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

Abstract

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 1 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 17α (H),22,29,30-trisnorhopane (Tm) usually exceeds the amount of 18α (H),22,29,30-trinorneohopane (Ts). The values of the ratio 22S/(22S + 22R) epimers of C32 homohopanes revealed that most samples reached equilibrium values. The 20S/(20S + 20R) and ββ/(αα + ββ) 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, Cabaços and Brenha Formations can be considered potential source rocks of hydrocarbons (gas and/or oil) in this section of the Lusitanian Basin.