Improved Pressure Prediction from Basin Modeling by Integrating Fault and Host Rock Mechanical and Diagenetic Properties

Marek Kacewicz¹, Russell K. Davies², Rob Knipe³, and Gavin Lewis^4
1Chevron Energy Technology Company , Sugar Land, TX, USA
²Rock Deformation Research USA Inc, McKinney, TX, USA
³Rock Deformation Research Ltd, University of Leeds, Leeds, United Kingdom
⁴Chevron North America, Houston, TX, USA

Hydrocarbon flow across and along faults is strongly controlled by lithology contrasts, bed thicknesses, and mechanical strengths of fault and host rocks. The mechanical properties in turn are a function of the mean effective stress related to burial, mineralogy, particularly clay content, fluid pressure, and diagenetic changes with burial. Fault properties change with time although models for fault rock properties are typically based on controls on the properties observed in the present day. This often leads to problematic pressure and charge predictions and is not sufficient if effective stress calculated by basin models in near-fault areas is used for reservoir quality prediction, which requires a good handle on effective stress history.

If exposed to similar temperature and effective stress conditions, fault rocks typically have greater cementation, increased brittleness, and lower permeability than host rocks due to mechanical damage and the resulting finer grain sizes and increased surface area of crushed grains in the fault zone. Rocks with higher clay content may also be brittle if overconsolidated by cementation or uplift. Tectonically reactivated brittle faults are likely to leak providing pressure communication across and along fault zones. Undisturbed quartz-cemented faults and faults in normally consolidated shale sequences will be sealing, i.e. serve as hydrocarbon migration and pressure barriers. Thus the flow along faults can be determined by the distribution of brittle or ductile behavior.

Three scenarios have been identified that are critical from the point of view of predicting flow properties and pressure in a structurally complex basin: 1. Host rock ductile - fault rock ductile, 2. Host rock ductile - fault rock brittle, 3. Host rock brittle – fault rock brittle. In Scenario 1 the system is typically characterized by decreased permeability in the fault zone. This is due to clay smearing and grain crushing. In Scenario 2 fault rocks experience more quartz cementation than host rocks and become more brittle. In Scenario 3 both fault and host rocks are equally prone to brittle failure (both are quartz cemented or overconsolidated). Proper analysis of the three scenarios and key controls due to effective stress and temperature history, and the associated fault rock properties are critical inputs for predicting flow and pressure using basin modeling methods.

Examples from the Gulf of Mexico demonstrate how correct identification of the three zones and associated fault rock properties lead to better pressure and hydrocarbon migration predictions through time and space.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.