A comparative study of confined vs. semi-confined turbidite lobes from the Lower Messinian Laga Basin (Central Apennines, Italy): implications for assesment of reservoir architecture

This study investigates the depositional architecture of turbidite lobes from the Southern Laga Basin (Lower Messinian, Central Apennines) and the external controls on its development. Physical stratigraphy and facies analysis of a 1700 m thick stratigraphic interval enabled us to unravel the hierarchical partitioning of lobes and compare the internal organisation of four units of different age and degree of topographic confinement. A four-fold hierarchy of depositional bodies recording short and long term variations in sediment input to lobes was recognized and includes: i) event beds, composed of sandstone-mud cap couplets deposited by single turbidity currents: ii) single lobes, comprised of few m thick bed sets with variable degree of amalgamation; iii) lobe sets, consisting of few ten of m thick stacks of single lobes that are typically depocentre-wide and iv) lobe complexes, representing high rank units of greater internal complexity. Field mapping of channel-lobe transition sectors and onlap terminations of lobes highlighted a significant stratigraphic change in size and morphology of the receiving basin topography which mainly resulted from sediment filling and onlapping of basinal slopes. The lobe depocentre evolved from an initial funnel-shaped trough of an approximate area of ?150 km2 to a wider morphology reaching a maximum area of over 500 km2, which suggest a notably more confined setting for the two older lobe complexes. Comparison of external shape and internal organisation of lobe sandbodies from the studied complexes points out how this different degree of confinement resulted in development of two main depositional architectures: i) sheet-like confined lobes, comprised of laterally contained sandbodies that in the axial part of the depocentre show flat geometry and little internal variability in facies partitioning and degree of amalgamation and ii) semi-confined lobes with a 'jigsaw' architecture, internally comprised of shingled to compensational patterns of building blocks with lobate to more complex planform and highly variable facies partitioning. The architecture of confined lobes is interpreted as having resulted from flow containment of incoming turbidity currents by lateral basinal slopes and deposition under confined accommodation space whereas that of semi-confined lobes would record deposition from freely expanding turbidity currents in a wider depocentre allowing for lateral migration of depositional lobes. The different architecture of confined vs. semi-confine lobes has important implications for the hydrocarbon industry as it results in significantly dissimilar structures of permeability and connectivity.