AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Reservoir characterization and potential of Lower Cretaceous fluvial units of the Lusitanian Basin (Portugal)

Abstract

This work addresses the reservoir potential of the lower Cretaceous fluvial units of the Lusitanian Basin (Portugal). These units present several oil-seeps in onshore areas, where no effective seal is present. In offshore areas, where exploration is going on for a few decades, these sandy units are considered to be a potential reservoir. However, porosity data are very scarce and restricted to dispersed indications in vintage wells. The aim of this work is to characterize directly the porosity and clay content of these units, based on onshore outcrop sampling, and to compare it with the same units analysed in exploration wells, as analogues for the offshore reservoirs. A preliminary approach has been made on 19 samples from Praia D’El Rey sandstones (close to Peniche), to test porosity estimation, based on three different methods: i) geotechnical essays (pycnometer method and paraffin method); ii) impregnated thin sections; and iii) image analysis with ArcGIS. From these analysis, it has been concluded that Petrographic study of impregnated thin sections and Point Counter would be the most time-efficient/accurate method to apply. In order to have a global perspective on the main characteristics of reservoir rocks of the lower Cretaceous fluvial units, 7 distinct locations were chosen, between Figueira da Foz (close to Coimbra, to the North) and Cabo Espichel (close to Lisbon, to the South), with a total of 23 samples. Petrographic porosity has been determined with the help of the Point Counter (around 300 points on each thin section). The average Porosity is around 15%, with a minimum of 1,3% and a maximum 20,3%. Clay content and mineralogy have been also addressed, considering its negative impact on reservoir properties. Both depositional and infiltrated clays have been identified and most of them considered to be non-expansive. The average clay content for the total 23 samples is 22%, ranging from an average of 8.5% (Torres Vedras) to an average of 29.3% (Ericeira). The clays are composed mainly by kaolinite (around 65-70%) and Ilite (around 25-30%), with minor Mixed-layers clays (around 5-10%). The same lower Cretaceous lithostratigraphic units have been studied in 6 onshore and 3 offshore exploration wells (access to data provided by ENMC), to determine its properties, submitted to different diagenetic conditions. Well logs and cores have been analysed in 9 wells and petrographic studies have also been performed in 2 wells (MRW-8 and Ms-1). The lower Cretaceous sequence of the MRW-8 well has a thickness of 77 meters, with Net-to gross of 20,5% and a porosity of 20,2%, while at the Ms-1 well it has a thickness of 54 meters, with a Net-to gross of 9,6% and a porosity of 6,3%. From this preliminary study, it may be concluded that the lower Cretaceous fluvial units of the Lusitanian basin show a reasonable to good porosity (mostly around 15%), and a probably good permeability related with moderate clay content (around 22%, mostly kaolinite and ilite). These characteristics point to a good potential of these units to act as a reservoir for hydrocarbons in offshore areas, where seal units are thicker and therefore much more effective than onshore.