The Iron Wash Fault Zone as a Natural Laboratory for Along Fault Fluid Flow: Implications for Long Term Integrity of Carbon Capture and Storage

Abstract

The Iron Wash Fault provides exceptional exposures of the internal structure of a seismic scale normal fault. It forms a useful analogue for faults in the overburden of Carbon Capture and Storage (CCS) reservoirs. It has a maximum offset of 120m, cutting a sequence of sandstones, shales, limestones and siltstones. In addition to its excellent 3D exposure, the outcrops contain evidence for multiple phases of both past and present-day fluid migration. This includes oil trapped in fractures and porous sandstone, bleaching of red sandstones, iron oxide and calcite cement.

At the exposure of the Iron Wash Fault, a regional seal unit is juxtaposed against another seal unit. The abundance of evidence for along-fault fluid migration documents how faults can provide pathways through low permeability formations. Most of the paleo-fluid flow evidence is found inside lenses in the fault zone, demonstrating that lenses are key to upwards migration of fluids through low permeability sequences, especially where sealing units are juxtaposed against sealing units.

Additional evidence for fluid flow is found in the damage zone. We characterize the damage zone fracture network and use paleo-fluid flow observations to show that a relatively small number of fractures is responsible for most of the fluid flow. In addition to acting as a pathway for vertical fluid flow, the damage zone can also act to facilitate fluid exchange between lenses inside the fault zone the surrounding host rock. If CO2 were to escape from a reservoir and migrate upwards, its route upwards would likely involve multiple fault zones connected by permeable formations. The length and complexity of the pathway affects the timescale of migration and the potential for local trapping. Understanding this exchange of fluids between the fault and the saline aquifers it intersects is key for predicting the migration of CO2 in the overburden.

To guarantee the long term integrity of CCS reservoirs, it is important to properly characterize the hydraulic properties of fault zones. We are developing methodologies for predicting the risk of high permeability lenses occurring, characterizing the fluid properties of fault damage zones and quantifying the potential for fluid exchange between fault zones and host rock.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018