Predicting the Hydraulic Behaviour of Carbonate-Hosted Extensional Fault Zones

Abstract

Predicting the sealing capacity of carbonate fault zones is complicated by the heterogeneity of intact carbonates and their respective fault rock textures, and the propensity for carbonates to respond to fluids and diagenetic processes. Carbonate-hosted extensional fault zones have been examined for locations of fault rock, types of fault rock produced and their influence on a fault's hydraulic behaviour. The location of fault rock affects fluid flow pathways across/along faults and is dependent on the fault's architecture. Fault zones with multiple slip surfaces often occur in weaker carbonates, distributing fault rock and preventing production of a continuous fault core at lower displacements, allowing fluids to flow across the fault. The sealing potential is also a function of the deformation mechanisms active during fault rock production. Lithological heterogeneity in a faulted carbonate succession leads to a variety of deformation mechanisms, generating several fault rock types with a range of microstructures along a single slip surface. The types of fault rock produced is a function of the host rock texture, specifically grain size, sorting, porosity and strength. Dispersed deformation creates large fracture networks within homogeneously fine-grained, weaker carbonates. In contrast, localised deformation occurs in heterogeneous, coarse-grained, stronger carbonates, creating cataclasite and cemented fault rocks. Each microstructure has different poroperm values, varying along-strike and down-dip. Permeability of all analysed fault rocks range from 0.0001 to >1000 mD and porosities vary from 1.6% to 34.7%. However, trends to the variable poroperm are observed, dependent on host lithofacies, juxtaposition and displacement. Mixing of different lithofacies at higher displacements increases the types of deformation mechanisms active, creating a variety of fault rocks, each with different poroperm values. This causes faults to have a negligible response when simulating reservoir models, with transmissibility multipliers of c. 0.86. Conversely, juxtaposition of similar lithofacies increases the fault rock homogeneity including their poroperm, and reduces the transmissibility multipliers to 0.001, causing the faults to significantly reduce flow. Understanding the deformation mechanisms active during faulting of a carbonate sequence aids prediction of the types of fault rocks formed, their hydraulic properties and influence during reservoir simulation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015