Reservoir characterization and modelling of the Louriñha Formation, Portugal : paralic sandstone bodies

Abstract

The Lusitanian Basin in Portugal is an Atlantic margin rift basin on the western side of the Iberian plate positioned north of Lisbon. The basin contains, among several other formations, the Upper Jurassic Louriñha Formation which has been the object of this study. The Louriñha Formation consists of tidally influenced fluvial deposits formed in a paralic environment. The succession includes several thin shell banks interpreted to represent marine flooding surfaces of limited duration. A total of 14 facies have been recorded which are grouped into 5 facies associations of channel fill deposits, floodplain fines, overbank deposits, inclined heterolithic stratification (IHS) and marine deposits. Intertidal flats have not been identified.

The paralic depositional environment is thought to be either an estuary or a tide-dominated delta, though due to little to no control in the third dimension within a rather small study area, a legitimate conclusion can not been drawn from the study. Mineral content and paleocurrent direction data indicate 1st order derived granitic detritus from the Hercynian Basement horsts in the northwest, which are areas presently exposed as the Berlengas and Ferilhões Islands. The burial depth of the Louriñha Formation does not exceed 3 km.

The succession can be divided into 5 section (S0-S1) based on the characteristic architectural styles where lower and upper boundaries are set to either paleosols or flooding surfaces. The sand:gross ratio and connectedness of the sandstone bodies can be linked to base-level fluctuations as these have exerted a major control on the accommodation space created

A stochastic object modelling is performed with the use of PetrelTM software where the studied stratigraphic succession is divided into 4 zones with different input parameters of channel-belt geometries, i.e. the fluvial deposits, in addition to the sedimentary logs. With the generated models it is possible to evaluate the properties of a theoretical reservoir with relevance to connectedness and heterogeneity on different levels. The models imply a possible stacked reservoir with high connectivity in Zone 1 and 3-4, though a possible thin barrier between Zone 3 and 4. Zone 2 composes a barrier between Zone 1 and 3.

Thin sections from rock samples collected from the study area shows that the porosity, and likely also the permeability, in the sandstone bodies are generally poor and not of reservoir quality due to extensive carbonate cement and mud filled pore throats.