Seismic interpretation of structural features in the Kokako 3D seismic area, Taranaki Basin (New Zealand) Edimar Perico*, Petróleo Brasileiro S.A. and Dr. Heather Bedle, University of Oklahoma.

Summary

The Western Platform represents an important tectonic unit The basin experienced different tectonic regimes over time in the Taranaki Basin and can considered a stable region. and the hydrocarbons traps are strongly related to Although, seismic characterization of Late Cretaceous to compressive structures (Coleman, 2018). King and Thrasher Early Miocene intervals reveals changes in the deformation (1996) highlight three main stages for the Taranaki Basin styles of normal faults. In some cases, the features vary development: (1) mid-Late Cretaceous to Paleocene intra- vertically from concentrated brittle zones (main planar continental rift transform; (2) Eocene to Early Oligocene discontinuity) to wide damage areas with deformation passive margin and (3) Oligocene to Recent active margin structures composed by smaller fault segments. In addition, basin. Muir et al. (2000) relates the origin of the basin with possible correlations were presented between structural NE and NNE basement faults formed during mid-Cretaceous features and siliciclastic deposits. Key takeaways from this due to the break-up of Gondwana. Considering the main research can be applied to areas with high hydrocarbon rifting processes, Strogen et al. (2017) recognize two phases: potential to guide the exploration of siliciclastic reservoirs NW to WNW half-grabens (c. 105 – 83 Ma) and an associated with faults. Lastly, the structural features extensional regime responsible for the formation of NE described were correlated with regional settings and help to depocenters (c. 80 – 55 Ma). Franzel and Back (2019) understand the geologically evolution of this particular summarizes four stages of development in the Taranaki region of the Taranaki Basin. Basin: (1) rift basin; (2) passive continental margin; (3) foreland basin and (4) back-arc basin. Introduction Moving from the basin to a regional scale, the Western The Taranaki Basin represents an important source of oil and Province is composed predominantly of Paleozoic gas in the western part of New Zealand´s North Island. The metasedimentary rocks with granitoids intrusions (Muir et study area is located within 20 km distance from the Tui oil al. 2000). According to King and Thrasher (1996), the field. The local geological entity is the Western Platform, a platform was not deformed by Neogene tectonic processes. stable block when compared to Central Graben terrain. Implications due to the presence of faults were also (Figure 1). discussed in late Tertiary deposits (Giba et al. 2010). During the Oligocene and Eocene the paleoshoreline was NE-SW oriented, with the proximal area situated in the south and the distal in the north (Higgs et al. 2012). DOI:10.1190/segam2020-3426487.1 The brief literature review presented shows how complex the geological evolution of the Taranaki Basin. Contemporaneous compressional and extensional regimes are responsible for developing different structures. The major objective of this study is to evaluate seismic reflectors related to Late Cretaceous to Early Miocene deposits and discuss the main observations with regional context. An important motivation for this study is the importance that fault characterization has in the petroleum systems. Faults represent a fundamental element to hydrocarbons migration and, as highlighted by King and Thrasher (1996), the majority of traps in the Taranaki Basin are structural traps.

Data and methods

The research was based on three main sources of

information: literature review; seismic interpretation and Figure 1: Location of the study area in the western part of New well logs. The mapping was based on the Kokako 3D marine Zealand. Note the predominance of NE-SW strucutral features and the main oil and gas fields. After MBIE (2015). seismic survey that was acquired in 2013 and covers an area of 593 km2 in the Tasman Sea (DownUnder, 2013). The Downloaded 11/04/20 to 68.97.118.233. Redistribution subject SEG license or copyright; see Terms of Use at https://library.seg.org/page/policies/terms

© 2020 Society of Exploration Geophysicists 10.1190/segam2020-3426487.1 SEG International Exposition and 90th Annual Meeting Page 1130 Seismic interpretation of structural features in the Kokako 3D seismic area

dataset used to map horizons and the faults is a full stack (PSDM volume converted to time). Two wells located in the area (Taranui-1 and Takapou-1) were correlated with seismic to define the age of specifics reflectors and the temporal distribution of some faults.

Seismic attributes were used to help the identification of faults. Variance (edge method) and energy-ratio similarity attributes highlighted structural discontinuities such as faults and also stratigraphic terminations (channel edges). Curvature attribute is represented by two components: k1 (most-positive) and k2 (most-negative curvature) that considered together, describe the shape of a surface.

Results Figure 2: Inline 1664 (full stack) with location of the interval of interest and the horizons mapped (A, B and C). The deformation if Three horizons were recognized in the interval of interest marked, essentially, by normal faults with small displacements. and two of them are the most important for next discussions (Figure 2). Horizon “A” was mapped as a peak (increase in impedance) and corresponds to Horizon 1 discussed in Franzel and Back (2019). Horizon “C” was mapped as a trough (decrease in impedance) and is related to the base of limestones from Tikorangi Fm. based on Taranui-1 well completion report (AWE Ltd, 2017). This last seismic surface can be considered a regional reflector that corresponds to Horizon 2 from Franzel and Back (2019), deposited about 19 Ma. The selection of the horizons to be

mapped was based on changes in distribution and geometry of the main faults. Furthermore, many discontinuities end close to Horizon C (yellow arrows in Figure 2).

The mapped faults show a predominace of N-S and NNW- SSE structures close to the "seismic basememt" as demonstrated with Horizon "A" (Figure 3). The dip azimuth

DOI:10.1190/segam2020-3426487.1 attribute highlighted these discontinuities in the surface. In contrast, the deformation in strata close to the Tikorangi Fm. is also marked by smaller fault segments oriented to NE-SW. Considering the style, in general, the faults are marked by a single deformational surface in the base of the structure that change in the upper part to synthetic and antithetic segments as noted in Horizon “C” (Figure 4).

More continuous features can be observed in the full stack (Figure 5A) and variance attribute, but small structures were better defined with curvature and energy-ratio attributes (Figure 5B). In some situations, these small faults can be restricted to specific strata, such as the interval between horizons B and C (red polygon). Changes in the shape of the seismic reflector can also reflect the presence of minor faults (Hart, 2011) and the curvature attribute seems to enhance Figure 3: Main horizons mapped in time domain (A, B and C) with these faults (small throw), as observed inside the black the corresponding dip azimuth attribute. polygon (Figure 5D).

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DOI:10.1190/segam2020-3426487.1 Figure 4: Composite line with an example of a commom fault geometry observed in the full stack volume and in the variance attribute data.

In specific situations it was possible to note a correlation between the fault geometry and the depositional style (Figure 6). Highly sinuous channels (younger than Tikorangi Fm.) show internal structures such as lateral migration features and seems to be related with more distributed faults (various sintetic and antithetic segments) in a wide deformatinal zone (yellow arrows – Figure 6). On the other Figure 5: (A) Full stack section with horizons and faults mapped. hand, there is a correlation between a straight channel and a (B) Energy ratio attribute of the same seismic section. Note the main fault segment (red arrows). This relation between fault improvement in the identification of small faults (red elipse). (C) distribution and depositional systems can be observed MapView of the energy ratio attribute. It’s possible to see NW-SE specially in the SW poriton of the study area, in the basal discontinuities.(D) MapView of curvature attributre used to enhance part of the channels. Over time, the siliciclastics deposits the identification of small curvatures in the reflectors that also tend to culminate with sinuous patterns. reflects discontinuities such as the NE-SW features (black elipse).

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Discussions

Deformation style in the normal faults changes over the time from a main fault discontinuity to smaller segments in the upper part (wide deformation zone). Vertical differences observed could be related to variations in the competence of the rocks and also variations in the stress field (lateral compression).

The changes in the direction of the structures over time seems to be in accordance with regional faults described by Strogen et al. (2017). For instance, NE-trending extensional faults can be related to the second rift event recognized by the authors (c. 80 – 55 Ma).

Considering the seismic attributes, the variance and the energy-ratio similarity highlighted the “big-picture” features and the curvature seems to enhance more detailed features. Changes in the curvature of the NE-SW oriented faults helps to delineate possible the discontinuities that possible have small displacements.

Conclusions

1. The use of different seismic attributes improve the mapping of structural features because each attribute

enhances a specific characteristics of the seismic dataset.

2. Late Cretaceous to Early Miocene faults impact the initial stratigraphic record of some deposits younger than the Tikorangi limestones.

3. Straight or highly sinuous channel system show local

DOI:10.1190/segam2020-3426487.1 agreement with the presence of a major fault segment or a wide deformational zone, respectively.

4. Other elements certainly interfere in siliciclastic deposits geometry (e.g. sediment supply, base-level variations), but structural elements can play an important control is some cases.

Acknowledgments

Special thanks to PETROBRAS for funding my master program at the University of Oklahoma and also to New Zealand Petroleum & Mineral (NZP&M) for making datasets public. The authors would like to thank to Dr. Mike Soreghan and Dr. Brett Carpenter for the constructive comments. We thank Dr. Marfurt and David Lubo-Robles Figure 6: Possible correlation between faults and depositional for the comments and support with the use of AASPI patterns. (A) Arbitrary line with two channels: a high sinuous software. Also, thank Schlumberger for Petrel licenses (yellow arrow) and a straight type (red arrow). (B) Arbitrary line with the same channels, in other position. (C) Time slice of the full provided to OU and AASPI. We thank all SDA members for stack with the two depositional systems. helpful comments. Downloaded 11/04/20 to 68.97.118.233. Redistribution subject SEG license or copyright; see Terms of Use at https://library.seg.org/page/policies/terms

© 2020 Society of Exploration Geophysicists 10.1190/segam2020-3426487.1 SEG International Exposition and 90th Annual Meeting Page 1133 REFERENCES

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