Preliminary results of ongoing analysis carried out in well-layered, sub-horizontal Cretaceous limestone rocks pertaining to the Altamura Fm. (Coniacian – Campanian inf.) are presented. Taking advantage of 3D exposures of the Murge area, southern southern Apennines foreland, Italy, the fracture network that pervasively crosscut the limestone rocks is accurately investigated by mean of 1:1 mapping, 1D scanline and 2D scanarea measurements of selected outcrops. The results of such detailed field survey shed lights on the type, dimension, geometry and scaling properties of fractures either confined within individual beds (stratabound fractures) or crosscutting several beds (no-stratabound fractures) within individual bed-packages as thick as a few meters. Moreover, the fieldwork also focussed on persistent fracture zones (small strike-slip faults) offsetting several bed-packages, and displaying incipient damage zones and, almost, no cataclastic fault cores. Stratabound fractures consist of bed-perpendicular joints arranged into two major orthogonal sets. Often, these bed-perpendicular joints abut against bed-parallel stylolites present within individual limestone beds, whose thickness ranges from 10 to about 60 cm. No-stratabound fractures are made up of sub-vertical sheared joints, which offset the limestone beds within individual bed-packages. Throws along these sheared joints is in the order of a few cm, the true horizontal displacement is up to a few tens of cm. Small strike-slip faults are characterized by throws up to a few tens of cm and true displacements in the order of meters. These features cut across many bed-packages, and are characterized by either individual or multiple slip surfaces surrounded by a discontinuous damage zone of joints and sheared joints. The main slip surfaces often recorded a strike-slip kinematics, and might be partially coated with precipitated calcite. Thin veneers of fragmented and brecciated limestone can also be present in the surrounding of the main slip surfaces within isolated pockets. After fieldwork, the mean orientation, size distribution, aspect ratio, aperture, intensity and fractal dimension of each fracture/fault set was used as input data to build up multiple DFN (Discrete Fracture Network) models of representative rock volumes. In particular, different DFN models were constructed based upon the investigated bed thicknesses (stratabound fractures) and bed-packages thicknesses (no-stratabound fractures). Differently, only one larger model was generated for small faults crosscutting several bed-packages. In the latter model were represented also stratabound and no-stratabound fractures within the host rock surrounding the smalle faults. The output of individual models were used to compute, at different scales, the overall P32, porosity and 3D permeability (Kx, Ky, Kz) values of the limestone rocks: from single beds to individual bed-packages and, finally, to multiple bed-packages. The results enable us to assess the multi-scale properties of the fracture network at different scales and, hence, to discuss the role exerted by brittle deformation on susburface fluid flow within tight layered carbonates.

Multi-scale fracture networks within layered Apulian carbonates, Italy

TONDI, Emanuele
2014-01-01

Abstract

Preliminary results of ongoing analysis carried out in well-layered, sub-horizontal Cretaceous limestone rocks pertaining to the Altamura Fm. (Coniacian – Campanian inf.) are presented. Taking advantage of 3D exposures of the Murge area, southern southern Apennines foreland, Italy, the fracture network that pervasively crosscut the limestone rocks is accurately investigated by mean of 1:1 mapping, 1D scanline and 2D scanarea measurements of selected outcrops. The results of such detailed field survey shed lights on the type, dimension, geometry and scaling properties of fractures either confined within individual beds (stratabound fractures) or crosscutting several beds (no-stratabound fractures) within individual bed-packages as thick as a few meters. Moreover, the fieldwork also focussed on persistent fracture zones (small strike-slip faults) offsetting several bed-packages, and displaying incipient damage zones and, almost, no cataclastic fault cores. Stratabound fractures consist of bed-perpendicular joints arranged into two major orthogonal sets. Often, these bed-perpendicular joints abut against bed-parallel stylolites present within individual limestone beds, whose thickness ranges from 10 to about 60 cm. No-stratabound fractures are made up of sub-vertical sheared joints, which offset the limestone beds within individual bed-packages. Throws along these sheared joints is in the order of a few cm, the true horizontal displacement is up to a few tens of cm. Small strike-slip faults are characterized by throws up to a few tens of cm and true displacements in the order of meters. These features cut across many bed-packages, and are characterized by either individual or multiple slip surfaces surrounded by a discontinuous damage zone of joints and sheared joints. The main slip surfaces often recorded a strike-slip kinematics, and might be partially coated with precipitated calcite. Thin veneers of fragmented and brecciated limestone can also be present in the surrounding of the main slip surfaces within isolated pockets. After fieldwork, the mean orientation, size distribution, aspect ratio, aperture, intensity and fractal dimension of each fracture/fault set was used as input data to build up multiple DFN (Discrete Fracture Network) models of representative rock volumes. In particular, different DFN models were constructed based upon the investigated bed thicknesses (stratabound fractures) and bed-packages thicknesses (no-stratabound fractures). Differently, only one larger model was generated for small faults crosscutting several bed-packages. In the latter model were represented also stratabound and no-stratabound fractures within the host rock surrounding the smalle faults. The output of individual models were used to compute, at different scales, the overall P32, porosity and 3D permeability (Kx, Ky, Kz) values of the limestone rocks: from single beds to individual bed-packages and, finally, to multiple bed-packages. The results enable us to assess the multi-scale properties of the fracture network at different scales and, hence, to discuss the role exerted by brittle deformation on susburface fluid flow within tight layered carbonates.
2014
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11581/369009
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