The discussion paper raises two main concerns:

1. The data provided and described are insufficient to recreate our case study models to examine them independently, and

2. Authors do not believe there is sufficient evidence for capillary seal in the case studies, specifically:

   1. Due to unaccounted uncertainty in vertical seismic resolution, the juxtaposition geometry might mean that fault rock is not supporting the column.

   2. Based on gamma ray values in one well log, the authors believe the Eumeralla Formation provides juxtaposition seal in the Boggy Creek field, rather than fault seal as per our original interpretation.

The main objective of our paper was to investigate the fluid property effects on fault-rock seal. We made a deliberate choice to demonstrate the uncertainty effects due to fluid properties, separate from the overall uncertainty of structural interpretations. The aim was therefore to show the working, which could be reapplied in other localities, rather than provide a comprehensive geological description of these already well-studied fields. However, the considerations suggested by the authors do not change the overall interpretation of our model.

Data availability

The data used for this study principally consist of historic well logs and reports, 3D models, and seismic surveys. The well logs and reports (GSV 2012) are publicly available, and contain all information needed to address these specific concerns: define stratigraphy, GWCs, flow units and V-Shale curves. The 3D models used in our study are described in detail in the original papers (Lyon et al. 2004, 2005a, 2005b, 2007; Ziesch et al. 2017). The models were combined with 3D seismic surveys supplied by CO2CRC (Balnaves, Haselgrove OGF93A, ONH01 and Curdie Vale). Seismic surveys were converted to depth using published velocity models (Lyon et al. 2004; Ziesch et al. 2017) to check the validity of key structures important for this study. We agree that our presented structural models cannot be recreated solely from our figures, but unfortunately we do not own the copyright to the 3D models and seismic surveys. However, the data can be accessed by contacting the original copyright owners. To the specific request of contour maps, the Penola Trough map is published in Karolytė (2018) (pages 110, 142).

Vertical uncertainty

The top Pretty Hill reservoir seismic interpretation from Lyon et al (2004) was tied to the true depth (from mudlogs) at 9 wells and shown to have a maximum mis-tie at depth of 180 m. A new velocity model was created which ties well log and seismic formation tops and the horizon (see Karolytė 2018 for full description). Five of the wells are within 1 km of the Ladbroke Grove fault, increasing the confidence that the fault/horizon intersection geometry is represented accurately. The vertical uncertainty can therefore be inferred from the remaining vertical difference (Δz) between the observed true depth at the wells and the apparent depth of the seismic picks. The values of Δz range from −28–20 m (Fig. 1a). This observed range represents a sample of all Δz values within the 3D horizon. It is reasonable to assume that the full range of Δz, as with most measurement errors, can be represented by a normal distribution. We present a Bayesian inference posterior distribution of the Δz (Fig. 1b), using non-informative priors and observed Δz values. The resulting probability distribution suggests Δz is −3 ± 38 m (2σ).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 1.

(a) The difference between true well depth and apparent depth of the seismic horizon pick (Δz) plotted against true well depths in Penola Trough. (b) Data from Figure 1a displayed as a histogram, compared to a simulated normal distribution representing the inferred full data spread, which incorporates more extreme values than captured by the data sample.

Our paper states that 31 m of gas column is supported by the fault-rock in the Katnook field (Table 1). Based on the above analysis, we can expect a range of different scenarios of gas column heights. They range from events where the GWC is higher than the top reservoir fault cut-off and therefore fault rock is not in contact with the column, to where the fault rock is supporting a larger column than we identified (Fig. 2). Taking this into account, our updated estimate of the mean gas column height is 28 m. By this analysis, there is a 7.7% chance that the GWC is above the fault cut-off and therefore fault-seal does not exist. However, we show there is 92.3% chance that the fault rock is supporting a gas column, and a 50% chance that this column is larger than our mean model (Fig. 2).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 2.

The 95% highest posterior density (HPD) interval of possible column heights in the Katnook reservoir against the Ladbroke Grove fault. The mean column height is 28 m. There is a 92.3% chance that the gas column is against the Ladbroke Grove fault, and a 7.7% chance that the GWC is above the top reservoir fault cut-off.

Regarding the authors’ second suggestion that the trap could be filled to spill, this would require the vertical uncertainty at our identified spill point (2890 m ± 38) to be outside the 2σ range. Using the posterior distribution of Δz outlined above, we calculate that the probability of the spill point being at or above the depth of GWC (2842 m) is 2%.

For Boggy Creek, the effect of vertical uncertainty is not significant, because the fault-supported column is sufficiently high (51 m) such that any uncertainty in the depth of the top reservoir fault cut-off relative to the GWC would still result in a gas column against the fault, and the spill point is sufficiently low (221 m below GWC), so the overall interpretation about the sealing structures is not impacted.

Lastly, while we did not formally discuss all the traps in the wider study area, but we assessed them and did not identify fault-rock seal.

Eumeralla Formation

The authors suggest that fault-rock seal does not exist in the Boggy Creek field, based on their interpretation of the Eumeralla Formation as a sealing unit. The Eumeralla Formation has been widely studied and documented to consist of interbedded lithic sandstones, siltstones and shales (Cockshell et al. 1995; Felton 1997), containing live and breached hydrocarbon columns and oil shows (Lisk 2004). The interbedded shales are considered to be ineffective seals, due to the presence of well-connected sandstones (Svendsen et al. 2004). Sandy layers cannot be easily detected by gamma ray logs due to high (up to 52%) volcanoclastic content (Svendsen et al. 2004). The mudlog (GSV 2012) analysis in the adjacent Buttress field shows that the bottom 25 m of the gas column is hosted by the underlying Eumeralla Formation. The 55 metres of Eumeralla Formation described in this mudlog (Fig. 3) demonstrate a high proportion of sandstone, similar to that of the Waare reservoir rocks. Taking all this into account, there is no compelling evidence to suggest that Eumerella can be considered a seal.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 3.

Annotated Buttress-1 mudlog. Depths in mRT (metres below rotary table), RT = 50.7 m. Grey indicates siltstone/clay, yellow – sandstone. Adapted from GSV (2012).

Summary

The purpose of our paper was to demonstrate the effect of uncertainty on fluid properties in fault seal. The discussion points raised by the authors do not change our interpretation of the case studies and the underlying data is available as explained in detail above. The vertical uncertainty derived by Bayesian inference demonstrates that the outcomes where the interpretation of fault-rock seal is different to that proposed in the original manuscript are restricted to the tail-end of the probability distribution in the Katnook field. Vertical uncertainty does not have an impact on the interpretation of the Boggy Creek structure. We therefore accept that there is a c. 8% probability that fault-rock seal does not support a gas column in one of our two case studies. However, we do not accept that the right approach to address this uncertainty is to select worst-case outcomes from the tail end of the probability distribution as an argument against the overall model validity. Further, the suggestion that the Katnook field may also be filled to spill requires a greater column of gas on the fault than we suggest. At Boggy Creek, there is no reason to believe the Eumerella is a sealing formation.

The concerns raised by the authors on the different technical aspects of the case studies all seem to point towards their preferred interpretation that fault-rock capillary seal does not exist. However, we show that the data suggest a high probability of fault-rock seal.