Palynofacies and source rock potential of Jurassic sequences on the Arruda sub-basin (Lusitanian Basin, Portugal)

• Type: Article in International Scientific Journal
• Date: 2015

Authors

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

Journal

Title: Marine and Petroleum Geology

Vol. 59

Pages: 575-592

ISSN: 0264-8172

Publisher: Elsevier

Indexing

ISI Web of Knowledge - 20 Citations

Scopus - 20 Citations

Scientific classification

FOS: Natural sciences > Earth and related Environmental sciences

Other information

Authenticus ID: P-009-Y49

DOI: 10.1016/j.marpetgeo.2014.10.009

Abstract (EN): The study of the organic matter present in sedimentary rocks is fundamental for the characterization of the potential source rocks as well as to define the depositional paleoenvironments. Parameters of organic petrology and organic geochemistry, and their variations along sedimentary successions are essential tools to achieve these goals. Two borehole successions (Freixial-1 and Benfeito-1) from Arruda sub-basin (Lusitanian Basin), that comprise siliciclastic-carbonate deposits of Jurassic age, have been studied in order to characterize the organic matter and its vertical variability as well as its hydrocarbon potential. Palynofacies and geochemical data indicate both, marine (including zooclasts) and terrestrial organic matter where the phytoclasts predominated despite an important contribution of the amorphous organic matter (AOM). Different types of solid bitumen were observed. Most of the analyzed samples have total organic carbon content (TOC) lower than I wt.% and Hydrogen Index (HI) values lower than 100 mgHC/ g TOC, revealing types III and IV kerogen. However, one set of samples exhibit TOC values higher than 1 wt.% and higher HI values depicting a type II kerogen. The biomarker data showed a dominance of n-alkanes with 15-30 carbon atoms having a unimodal distribution. Pristane/Phytane ratio varies between 0.04 and 1.84 indicating an alternation of oxic-anoxic conditions along the sequences. Typical terpane distributions are characterized by the presence of tricyclic and tetracyclic terpanes as well as pentacyclic terpanes (rare in carbonated samples from Freixial-1 borehole). 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 values of the ratio 22S/(225 + 22R) epimers of C32 homohopanes revealed that most samples reached equilibrium values. The 20S/(20S + 20R) and beta beta/(alpha alpha+ beta beta) isomerization ratios in C29 sterane, calculated for the studied samples, range between 0.19-0.61 and 0.20-0.70, respectively, with several samples reaching the equilibrium values. The vitrinite reflectance values are higher than 0.5% indicating that the organic matter reached the oil generation window. According to the data obtained in this study, the Abadia, Montejunto, Cabacos and Brenha Formations can be considered potential source rocks of hydrocarbons (gas and/or oil) in this section of the Lusitanian Basin.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 18