Paleoenvironmental characterization of a Jurassic sequence on the Bombarral sub-basin (Lusitanian basin, Portugal): Insights from palynofacies and organic geochemistry

• Palynofacies data indicate a terrestrial origin for the organic matter.
• The organic matter was preserved in an oxic and proximal to distal environment.
• Rock-Eval pyrolysis reveals a type III kerogen in an immature to mature stage.
• The sedimentary sequence studied does not reveal a potential source rock for oil.

A Middle–Upper Jurassic sedimentary succession has been cut by the Ramalhal-1 borehole in the Bombarral sub-basin (Lusitanian Basin, Portugal). The succession comprises the Brenha, Montejunto, Abadia and Lourinhã Formations, which are composed of limestones, marls, sandstones and clays. The main objective of this study was to characterize the organic matter using palynofacies and geochemical analyses to determine the depositional paleoenvironments and the hydrocarbon generation potential.The Total Organic Carbon content is generally less than 1 wt.% but reaches 4.1 wt.% in one sample, due to the presence of perhydrous coal. The Rock-Eval pyrolysis results reveal lower values HI, usually nether to 100 mgHC/gTOC, showing that the studied samples have type III kerogen and a low potential for oil generation. The Tmax values (varies between 319 and 443 °C) and biomarker data obtained for these samples indicate a maturation ranging between immature and, eventually, mature. The palynofacies data indicate predominance of the Phytoclast group (> 80%), with different degradation degrees and a modest contribution of Palynomorph group, especially sporomorphs, and the Amorphous Organic Matter (AOM) group. Solid bitumen was observed in some samples from the Montejunto and Brenha Formations. Determining the relative percentages of the kerogen groups allowed for the identification of seven intervals that established variations in the depositional environment.

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