Hierarchies of stratigraphic discontinuity surfaces in siliciclastic, carbonate and mixed siliciclastic-bioclastic tidalites: Implications for fluid migration in reservoir quality assessment

Abstract

The hierarchies of the stratigraphic discontinuity surfaces observed in ancient tidalites are qualitatively assessed, aiming to evaluate their role as possible preferential conduits for fluid migration. Three outcrop examples are presented from microtidal settings of southern Italy: (i) siliciclastic tidalites consisting of quartz-rich cross-stratified sandstones generated by strong two-directional tidal currents flowing along a tidal strait; (ii) carbonate tidalites, which accumulated in a Cretaceous lagoon and tidal flat where peritidal cycles formed vertically-stacked sequences of biopeloidal and fenestral packstones, wackestones and bindstones during repeated phases of Milankovitch-scale sea-level changes; (iii) mixed, siliciclastic-bioclastic tidalites, deposited in a bay and recording offshore-transition, to shoreface wave-dominated and tide-influenced environments. Observations made during this study suggest that fluid movement can be controlled by the presence of main bounding surfaces that occur at different dimensions, from large (hectometre)-scale, to medium (decametre)-scale, to smaller (metre)-scales. These surfaces produced either by depositional or erosional processes, are characterised by different features and geometries in siliciclastic, carbonate and mixed siliciclastic-bioclastic tidalites arguably revealing complex internal pathways for fluid flows. These results suggest that fluids propagating along the main discontinuities follow a dominant sub-horizontal direction of propagation, associated with minor sub-vertical movements, due to local internal surface geometries and interconnections and a general lack of fractures. This surface-based approach to the study of fluid-flow transmission within stratified rocks represents a conceptual attempt to predict fluid mobility and reservoir potential in tidalite-bearing siliciclastic, carbonate and mixed reservoir rocks.