Depositional architectures and facies distribution in turbiditic sandstone channel-fills. A case-study from the Gorgoglione Flysch Formation (Basilicata, Southern Italy)

Turbidite channels are among the most important deep-water hydrocarbon reservoirs currently being explored. Recent advances in the understanding of these depositional systems result from significant improvements in modern 3-D seismic imaging, showing their three-dimensional stratal complexity and architectures at multiple scales. However, lateral and vertical variability of reservoir properties are associated with differences in the nature of channel fill and their stacking patterns, which commonly are at scales below the resolution of even high resolution 3-D seismic datasets. For this reason, outcrop studies are powerful tools to investigate the properties of channel-fill deposits and improve the characterization of channel systems. A field-based study is presented from a key stratigraphic interval of the Gorgoglione Flysch Fm, a Miocene turbiditic succession exposed within a piggy-back basin in the Southern Apenninic Chain of Italy. This formation includes 10-to-60-m-thick amalgamated sandbodies, systematically offset-stacked to form channel complexes that are laterally associated with heterolithic overbank deposits. There is an upward evolution from these laterally-migrating amalgamated channelized sedimentary bodies into isolated sand-filled channels, 4-to-35 m thick, displaying considerable lateral thickness variability. Systematic bed-scale stratigraphic measurements on the sandbodies reveal a recurring motif of three main coarse-grained facies forming a distinctive channel-fill facies association. The basal LC1 lithofacies consists of pebble- to cobble-sized conglomerate within a poorly-sorted, very coarse-grained sandstone matrix, interpreted as a channel-base lag resulting from the bypassing of high-density turbidity currents, characterizing the earlier phases of the channel formation. LC1 lithofacies is commonly overlain by LC2 lithofacies, composed of normally-graded, coarse-to medium-grained sandstones, predominantly structureless or plane-parallel laminated. This lithofacies has been interpreted as the sedimentary product of collapsing, sand-rich high-density turbidity currents during the backfilling stage. LC3 lithofacies is dominant towards the top of the sandbodies; it is characterized by multiple sets of 3D dune-scale cross stratification in medium- to coarse-grained sandstones, whose deposition is related to the influence of persistent energetic traction currents at the end of turbiditic events that reworked the topmost part of the channel-fills. Geometry, continuity, and distribution of sandstone facies at the scale of channel-fills, combined with palaeoflow analysis, provide insights for the identification of the different parts of the channels (axis / off-axis / margin) and locally the characterization of the degree of sinuosity. The detailed description and interpretation of these deposits can help to better understand the processes involved in the evolution of turbiditic channels for an accurate characterization of deep-water reservoirs.