyses (Total Organic Carbon, Rock-Eval pyrolysis and molecular biomarker analysis). The palynofacies data indicate that the sequence contains mainly phytoclasts (non-opaque phytoclasts). However, the Middle Jurassic samples are dominated by Amorphous Organic Matter (AOM). Continental and/or marine palynomorphs are present in all the samples. The Cretaceous samples are characterized by small amounts of kerogen that have high contents of solid bitumen. The Total Organic Carbon (TOC) content is generally less than 1 wt.%. The Rock-Eval S1 and S2 parameters vary from 0.01 to 3.50 mgHC/g rock and 0.15 to 34.03 mgHC/g rock, respectively, with the highest values corresponding to the Cretaceous samples. The hydrogen index (HI) and oxygen index (OI) values range from 35 to 552 mgHC/g TOC and 4 to 180 mgHC/g TOC, respectively. The T-max values range from 416 to 437 degrees C. The biomarker analysis showed that n-alkanes with 15-30 carbon atoms are present and usually have a unimodal distribution with a predominance of low to medium molecular weight compounds. The CPI values range between 0.63 and 3.65, and the pristane/phytane ratios vary between 0.48 and 1.64, indicating alternation of oxic-anoxic conditions along the sequence. The distribution of terpanes shows small amounts of tricyclic and tetracyclic terpanes in most of the samples (except for some Cretaceous samples) and a predominance of pentacyclic terpanes. The amount of 17 alpha (H),22,29,30-trisnorhopane (Tm) usually exceeds the amount of 18 alpha (H),22,29,30-trinorneohopane (Ts). The 20S/(20S + 20R) and alpha beta beta/(alpha alpha alpha + alpha beta beta) ratios of the C29 steranes generally have values below the range of equilibrium, indicating an immature stage of the OM.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 15