Petrographic characterization and Reservoir potential of Hybrid Deposits from the Upper Jurassic of the Lusitanian basin (Portugal)

Abstract

The Upper Jurassic hybrid successions of the Lusitanian Basin, in west-central Portugal, have attracted much interest of sedimentologists, paleontologists, stratigraphers and even explorationists, due to its conspicuous small-scale facies variety. However, a detailed petrographic analysis of these hybrid deposits is lacking, even more so in terms of reservoir potential, which is the focus of the present study. In fact, an insight on hybrid deposits is crucial for hydrocarbon exploration, since carbonate and siliciclastic components play different roles in the porosity and permeability of the reservoir. Two costal cliff outcrops, have been targeted until now: i) a 200m thick section between the Consolação and São Bernardino (C-B), around 65 km NNW of Lisbon, composed of coastal marls and sandy carbonates; ii) a 30m thick section at Praia Azul (PA), around 45km NNW N of Lisbon, composed of marly clays and bioclastic and sandy carbonates. Field observations, including sedimentology, paleontology and high-resolution sequence stratigraphy, suggest that the depositional environments correspond to a shallow water marine shelf, fed by fine to coarse terrigenous inputs, bordered by coastal bays and/or lagoons, occasionally exposed. Detailed fieldwork has led to the identification of several decimetric hybrid layers, with siliciclastic and calciclastic grains in a carbonate matrix or cement. 31 thin-sections were performed, impregnated with blue resin, petrographically analysed and compositionally determined through the modal point counting method, with 500 points/thin section. The petrographic analysis of 21 thin-sections from the C-B outcrop, indicate a broadly regressive trend, represented (from base to top) by Bio- and Pelmicrites, sandy BioPelmicrites, sandy Pelmicrites and Micrites, intrasparitic Sandstones, sandy Sparites, and finally sparitic Sandstones. The porosities are generally low and ineffective, mostly vuggy and fracture. Micrite rich samples show porosities between 1 and 8%, samples with abundant siliciclastic grains in a sparitic cement have porosities <6% and sparite rich samples porosity ranges from 0% to 1%. The PA outcrop petrographic analysis, performed on 10 thin-sections, exhibits: sandy Micrites and Biomicrites, micritic and intramicritic Sandstones, sparse and packed Biomicrites, packed Biomicrosparite. The sandy rich samples show alternating porosities of effective and ineffective, vuggy and sometimes primary type, with values of 11 to 13%. The micrite rich samples show varying porosities from 2 to 11%, increasing with the increase in extrabasinal content, along with the pores connection degree. Finally, the samples mainly composed of bioclasts (up to 60%) exhibit the lowest porosities, less than 1%, except when microsparite cement is present (porosity of 5%). At this stage of the research, the analysis of these two outcrops shows that reservoir potential in hybrid deposits is sensitive to siliciclastic components abundance, but also to the kind of carbonates involving them - either micritic matrix or sparitic cement. The simplistic idea that the higher the siliciclastic content, the higher the porosity, is only valid, at least in these particular case-study, when the grains are surrounded by micritic matrix (with porosities up to 13%), because in the presence of sparitic cement, the porosity is reduced to almost 0%.

AAPG Datapages/Search and Discovery Article #90325 © 2018 AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin, Lisbon, Portugal, May 2-3, 2018