Sedimentological and petrophysical analysis of the Monte Sant'Angelo Megabreccias (Cenomanian), Gargano, southern Italy

Abstract

The Monte Sant'Angelo Megabreccias (Cenomanian) is one of three distinct and impressive Cretaceous megabreccias exposed on the flank of the Apulian platform, Gargano Promontory, southern Italy. This thesis describes and interprets stratigraphic and petrophysical data collected from two successions on the northern and southern Gargano peninsula, respectively. The northern succession displays a prograding (outcrop scale) package of clast-supported and matrix-supported megabreccias which grade upwards into proximal calciturbidites. The southern succession displays an aggrading (outcrop scale) package of stacked, matrix-supported megabreccias. Sedimentological mapping led to the recognition of subfacies which suggest that marginal platform areas were source of these allochthonous debrites. Sediments were lithified prior to erosion and re-sedimentation. Data support previous speculations on synsedimentary tectonics as a controlling factor for the repeated collapse of these marginal areas. Changes in accommodation space controlled the depositional patterns. One hundred and seventeen plug samples with corresponding thin-sections, permeability- and porosity measurements are classified according to three porosity classification systems. All three systems fail to produce adequate correlations of permeability and porosity for their respective groupings. Poroperm distributions also form the basis for petrophysical interpretations in terms of stratigraphic impact, and correlations between poroperm signatures and stratigraphic properties of the Monte Sant'Angelo Megabreccias are discussed. Stratigraphic and petrophysical analysis supports the subdivision of the mapped megabreccias into 4 distinct flow-units, which display important differences in terms of depositional pattern, internal fabric and petrophysical properties. Important differences are also mapped in the comparison of the northern and southern succession. These differences are underpinned by statistical analysis of petrophysical data, and coincide with the stratigraphic data. The diagenetic history and its impact on reservoir properties is described and discussed.