Fault Seal

An accurate and comprehensive understanding of the full stress history of a fault is critical to evaluating fault seal risks by geologists and flow barrier compartments by reservoir engineers. While the stratigraphy on either side of a fault can be a contributing factor, it is more important to know if, at critical times, the fault has been pulled apart, is open and leaking, or pushed shut and sealing.


Hydrocarbon Seeps

Many seeps are indicators of non-sealing faults. The relative advance of Timor over the northerly directed Australian continent has caused loading to depress the Timor Sea Trough by 3000 m (Figure 1). The northwestern Timor ‘wall’ of the trough comprises numerous stacked reverse faults while the Australian margin is normally faulted caused by bending of the Australian leading edge. This loading causes the normal faults to propagate from the surface downwards and in several instances to cut the Jurassic reservoir. Vertical migration up the extended and open fault planes generally occurs sub-parallel with the axis of the trough and is expressed at the seabed by reefs nourished by the escaping hydrocarbons (light blue areas in Figure 1). The distribution of productive fields (Laminaria-1) and discoveries (Oliver-1ST1) amongst paleo accumulations (Cockell-1ST1 and Keppler-1) suggests a complexity of sealing and non-sealing faults.


Fault History

Fault planes can be considered as zones of shear and range between two end members: extensional normal faults (N) where cohesion along the fault zone is lost and the fault is non-sealing, and compressional reverse faults (R) where minor intermittent fault breaks are healed due to essentially no loss of cohesion (Figure 2). Between these end members there are varying components of shear in both horizontal and vertical planes: extensional strike slip (ESS, left clay model of Figure 2) where the divergent component of the fault plane results in greater leaking potential or a weak seal, through ‘parallel’ strike slip (SS) to compressional strike slip (CSS, right of Figure 2) where cohesion is temporarily lost during intermittent fault growth and then healed by strongly sealing brecciation, resulting in moderate to strong seal. At any time in the history of every fault it may experience any of these stress states. The stress state can be determined and quantified and seal risk can be expressed as a percentage for prospects (fault seal) and for fields (flow barriers/compartmentalisation).


Normal Faults – Horizontal

A deformation ellipse has been placed on Figure 1 with the normal fault parallel with the reefs. The en echelon distribution of the reefs implies a left lateral shear in the horizontal plane. A differently oriented deformation ellipse is required to account for the reverse faults on the Timor wall of the trough. More likely there is a myriad of ellipses required to explain the details of each fault or fault set over the entire area today and in the past.


Normal Faults – Vertical

While the concepts of compression and grain crushing by reverse faults leading to sand against sand seals are easy to grasp, the fact that normal fault dip in the vertical plane is a key determinant of fault seal or non-seal, requires explanation. In Figure 3(a) the deformation ellipse in Figure 1 has been flipped into the vertical plane, which is the correct plane to use as a ‘strain’ ellipse for analysing normal fault formation provided SV and SH are swapped around. The deformation ellipse, on a single strain diagram, shows the change in dip of a normal fault plane from vertical at the surface (rigid basement) through lower angle shear (strike slip or ‘conjugate shears’) to the near horizontal dip in ductile lower crust. The same progression can be seen in a sedimentary basin from surface sediments to deeper ductile shales and salt. This fault dip change, and increasing cohesion with depth, can be expressed as an arc in Figure 3(b) or, after Anderson, as the shaded fault plane in Figure 3(c).


Fault Stress Analysis

Figure 4(a) is an arbitrary line between Oliver- 1ST1 and Cockell-1ST1 (Figure 1) from a small 3D survey on the Northwest Shelf of Australia. The Oliver Fault is growing as a normal fault today, but the fault plane is not a simple curve and seal potential will vary significantly due to this geometrical factor.


Since increasing compression favours increasing fault seal, Figure 4(a) has been mapped at horizons which are globally more likely to have experienced some compression during deposition. The shallow data are good but become ‘fair’ at the Mid Jurassic (Callovian) Plover Formation which contains a 174 m gas and minor oil leg. Software is used to form isochores between successive pairs of horizons to create the stratigraphic section equivalent to Figure 4(a) as Figure 4(b); it then converts the isochores and any normal and reverse faults to extended Anderson stress states to form a 3D stress volume from which Figure 4(c) was created, also between Oliver-1ST1 and Cockell-1ST1. It is now possible to determine the full stress state history on a fault by relating seal potential or risk via Figure 2.


Fault Seal/Flow Barrier Confidence

The red curves in Figures 5(a) and 5(b) are the paleo stresses from the Oliver and Cockell faults (Figure 4(c)) and are presented on fault seal graphs. The Top Triassic (LTr) was a reverse fault stress state at Oliver which was averaged along its length at 89% fault seal confidence (the Cockell fault was cut at this horizon by the edge of the survey). Both faults show some strike slip, low to moderate seal potential (47% versus 44%) in the Late Jurassic Vulcan Formation regional seal. Both experienced stronger seal potential beginning in the Late Paleocene compression (70% versus 95%) due to Australia commencing its northward plate tectonic advance (see the previous article ‘Stress from Seismic’). Normal faulting to extensional strike slip has imparted no to low sealing potential (12%) since the latest Oligocene collision with Timor on both faults. The Oliver Fault had a minimum of 47% seal confidence across thick Vulcan sealing shales. The actual value would be greater due to the extra gouge formed by the Late Paleocene compression (70%); likewise the Cockell fault would have been higher than 44% due to the same compression (95%) and due to Top Vulcan compression (79%). Hydrocarbon migration in the area is estimated to have been Late Cretaceous – Early Tertiary indicating hydrocarbons were probably sealed in both structures by the Mid Jurassic Plover Formation at that time. The Plover had weak seal potential (minimum of 28% at Oliver versus 18% at Cockell) indicating no flow barrier seal and no compartmentalisation.


Last Extension

Late normal faulting could have disrupted the seals with leakage to the M. Eocene potential thief zone, or to the seabed. The black curves on Figures 5(a) and 5(b) (‘Last Extension’) examine these possible fault leak disruptions across the Vulcan seal. In the case of Oliver the normal fault cuts the seabed at 70° dip (bottom axis, Figure 5(a)). The dip is 51° across the Late Jurassic Vulcan with fault seal confidence of 54%. This is shown by the dotted black line and fault seal is actually higher than the ‘red’ minimum paleo stress state seal confidence (47%). The late normal fault did not disrupt the seal, hence the Oliver discovery.


In the case of Cockell, the normal dip was uniformly steeper at 66° with less than 35% fault seal confidence across the Vulcan sealing unit, a reduction on the red curve (44% minimum), hence late fault seal failure probable. This was confirmed by the paleo accumulation shows in the well, a reflection of the regional Timor-induced problem. Note the southeasterly trending row of small reefs at Cockel-1ST1 on Figure 1, but not at Oliver-1ST1.


Also note that the late normal faulting would have had negligible influence on any potential flow barriers at Oliver below the Vulcan seal as stress-induced seal confidence is only 28% and extension related seal confidence is only 34% in the Plover Formation reservoir.


Considering fault history in terms of stress and fault geometry allows geologists and engineers to quantify the principal factor that controls fault seal/flow barriers.





Fig. 1. Recent thrusting of Timor and extensional normal faulting outlined by reef growth resulting from hydrocarbon escape to the seabed on the Australian margin.




Fig. 2. Extended Anderson stress states with extensional strike slip or divergent shear clay model with negative relief (left) and compressional strike slip or convergent shear clay model with positive relief (right).




Fig. 3(a) The deformation ellipse for a normal fault, (b) Diagramatic normal fault in section with changing deformation with depth, and (c) Normal fault plane (after Anderson).






Fig. 4(a) Arbitrary seismic line between Oliver-1ST1 and Cockell-1ST1. (b) The stratigraphic section between Oliver-1ST1 and Cockel-1ST1. (c) The stress section for the arbitrary line between Oliver-1ST1 and Cockell-1ST1.




Fig. 5(a) Fault seal confidence graphs for Oliver-1ST1 and (b) Cockel-1ST1.