D. Healy

Fracture patterns in carbonate fault zones and their influence on petrophysical properties

Maltese field examples of variable carbonate lithofacies in fault displacements ranging from ~20cm up to 100m are used to examine how different carbonates deform, and to understand evolution of the fault zone architecture and ultimately their petrophysical signatures. The evolution of fault zone architecture in time and space, and the associated changes in deformation mechanisms, exert an important control over the sealing potential of faults. It has been suggested that faults in carbonates could form seals after as little as ~20 m displacement, especially when juxtaposed next to a different formation. However, as seen in the Maltese examples, the complete opposite may occur. An intense zone of deformation is formed, which enhances both the porosity and permeability on an outcrop scale. This also prevents the localisation of deformation onto one slip surface, stopping the formation of a continuous, impermeable fault core. Examination of how the rocks deform, through different deformation mechanisms, can potentially help unravel the relationship between fault zone architecture and petrophysical properties. It can also help to indicate the potential evolution of the petrophysics through understanding scaling of the damage zone relationships with displacement of different carbonate lithofacies.

Geomechanics and geology: introduction

presented at the Geology of Geomechanics Conference, 28–29 October 2015, Geological Society, London. Heidbach, O., Rajabi, M., Reiter, K., Ziegler, M. & WSM TEAM 2016. World Stress Map Database J. P. TURNER ET AL. 4

Permeability Estimation in a Multi-fractured Top Seal

Permeability of fracture media is one of the most important parameters characterising fluid flow, but it requires a detailed knowledge of fractures and fracture networks distribution. The unique exposures north to Santa Cruz represents a rare opportunity to observe and fully investigate a recently active bitumen-bearing fractured top seal. Permeability of a fracture network depends on the statistical distribution of fracture length, aperture, orientation, and density. Those fracture attributes are related to the permeability properties though a tensor (the Permeability Tensor). The statistical methods presented here for collecting and analysing fracture attributes shows how to obtain a more accurate data set directly collecting fracture attributes from fields, and how those features are fundamental for estimating permeability in a multi-fractured system.

Diagenetic and tectonic evolution of pore networks in carbonate normal fault zones and their effects on permeability

This study quantifies changes in carbonate fabrics and pore network characteristics in fault zones using field analogues with the ultimate aim of understanding fluid flow around carbonate hosted normal fault zones. The shallow water carbonate sequence in Malta, which is dissected by an array of normal faults of varying displacements, is the chosen field analogue. The study reveals a wide range in petrophysical properties on the core plug scale. Porosity ranges from less than 5 % to greater than 35 %, permeability varies by seven orders of magnitude from 0.001 mDs to 1000 mDs and ultrasonic p-wave velocity ranges between 2 and 6 km/s. The range in these petrophysical properties is in part due to primary depositional fabric. However, modifications of the primary fabric during subsequent diagenesis and deformation are important in shaping the petrophysical properties of the rock. Pore throat size and pore type are important characteristics of the pore network which control the permeability. The changes in the carbonate fabrics into fault zones results in pore throat size and pore type changes and hence modifies the permeability. Total porosity and rock fabric are important controls on the p-wave velocity and can allow for predictions of pore network characteristics.