Characterisation of the 2D and 3D density and connectivity attributes of fracture systems in carbonate reservoir analogues: implications for fluid flow

Abstract

Fault zones, hosted in fractured carbonate reservoirs, can behave as either high porosity and permeability conduits, favoring the migration of fluids; or, conversely, as low porosity and permeability barriers, retarding fluid flow, due to the presence of fine-grained fault gouges (Agosta and Aydin, 2006, Agosta and Kirschner, 2003). Due to these reasons, fault zones can have great economical importance for the hydrocarbon industry. Furthermore, within fault zones, the cyclic accumulation and sudden release of trapped, high pressure fluids can trigger earthquakes and aftershocks (Miller et al. 2004).

In this project, we referred to the classical fault zone architecture models defined by Sibson (1977) and Chester et al. (1993), in which faults are built up of a fault core (where most of the displacement is localised), a damage zone (containing fractured host rocks) and the protolith (the unfractured host rock). Faults, with displacements ranging from cm- to km-scale have been studied within two study areas, Flamborough Head, UK and the Gubbio fault in the Northern Apennines, Italy. Flamborough Head is a peninsula in East Yorkshire, which represents analogues for hydrocarbon rich, fractured North Sea chalk reservoirs; whereas the Gubbio fault is a regional scale, seismically active normal fault, characterized by complex fault zone architectures, cutting through different types of carbonates.

At both study areas, field-based, outcrop-scale structural observations were completed in order to explore the internal architecture and infer the fluid transmissibility of the fault zones. Additionally, microscale structural observations were made using representative thin sections, collected from the different fault zone domains of the studied fault zones. Qualitative structural observations were complemented with quantitative analyses to study the variation of fracture and vein density and connectivity patterns across the fault zones, which were later used as a proxy for fluid transmissibility. These analyses included established 1D (transects) and 2D (image analysis) methods and a newly developed workflow for the modelling of fracture networks in 3D, based on LiDAR data. 3D modelling of fracture networks was developed using different fracture height/length aspect ratios. The quantitative comparison of different aspect ratio 3D models with established 1D and 2D results, by using misfit graphs, enabled to validate the different 3D models and to estimate the mean aspect ratio of fractures within the fault zones. Qualitative and quantitative results were integrated in conceptual fault zone architecture and fluid flow models.

At Flamborough Head small (cm-scale) and larger (up to 20 m) displacement normal faults were studied in two different types of chalks: one characterized by cm-scale interlayered marl horizons and another one, absent of it. Within the marl-free host rock, in the fault zones of both the small and the large displacement faults, fluid assisted deformation features, such as veins, are often observed. On the contrary, in marl-rich units, fluid assisted deformation features are absent, while fractures filled with intruded marl from the interlayered horizons are common. This suggests that the occurrence of fluid flow in this lithology is primarily controlled by the protolith.

1D quantitative analysis at Flamborough Head showed that, as also predicted by classical fault zone models, vein density progressively increases in the damage zones of faults moving from the protolith towards the fault core. 2D quantitative analysis showed that fracture connectivity remains as low as background values in the outer parts of the damage zones, whereas it increases rapidly in the inner parts. By comparing the fracture density and connectivity patterns measured from different aspect ratio 3D models with results measured from 1D and 2D analyses showed that the most realistic model is the 1/5 fracture aspect ratio one.

The Gubbio fault cuts through a carbonatic multilayer containing carbonates with different marl content. In the Marne a Fucoidi formation marl is homogenously distributed, while in the overlying Scaglia Group marl is absent. Within the damage zone, hosted in the Marne a Fucoidi formation, fluid assisted deformation features are rare and are only present in the damage zones of subsidiary faults that entirely cut through the formation, linking the under and overlying marl free carbonates. On the contrary, within the damage zone, hosted in the Scaglia Group, fluid assisted deformation features are common, especially close to the fault core of the Gubbio fault and in the damage zone of subsidiary faults. Similarly to Flamborough Head, this suggests that the occurrence of fluid flow is primarily controlled by the nature of the protolith.

As predicted by classical fault zone models, 1D quantitative analysis across the Gubbio fault showed that vein density increases in the damage zone moving from the protolith towards the fault core. Similarly to results from Flamborough Head, 2D quantitative analysis showed that fracture connectivity is low in the outer parts of the damage zones, but increases rapidly within the inner parts, and the comparison of 3D models with 1D and 2D results showed that the most realistic model is the 1/5 aspect ratio one.

The conceptual fluid flow models, built for the study areas, highlights: a) the importance of different marl content host rocks controlling the initiation of fluid flow; b) the development of smaller and larger displacement normal faults and the effects of their displacements on fluid transmissibility; c) the effects of fault damage zones, positioned in an overlapping geometry, resulting in the development high and low fracture connectivity subdomains and fracture corridors; d) the differences in the relative variation of fracture/vein density and connectivity throughout the damage zone compared to background values; e) the fluid transmissibility of the different fault rocks, located within different subdomains of the fault core and f) the anisotropy of fluid transmissibility in the fault core.