From fracture analysis to flow simulations of fractured carbonates: the case study of Roman Valley Quarry

The Roman Valley Quarry located at the northern termination of the Majella anticline in central Italy contains an excellent exposure of bitumen-bearing faulted carbonates, and therefore provides the opportunity to assess the role of stratigraphic and structural heterogeneities on subsurface flow. The vertical walls of this quarry expose in 3D the inner structure of two oblique-slip normal faults oriented WNW-ESE (called the SW and NE Faults). These faults crosscut the Oligo-Miocene Bolognano Formation, which is a medium- to high-porosity limestone (Cilona et al., 2014). The SW Fault has a seismically detectable throw of 40 m, and consists of a continuous main slip surface, with fault rocks that vary along strike from clast- and cement-supported cataclastic rock to un-cemented breccia. Using the bitumen distribution as a tracer of hydrocarbon migration, it is inferred that this fault behaved as both a barrier-conduit and distributed conduit for fluid flow. Conversely, the NE Fault, which has a sub-seismic throw of 8 m, consists of a fractured zone where several smaller slip panels interact, forming a continuous damage zone. This fault is composed of discontinuous pods of intensely deformed limestones and small slip surfaces surrounded by less-deformed rocks. Each slip surface represents a likely conduit to fault-parallel fluid flow, and taken together, these planes constitute a distributed conduit permeability structure (Agosta et al., 2010). Laboratory measurements and very detailed Discrete Fracture Network (DFN) models are integrated to quantify matrix and fracture contribution to porosity and permeability within each of the lithofacies cropping out in the study area. DFN models were constrained by spatial and dimensional properties of fractures obtained by scanline surveys. These models, calibrated with the field observations, were used to calculate fracture permeability and porosity based on the Oda upscaling method. Finally, the obtained hydraulic properties were used to build an outcrop-scale static model of both the matrix and the fractures, accounting for their stratigraphic and structural heterogeneities. The model served as input to dual porosity-dual permeability flow simulations to test fluid pathways for various flow scenarios. We show that fluid crossing a fault characterized by a discontinuous low-permeability fault core may bypass reservoir fluid for a range of fault core permeabilities. References: Agosta, F., Alessandroni, M., Antonellini, M., Tondi, E., Giorgioni, M. 2010. From fractures to flow: A field-based quantitative analysis of an outcropping carbonate reservoir. Tectonophysics 490, 197–213. Cilona, A., Faulkner, D. R., Tondi, E., Agosta, F., Mancini, L., Rustichelli, A., Baud, P., Vinciguerra, S. (2014). The effects of rock heterogeneity on compaction localization in porous carbonates. Journal of Structural Geology, 67, 75-93.