Topology of normal fault damage zones in carbonate rocks, Malta - implications for the development of connectivity in evolving fault networks

Abstract

Fault damage zones are known to be more complex in linkage zones, relays and at fault tips. From a reservoir perspective, these damage complexities may act as conduits or barriers for fluid flow, hence they need to be understood and quantified. This study quantifies the fracture network properties of three different damage zone types: damage occurring around stage 2 to 4 relay ramps, splay faults and fault tips. Specifically, the connectivity is quantified by the use of topology. Topology uses fracture intersections and endpoints to quantify the relationships between fractures within a fracture network, by the use of nodes and branches. A topological approach is used on 18 damage zones in the study area, Ras ir Raheb, on the west coast of Malta. This area shows excellent outcrops of small-scale (throw less than 1m) normal faults formed in carbonate rocks. Analysis of the topology and geometry of the studied damage zones show that: the connectivity is lower in stage 2 relay damage zones and fault tips compared to stage 3 relay damage zones and splay fault damage zones. The greatest connectivity is documented in stage 4 relay damage zones, which indicates that connectivity develops with maturity. Circular sub-samples, fracture intensity- and connecting node frequency maps are used to analyse spatial variability within the damage zones. Results show that the higher values are localised in areas of bifurcation and linkage. The topological characteristics of studied damage zones may be used as proxies for different stages of fault and damage zone evolution, with fault tips representing an early stage, and hard-linked relays and splays representing further progressed stages of growth and linkage. Using this as a basis, a model for the development of connectivity in evolving extensional fault networks is presented: 1) fault initiation stage; multiple isolated faults are active, resulting in numerous fault tip damage zones. The fault network is immature at this stage, hence a low connectivity. 2) Interaction and linkage stage; faults start to interact and link, ergo the fault network becomes more connected and show a medium connectivity. 3) Through-going fault zones; deformation continues and becomes localised along a few through-going faults, consequently the fault network shows the highest degree of connectivity. The use of topology increases our understanding of network properties of damage zones and allow us to explore the evolution of connectivity, both spatially and temporally in damage zones and other fracture networks.