Austral Pacific Energy Ltd. 2007. Petroleum Modelling in PEP38258 Offshore Canterbury Basin New Zealand. New Zealand open-file Petroleum Report 3504.Wellington: Ministry of Economic Development.

Cartwright, J.A. and Dewhurst, D.N. 1998. Layer-bound compaction faults in fine-grained sediments. Bulletin of the Geological Society of America: 110, 1242-1257.

Cartwright, J. 2011. Diagenetically induced shear failure of fine-grained sediments and the development of polygonal fault systems. Marine and Petroleum Geology: 28, 1593-1610.

Gay, A., Lopez, M., Cochonat, P. and Sermondadaz, G. 2004. Polygonal faults-furrows system related to early stages of compaction - upper Miocene to recent sediments of the Lower Congo Basin. Basin Research: 16(1): 101-116.

Leary, R.O. and Mogg, W.G. 2008. The petroleum potential of the offshore Canterbury Basin: insights from petroleum system modelling. PESA Eastern Australian Basins Symposium III: Sydney: 349-368.

7. References

Figure 1. Location map of the offshore Canterbury Basin showing 3D (Waka 3D) and 2D seismic data coverage and drilled wells.

Canterbury Basin is one of the southeast frontier basins in New Zealand and is under active

exploration. Marine mudstones within the Cretaceous and Paleocene succession are the main seal

rocks for the underlying Cretaceous reservoir units in the basin.

High-quality 2D and 3D seismic data reveal the presence of extensive Paleocene and Eocene

Polygonal Fault Systems (PFS) in the offshore Canterbury Basin.

These extensive layer-bound normal fault systems typically develop in fine-grained (hemi-pelagic)

sediments during early stages of burial and compaction (Cartwright and Dewhurst, 1998;

Cartwright, 2011). The presence of PFS provide information on the depositional environment in

areas with limited well coverage.

Timing of development of PFS in relation to the petroleum expulsion is critical for understanding

fluid migration and basin prospectivity.

In this study we show the distribution of polygonal faulting in the basin, utilising the detailed

geometry of the faults in the Waka 3D seismic volume. We also discuss their significance for

interpreting paleo-depositional environments and lithologies, and implications for fluid migration.

1. Introduction

3. Characterisation of the Polygonal Fault Systems (PFS)

2. Data and methods

— Polygonal Fault Systems (PFS) in the offshore Canterbury Basin occur in three layer bound fault tiers and are widely distributed in the Paleocene and Eocene intervals.

— Presence of PFS indicate a mudstone-dominated facies in an outershelf–bathyal depositional environment.

— Especially during early stages of compaction, PFS in the Paleocene and lower Eocene intervals may have acted as conduits (seal bypass systems) for petroleum expelled from the mid Cretaceous source rocks.

— At present there is a lack of clear evidence of non-intrusive related fluid escape features within polygonal fault intervals and it may indicate a sealing nature of these faults. Further analysis is required to determine whether PFS negatively impact on seal integrity.

6. Conclusions and uncertainties

Acknowledgements

This project was carried out as part of on-going research into the evolution of New Zealand's petroleum basins, as part of the Petroleum Basin Research (PBR) programme at GNS Science, funded by direct core funding provided to

GNS Science by the New Zealand Government. We would like to thank Paradigm for providing licenses of interpretation software for this work.GNS Science

GNS Science1 Fairway DriveAvalonLower Hutt 5010PO Box 30368Lower Hutt 5040New ZealandT +64-4-570 1444F +64-4-570 4600

Polygonal Fault Systems in the Canterbury Basin, New Zealand

1Tusar R. Sahoo , Hannu Seebeck and Andy Nicol1t.sahoo@gns.cri.nz

4. Implications on facies and depositional environment interpretation 5. Implications for fluid migration

— PFS are mainly observed in the mudstone dominated layers (e.g. Clipper-1 in Figure 7).

— All mudstone-dominated layers do not contain polygonal faults (Figures 6 and 7). Polygonal faults may be confined to mudstones dominated by clay-sized particles.

1 km

top Paleocenetop Paleocenetop Paleocene

intra Eocene 1intra Eocene 1intra Eocene 1

intra Eocene 2intra Eocene 2intra Eocene 2

intra Eocene 3intra Eocene 3intra Eocene 3

Figure 2. A 3D image showing examples of coherency attribute along interpreted horizons and seismic intersections in the Waka 3D seismic survey.

Figure 3. Coherency attribute map at top of the Paleocene showing presence o f Po lygona l Fau l t Systems (PFS).

Figure 4. Seismic section showing interpreted horizons and PFSs. Three tiers of faults are observed; tier-1 occur mainly within the Paleocene interval; tier-2 occur within the lower Eocene interval; tier-3 occur within the upper Eocene interval. Polygonal faults in the upper Eocene (tier-3) are highly faulted, well connected, and have more vertical extension compared to other fault tiers.

Figure 5. A vertical stack of horizon attributes across fault tiers showing a network of polygonal faults. VE: vertical extension of faults, FT: fault throw; FS: fault spacing. PFS in horizon slices at intra Eocene 2 and intra Eocene 3 are with higher fault intensity (least fault spacing 150–350 m) compared to older horizon slices at intra Eocene 1 and top Paleocene.

Figure 6. Coherency map showing distribution of polygonal faults in the Waka 3D seismic data. Absence of tier-2 PFS in the northwestern part of the Waka 3D may be due to lower proportion of clay-sized particles.

Figure 7. Well Clipper-1 showing cuttings lithology and equivalent zones containing PFS in deeper parts of the basin.

Figure 8. Gross depositional environment map at top Paleocene level. PFS distribution has been used to define mudstone-dominated layer deposited in an outer shelf-uppermost bathyal environment.

— Previous modelling suggests that expulsion from the mid Cretaceous (Clipper Formation) source rocks commenced as early as the Paleocene (Austral Pacific Energy Ltd, 2007; Leary and Mogg, 2008).

— Global examples of PFS suggest that they can develop within 100 m of the seabed in water depths of >1000 m (Gay et al., 2004).

— The presence of a pervasive network of polygonal faults in the Paleocene and Early Eocene intervals may have contributed to a increased rate of loss of petroleum generated from the mid Cretaceous source rocks.

Waka 3D has been used to characterise the fault geometry and displacement of the polygonally faulted intervals. This provides the basis for mapping the distribution of PFS throughout the basin using the available 2D seismic data.

Figure 9. Cumulative expulsion rate history for a 100 m thick interval of mid Cretaceous source rocks. Expulsion of oil and gas started during the Paleocene (modified f rom Austra l Paci f ic Energy Ltd, 2007).

172°E

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°S

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Resolution–1Resolution–1Resolution–1

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Galleon–1Galleon–1Galleon–1

Clipper–1Clipper–1Clipper–1

0 50 km

Legend

Study area for facies analysis

Petroleum exploration wells

+ Gas shows

0 Gas & Condensate shows

ª Dry hole

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2D Seismic lines

Waka 3D Seismic survey

Waka 3D

Figure. 4

Mapping of age markers and key horizons through PFSs

3D attribute volumes

Attribute analysis along horizons and horizon slices

Mapping of age markers and key

horizons through PFS

Characterisation of PFS

3D attribute volumes

Attribute analysis along horizons and horizon slices

Implications on facies, depositional environment and

fluid migration

200 m

s

3 km

PFSs (tier-3)

PFSs (tier-1)

PFSs (tier-2)

top Paleocenetop Paleocenetop Paleocene

intra Eocene 1intra Eocene 1intra Eocene 1

intra Eocene 2intra Eocene 2intra Eocene 2

intra Eocene 3intra Eocene 3intra Eocene 3

top Eocenetop Eocenetop Eocene

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Crossline 1410

highly faulted and connected network of polygonal faults

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Shoreface-shelfal

Shelfal (mudstone & siltstone)

Outer shelf–uppermost bathyal;

Bathyal (mainly mudstone)

Igneous intrusions/volcanics

(sandstone & siltstone)

(mudstone) presence of polygonal faults

Resolution–1Resolution–1Resolution–1

Cutter–1Cutter–1Cutter–1

Endeavour–1Endeavour–1Endeavour–1

Galleon–1Galleon–1Galleon–1

Clipper–1Clipper–1Clipper–1

Presence of PFS at the Paleocene level

Waka 3D

60 50 40 30 20 10 0

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top Paleocene (tier-1)

Horizon slice 680 ms below top Eocene (tier-2)

Polygonal faults observed in the Waka 3D seismic data

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zones containing polygonal faults in deeper parts of the basin

Clipper-1 Gross depositional environment map at top Paleocene (56 Ma)

gas

oil

Age (My)

Tier-3 PFSVE: 400 msFT: 10–20 msFS: 150–350 m

Tier-2 PFSVE: 150 msFT: 10–20 msFS: 200–450 m

Tier-1 PFSVE:150 msFT: 10–20 msFS: 350–600 m

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Horizon slice 180 ms below top Eocene (tier-3)