Geological Society of Australia - WA division
20201104 GSA William Power. Fluid Flow and Faults – Constraints from outcrop and fault seal analyses
Abstract
Recent research suggests that faults are neither singularly sealing nor singularly leaking. Instead, their behaviour is variable, depending on a patchwork of conditions that result from cross-fault juxtapositions and fault rock type. Preparation of a representative 3D solid geology model prior to assessing fault-related fluid flow is essential. Scaling relationships for faults, for example, the fault length to fault throw ratio, and the fault length to fault thickness ratio can provide constraint when direct, outcrop scale observation of the faults is not possible. Faults are best approximated as thin, approximately planar zones, with gouge zone or core zone thickness at 0.1-10% of their horizontal or vertical dimension. Fractured damage zones are somewhat thicker. Core zone/gouge zone thickness is actually “paper thin”, in fact the ratio of size to damage zone thickness is approximately analogous to an A4 size sheet of paper. However, when examined closely most fault zones also have "holes" or zero thickness positions where the fault damage zone or gouge/fault rock has negligible thickness.
In groundwater analysis, it is essential to consider the complete hydrological system in order to calculate risks. Cross fault flow between aquifers is controlled by the juxtaposition area and the thickness of the fault zone. For fault juxtapositions, the juxtaposition area is large in comparison to the damage zone thickness, which means there is a very short flow path over a large cross sectional area. In contrast, flow paths along fault zones through fractures can be controlled by either the length of the pathway, or the cross sectional area. Hydrogeological modelling has only just begun to incorporate these details into flow models, which suggests that many hydrological risk assessments that have been undertaken may be flawed if faults are present. The creation and preservation of conventional oil and gas accumulations requires both a source pathway to charge a structural trap, and also a coherent and long-term top and lateral seal. Although widely used, fault seal calculations using shale gouge ratio or shale smear algorithms are generally inaccurate. This is very likely because these algorithms typically assume capillary seal and a continuous membrane, which is usually not warranted (fault gouge zones are actually paper thin and typically have holes). A large number of case studies of known hydrocarbon accumulations are summarised in the talk and a recent publication (below). These show that analysis with simple juxtaposition seal rather than juxtaposition with additional clay smear seal provides the best match to the independently observed hydrocarbon fill levels.
Recent Publication: https://www.researchgate.net/publication/335040180_Validation_and_Analysis_Procedures_for_Juxtaposition_and_Membrane_Fault_Seals_in_Oil_and_Gas_Exploration?fbclid=IwAR0khfnonmIgOWwKTXQLOUmyIAytrRCTTl-oeBgZ5yspcgScuN4BBNU7IQU
Biographies
William Power is a Perth-based structural geologist/consultant and independent researcher with over 30 years experience in work with geological faults, fault zones, and fractured rock masses. He has worked in many areas related to geological faults including earthquake hazards, structural geology, mineral exploration, hydrogeology, insitu stress estimation, and oil and gas related geomechanics and exploration.
Titus Murray is a NSW-based structural geologist, consultant, and independent researcher with over 30 years experience in structural geology. Titus has specific and intensive experience in the fields of oil and gas and groundwater. He has spent much of the last 15 years developing methods for the analysis of fault seal, and conducting independent research on the character and geometry of fault zones. He also has significant experience with the development and constraint of trap structures in oil and gas exploration.
