Subsurface S_HMAX determined from a borehole image log, onshore southern East Coast Basin, New Zealand

ABSTRACT

Understanding the in-situ stress field in the subsurface provides insights on the tectonic processes and seismic hazards of a basin. This paper presents observations and interpretations of present day maximum horizontal stress (S_Hmax) orientation from well Ranui-2 located in the onshore southern East Coast Basin. Fractures identified in the Ranui-2 image log have been divided into distinct subsets based on fracture orientation. These subsets correspond in a general sense, to changes in lithology in the well, which are associated with changes in formations. Analysis of the image log shows the present day S_Hmax in the borehole has two primary stress directions, W–E and NW–SE, and most of the fractures strike NE–SW. The change in S_Hmax occurs at a depth of circa. 1130 m measured depth (918 m true vertical depth subsea), below the depth at which the borehole changes from vertical to ≥10° deviation. When these data are compared to the regional stress field obtained using other stress indicators such as earthquake focal mechanisms, the results are consistent with each other. This study shows that shallow subsurface indications of present-day S_Hmax in southeastern North Island reflect S_Hmax orientations from deeper within the subsurface.

KEYWORDS:

• Subsurface stress
• East Coast Basin
• Ranui-2
• FMI^TM image log
• S_Hmax
• borehole breakout
• drilling-induced tensile fractures

Introduction

Petroleum exploration wells have been drilled in the East Coast Basin since the late nineteenth century (Figure 1). Most of the geological information obtained from these wells has consisted of cuttings and conventional wireline log data (e.g. natural gamma, resistivity, sonic, and density logs can provide information on the lithology in the well) (e.g. BP Shell Aquitaine Todd Petroleum Developments Ltd 1976; Biros et al. 1995; Tap Oil Limited 2004). In recent decades, borehole imaging tools have been used in many wells overseas, providing an opportunity to compensate or complement information obtained from cuttings, core, and traditional wireline data sets taken in the well (e.g. Bell and Gough 1979; Prioul and Jocker 2009; Nie et al. 2013). Using image logs has in the past been considered an added expense, so are often not used in this country. Some recent wells however, including some in the Taranaki and East Coast basins, and the Taupo Volcanic Zone, have had borehole televiewer (BHTV) or resistivity imaging tools run in them and the data acquired from these imaging tools can provide valuable new information on the subsurface geology and structure (e.g. NZEC 2012a, 2012b; Rajabi et al. 2016; McNamara et al. 2017; Griffin 2018).

Figure 1. Map showing the location of the Ranui-2 borehole and other imaged wells in the southern onshore East Coast Basin, the active and inactive faults present in the area, and the sub-belts and structural blocks of Moore (1988a). The inset map shows the present plate margin boundary (HMST – Hikurangi Margin subduction thrust) and the relative plate motion vectors from Nicol et al. (2007). Boundaries are approximate.

Borehole image logs provide an oriented image of the inside of the borehole at resolutions of ≤1 cm (Lagraba et al. 2010). Image logs are best used for adjusting core depths to log depths, identifying bed contacts, bedding dips, and structural and sedimentary features. Image log interpretation can measure in-situ bedding, fractures, and in-situ stress in the borehole, which are features that cannot be obtained from the conventional wireline log suite or conventional core (e.g. Plumb and Hickman 1985; Prensky 1999).

Understanding the in-situ stress field in the subsurface from borehole image log data provides valuable observations and constraints for models of tectonic processes and the driving mechanism of plate motion that are not easily obtained from other methods, for example, in Iran (Rajabi et al. 2010), north China (Nie et al. 2013), and New Zealand (Rajabi et al. 2016). These data can be compared with surface geology observations and together, can assist in assessments of long-term seismic hazards (Hickman et al. 1985; Finkbeiner et al. 1997; Prensky 1999; Rajabi et al. 2010). In-situ stress data from borehole image logs when used in conjunction with the natural fracture distribution also obtained from the image logs, enables the relationship between fracture formation, the current stress field, and the regional tectonic history to be established (Hickman et al. 1985; McNamara et al. 2015; Massiot et al. 2017). Fractures interpreted from borehole image logs, that are most prone to tensile or shear failure in the present-day stress, typically those striking approximately parallel or within 30° of the maximum principal stress orientation, are often observed to transmit the largest volumes of fluids (Barton et al. 1995; Sibson 1996). This may allow enhanced fluid flow within the rock, making it a suitable candidate for unconventional resources.

Since the 1990s, image logs have been acquired in several onshore and offshore petroleum wells in the southern East Coast Basin, eastern North Island (Biros et al. 1995; Tap Oil Ltd 2004; ECEV III Ltd 2012; NZEC 2012a, 2012b). One of these wells Ranui-2, located near Masterton (location 40.94231667°S, 175.91781944°E; Figure 1), was drilled as a stratigraphic borehole in 2012 by East Coast Energy Ventures (ECEV III Ltd), a subsidiary of New Zealand Energy Corporation (NZEC), mainly to assess the unconventional petroleum potential of the area. This paper presents observations and interpretations of the image log dataset from this well, and details the in-situ stress features and fractures present, and compares them to the present-day subsurface stress and nearby active faults at the surface within the southern onshore East Coast Basin.

Geological setting

The Wairarapa region occupies a now-uplifted part of a late Neogene forearc basin and associated inboard margin of an accretionary wedge, which are all collectively part of the actively deforming Hikurangi subduction margin (e.g. Wallace et al. 2004; Nicol et al. 2007; Bailleul et al. 2013) (Figure 1). The ‘Hikurangi margin’ presently accommodates oblique subduction of the Pacific plate beneath the Australian plate in New Zealand's North Island (De Mets et al. 1990, 1994) (Figure 1). The margin forms the transition from subduction of the Pacific plate along the Tonga-Kermadec trench (and associated back-arc extension in the Lau-Havre Trough) to continental collision and strike-slip movement along the Alpine Fault (Nicol et al. 2007). Subduction along the Hikurangi margin is inferred to have commenced about 24–40 Ma (Ballance 1976; Rait et al. 1991; Kamp 1999; Stern et al. 2006; Mortimer 2018) and currently accommodates a relative plate-motion vector of 42–48 mm/yr trending at c. 50° to the trend of the margin (Nicol et al. 2007) (Figure 1). Plate convergence (i.e. margin-normal motion) is principally accommodated on the subduction interface, while the remainder of the margin-normal motion and most of the margin-parallel motion (>50%) are accommodated in the upper plate through a combination of reverse faulting, strike-slip faulting, and vertical-axis clockwise rotations (Walcott 1989; Cashman et al. 1992; Beanland 1995; Barnes and Mercier de Lépinay 1997; Beanland and Haines 1998; Nicol and Beavan 2003; Wallace et al. 2004; Nicol et al. 2007; Lamb 2011).

Moore (1988a) divided the onshore East Coast Basin into two structural sub-belts (the Western and Eastern sub-belts) and seven structural blocks (Figure 1), based on the Cretaceous–early Paleocene succession. Each sub-belt and block is characterised by differing basement rocks, sedimentary fill, geological histories, and structural styles (Moore 1988a). Borehole Ranui-2 was drilled within the southern part of the Pongaroa Block of the Western Sub-belt (Figure 1). This block is bounded along its eastern side by the Adams-Tinui Fault complex, and comprises a gently-folded structure (at least within the Miocene succession), with a relatively complete succession of Early Cretaceous to Late Cretaceous-Paleogene age, unconformably overlain by middle–late Miocene and Pliocene strata (ECEV III Ltd 2012). Basement rocks comprise Early Cretaceous metasedimentary strata of the Pahau Terrane, although these were not intersected in borehole Ranui-2. The area around the Ranui-2 borehole is also bisected by several WSW–ENE-trending active strike-slip faults, such as the Carterton Fault (Langridge et al. 2016) (Figure 1). The strike of many of these faults is near-parallel to the strike of the Pacific-Australian plate convergence direction.

Within Ranui-2, the main imaged interval (between 843 and 1422 metres) intersected the middle Miocene Karamu Sandstone, latest Cretaceous–Paleocene Whangai Formation (Haumurian to early Teurian), Late Cretaceous Tangaruhe Formation, and the late-Early Cretaceous to early-Late Cretaceous Springhill Formation (Motuan to Ngaterian) (Figure 2, ECEV III Ltd 2012). Three intervals were cored, at 921–927 m, 1059–1061 m, and 1377.10–1383.10 metres measured depth (m md). The Whangai Formation, a highly fractured siliceous mudstone, is considered to be a possible unconventional reservoir unit in East Coast Basin (IGNS 2005). The formation, typically a 300–500 m-thick non-calcareous to calcareous mudstone to fine sandstone, was deposited over virtually the entire East Coast region during the Late Cretaceous to Middle Paleocene (Moore 1988b).

Figure 2. Schematic diagram showing the stratigraphy encountered in borehole Ranui-2, and the different data and depths at which they were acquired in the borehole. A schematic regional chronostratigraphic panel is to the right, illustrating the relationship between the stratigraphy in the borehole and the regional stratigraphy of the Western and Eastern Sub-belts. All depths are in metres measured depth (md). Borehole data from ECEV III Ltd (2012). Chronostratigraphy adapted from Bland et al. (2015). Stratigraphy not to scale.

Data and Methodology

Image and wireline log acquisition

Ranui-2 was planned as a vertical borehole, but the borehole deviated to a maximum of 25° to the southeast at 1420 metres measured depth (m md) (ECEV III Ltd 2012). Total depth drilled in the borehole was 1440.70 m md. A conventional wireline log suite was run in the borehole, along with a resistivity-based imaging tool (FMI^TM), with data acquisition over most of the well. The main image logged interval was between 843 and 1422 m md (631–1210 metres true vertical depth relative to sea level, m TVDss), with a repeat run logged over 840–940 m md (Figure 2, ECEV III Ltd 2012).

The conventional suite of wireline logs included gamma ray, calliper, acoustic, density, array resistivity, and neutron porosity. Most of the suite covered the interval 395.00–1440.18 m md (Figure 2, ECEV III Ltd 2012). The borehole image data were acquired using Schlumberger's FMI^TM resistivity imaging tool, a four-arm, eight-pad-based resistivity measurement device (for details see Schlumberger 1986). The pads are pressed against the borehole wall, which enables the array of electrodes organised in rows on each pad to record changes in micro-resistivity of the formation as the tool is slowly pulled up the borehole. The tool is sensitive to any spalling at the borehole wall (Zoback 2007), which makes the resulting dataset useful for determining any borehole breakout directions. The thin grey areas in Figure 3A and B represent the gaps between the electrode arrays on the pads of the tool where no data were collected.

Figure 3. Borehole breakouts (white boxes) from A, Above and B, below the change in deviation at c. 940 m md in the FMI resistivity image log from borehole Ranui-2. A low confidence DITF (dashed white line) has been interpreted at c. 1009.50 m. The breakout orientation does not coincide with the high side of the borehole (indicated by the red ‘H’ on the images), eliminating the possibility that the breakouts are keyseats. Sinusoidal features in the images have been interpreted as fractures. The grey lines represent the space between the pads on the FMI imaging tool. C, Schematic cross-sections of borehole breakout and DITF formation (adapted from Hillis and Reynolds 2000). Borehole breakouts form when circumferential stress around the borehole exceeds the compressive rock strength and thus are oriented parallel to S_hmin (σ_h). DITFs form when the circumferential stress exceeds the tensile strength of the borehole wall, thus are oriented parallel to S_Hmax (σ_H).

Image log processing and quality control

The raw acquisition imaging data from the borehole were loaded into Landmark Recall^TM 5.4 software, and rigorous quality control, including but not limited to, assessment of tool acquisition parameters and orientation data, speed corrections to account for tool stick, and static and dynamic image normalisation, was undertaken to assess the suitability of the raw data for processing and interpretation (see online supplementary material). Once this was completed, the images were processed using Recall^TM, which included: (1) standardising the image data, (2) applying a speed correction to remove speed irregularities induced as the tool was pulled up the borehole, (3) normalising the data statically to illustrate the gross variations throughout the imaged interval, and (4) re-normalising the data dynamically using a sliding window (1.0 m), to highlight more detailed features (Hansen and Buczak 2010; García-Carballido et al. 2010). The resulting images were displayed unwrapped with 0° (North, ‘N’) at the left-hand edge, 180° (South, ‘S’) in the centre, and 360° (N) at the right-hand edge (Figure 3). The reader is referred to Griffin (2018) or the online supplementary material for more detail on the image data acquisition and processing procedures used in this study.

Processed image log data provide information from inside the borehole including lithology, structural features (e.g. fractures, fault orientation, width and fill), and in-situ horizontal stress orientations (from induced features) (Lagraba et al. 2010).

Image quality

In this study, image log data quality was deemed good where >75% of the image was interpretable, moderate where 50%–75% was interpretable, poor where <50% was interpretable, and bad where the image quality precluded any attempt at interpretation. Image quality is generally moderate to good in the borehole, however poor hole conditions (indicated by the four-arm callipers) in the borehole has affected the image quality throughout the imaged interval (e.g. between 998–999.50 m, 1016–1017 m, 1025–1032 m, 1120–1127 m, and 1169–1380 m md) (see the Ranui-2 structural summary plot in the online supplementary material for the image quality log). Just under half (43%) of the imaged interval in borehole Ranui-2 has moderate (24%) to good (19%) image quality, and of the remainder, 47% is of poor quality. Where the image is interpretable (e.g. between 843–998 m, 1032–1120 m, 1127–1169 m, and 1380–1405 m md), confidence in the interpretation is high. Determining the quality of the images is important, as it gives confidence in the identification of features and subsequent interpretation.

Feature identification

Borehole image logs provide 360° coverage of the borehole wall based on rock property contrasts, allowing both natural (bedding, fractures, faults) and induced features (borehole breakouts, drilling induced tensile fractures (DITFs)) to be identified (Rider 1996). The image logs represent contrasts in resistivity between borehole lithology and the identified feature (Rider 1996). Features that appear dark on the processed images are of low resistivity (i.e. are conductive), whereas bright features are resistive.

Borehole breakouts appear as broad, parallel, dark (conductive/low resistivity), vertical, locally irregular features that occur 180° apart (i.e. opposite sides of the borehole) (Figure 3A and B). DITFs appear on resistivity image logs as paired or singular (depending on the gaps between the pads) narrow, well-defined dark (conductive) fractures that are vertical to sub-vertical, occurring 180° apart (Figure 3B). In this study, the breakouts and DITFs were more visible on the statically-normalised images (Figure 3A and B), however many of the DITFs are of low confidence due to the singular appearance and/or poor image quality at the depths they were identified at (e.g. 879, 966.50, 1067, 1086, 1149, 1153, 1169, and 1185 m) (see structural summary plot in the online supplementary material). Borehole breakouts and DITFs are orthogonal to one another (Figure 3C).

For this study, borehole breakouts and DITFs have been interpreted from the higher resolution image log alone, rather than the four-arm calliper tool run in the borehole. By analysing the logging run parameters, it appears that the calliper tool did not rotate in the borehole, meaning that the callipers did not encounter breakouts in the borehole.

Natural fractures and faults were also observed on the image logs (Table 1). These features were picked by matching sinusoids to observed geological features on the images (Rider 1996). Sinusoids were only picked where geological features could be recognised with confidence (Figure 3A and B). Both full and partial sinusoids have been included in the fracture strike orientations in the borehole. A full sinusoid is picked on a feature that can be correlated for more than >75% of the borehole circumference, whereas partial sinusoids are used for features that can be traced for less than 75% around the borehole. Details of the different features (both natural and induced), and their associated dip-type definitions can be found in the online supplementary material (Table S1) and in Griffin (2018).

Table 1. Summary of fracture and in-situ stress feature counts by formation, for the Ranui-2 borehole.

Formation	Formation depths (m md) (imaged interval only)	Low resistivity fractures	High resistivity fractures	Total fractures for formation	Faults	In-situ stress features
Karamu Sandstone	843.00–901.40	58	2	60	1	5 (BO)
Whangai	901.40–1122.50	272	11	283	–	12 (BO), 2 (DITF)
Tangaruhe	1122.50–1171.40	42	0	42	–	6 (BO)
Springhill	1171.40–1422.00	206	1	207	–	18 (BO)
Total		578	14	592	1	43

BO = borehole breakout, DITF = drilling induced tensile fracture. Formation depths are taken from the lithological descriptions in the well completion report (ECEV III Ltd 2012).

Determination of the S_Hmax orientation from drilling-induced tensile fractures and borehole breakouts

Bell and Gough (1979) first recognised the possibility of determining in-situ stress orientation from failures in borehole walls using four-arm dipmeter data acquired in oil wells. The data showed that many boreholes in Alberta, Canada, had intervals that were non-circular as a result of caving of the borehole walls. They demonstrated that this was the result of stresses around the borehole exceeding the strength of the rock, causing pieces of the borehole wall to spall off. The cavings, or breakouts, elongated the boreholes in a consistent direction both within the holes, and field wide.

The concentration of stress around a borehole can lead to compressive failures known as borehole breakouts and/or drilling-induced tensile fractures (Zoback 2007). Assuming that the vertical stress (S_v) is one of the principal stresses, borehole breakouts are indicators of the minimum horizontal stress (S_hmin) orientation in a vertical borehole. The breakouts form by compressional failure at azimuths where the concentrated stress in the borehole is most compressive (Figure 3C, Bell and Gough 1979; Barton et al. 2009). The maximum horizontal stress direction (S_Hmax) is perpendicular to S_hmin (Figure 3C), and forms when the minimum circumferential (hoop) stress around the borehole wall is smaller than the tensile strength of the rock (Zoback et al. 1985; Aadnoy 1990; Bell 1990; Barton et al. 1997; Hillis and Reynolds 2000; Barton and Moos 2010). DITFs occur in the borehole wall where the circumferential (hoop) stress is negative (extensional) and exceeds the tensile strength of the rock (Figure 3C, Hillis and Reynolds 2000; Barton et al. 2009). DITFs can be interpreted from image logs, where they appear as a set of conductive (dark) and sub-vertical fractures on opposite sides of the borehole, however they cannot be interpreted from oriented calliper logs (Rajabi et al. 2016).

In deviated wells, it can be difficult to identify borehole breakouts from oriented calliper logs, as they need to be differentiated from other borehole enlargements that are non-stress-related, such as key seats and washouts (Plumb and Hickman 1985; Zoback 2007; Heidbach 2016a). In addition to this, breakout occurrence, width, and depth can be a function of the rock strength, remote stress state, mud weight relative to formation fluid pressure, and borehole orientation relative to the principal stresses in an inclined borehole (Zoback 2007; Barton et al. 2009; Davatzes and Hickman 2010). Therefore, the position of the breakouts depends on the magnitude and orientation of principal stresses, as well as the orientation of the well with respect to the stress field, and do not necessarily align with the geographical orientations of the horizontal stresses (Peška and Zoback 1995; Zoback 2007; Barton et al. 2009). Therefore, S_hmin can only be reliably estimated from breakouts in vertical boreholes (with <10° deviation) (Heidbach 2016a). Corrections and/or errors to the stress orientations determined from induced features can be done for wells deviated >10° (Peška and Zoback 1995; Zoback et al. 2003). Determining the stress magnitudes of the three principal stresses (i.e. S_v, S_Hmax, and S_hmin) is beyond the scope of this study, therefore the weighted average in-situ stress feature orientations and standard deviations used in this study have not been corrected for well deviation. However, modelling has been used to determine the direction of borehole breakouts in a deviated well, based on different principal stress magnitudes (see Results section below).

This study has inferred the S_Hmax orientation in Ranui-2 from borehole breakouts observed in the image logs. Due to the few reliable DITFs identified in the well, estimates of present day S_Hmax direction in the borehole have been assumed to be perpendicular to the observed S_hmin orientation. Due to the shallowness of the borehole (circa 1440 m md), it has been assumed that in the deviated section, one of the principal stresses is vertical. As such, the findings in the lower deviated part of Ranui-2 are preliminary and may be less reliable indicators of S_Hmax. Further work is needed to understand the S_v, S_Hmax, and S_hmin magnitudes of well Ranui-2.

World stress map

The World Stress Map (WSM) allows comparison between different indicators of stress orientation (e.g. focal mechanism solutions, borehole image logs, four-arm calliper logs) on a global scale, using a quality ranking scheme (A–E), with A being the highest quality and E the lowest (Heidbach 2016a). The ranking scheme is based mainly on the number, accuracy, and the depth of the measurements, and has undergone many refinements over the years since it was introduced by Zoback and Zoback in 1989 (Heidbach 2016a). Data in this study is compared to existing stress orientation data in the WSM (e.g. Townend et al. 2012; Heidbach et al. 2016b), and using the WSM ranking criteria allows reliability and global comparability of stress data, to further refine the stress regime of the East Coast Basin.

Borehole breakouts and DITFs provide two stress orientations for a borehole, so each needs to have its own quality ranking. A circular statistical method, detailed in Heidbach (2016a), was used to calculate the mean S_Hmax azimuths and standard deviations for both the identified breakouts and DITFs from Ranui-2, with the results compared against the WSM quality rankings.

In this study, the length (L), rather than the number, of the individual in-situ stress features has been used to determine the quality ranking of Ranui-2. The standard deviation of the Length method has been used to classify the mean S_Hmax, following the borehole breakout classifications in Heidbach (2016a).

Results

S_Hmax

The observed in-situ stress data obtained from the image logged interval in Ranui-2 is shown in Figure 4A, prior to applying the circular statistical method of Heidbach (2016a) to the data. The breakouts (n = 41) have a general N–S to NE–SW orientation, while the two DITFs are oriented W–E. These orientations are similar to the breakout and DITF interpretations in ECEV III Ltd (2012), with strikes of NNE–SSW (n = 52) and WNW–ESE (n = 7), respectively. As indicated above, mean S_Hmax azimuths were inferred from the borehole breakout data so all numbers of data points and statistics from herein refer to the breakouts. The mean S_Hmax azimuth (uncorrected for well deviation) for the entire imaged interval 843–1422 m md in Ranui-2 is 128° (±29° standard deviation, n = 41, combined length of features = 177 metres) (Figure 4B, Table 2). This equates to a quality ranking of D (based on the standard deviation), using the WSM quality ranking method of Heidbach (2016a). However, the inclinometry data in the well suggests that the borehole is deviated ≥10° below ∼940.00 m md (728 m TVDss). Therefore, the breakouts below this depth need to be corrected for the effect of wellbore deviation.

Figure 4. Rose diagram showing the A, all the observed in-situ stress features in the entire imaged interval, prior to applying the circular statistical analysis of Heidbach (2016a). Borehole breakouts are in black (n = 41), while DITFs are in grey (n = 2). B, C, and D, Scaled rose diagrams showing the mean S_Hmax azimuth directions determined from borehole breakouts identified from the imaged interval in borehole Ranui-2, using the circular statistical method. B, The entire imaged interval (843–1422 m md). C, The interval above the change in deviation (843–c. 940 m md). D, The lower deviated (≥10°) interval (c. 940–1422 m md) in the borehole. N = number of data points. The standard deviation is given in the brackets for each interval. The quality ranking follows the World Stress Map criteria of Heidbach (2016a).

Table 2. Stress indicators from the analysed resistivity borehole image in Ranui-2, based on the changes in mean S_Hmax azimuth (shallow and deep) and deviation in the borehole (shlwdev and deepdev).

Borehole ID	Latitude/Longitude (GD2000)	Azimuth (deg)	Fracture type	Depth (m TVDSS)	Quality	Number of features (n)	Length of features (L)	S.D. (deg)	Weighting	Uppermost feature depth (m TVDss)	Lowermost feature depth (m TVDss)	Top of imaged interval (m TVDss)	Bottom of imaged interval (m TVDss)
Ranui-2 entire	−40.9423/175.9178	128	BO	916	D	41	177	29	L	639	1192	627	1210
Ranui-2 entire		97	DITF	776	E	2	2	6	L	754	798	627	1210
Ranui-2 shlwdev	−40.9423/175.9178	100	BO	661	n/a	5	24	13	L	639	683	627	728
Ranui-2 shlwdev		n/a	DITF	n/a	n/a	n/a	0	n/a	L	0	0	627	728
Ranui-2 deepdev		134	BO	977	n/a	36	152	27	L	761	1192	728	1210
Ranui-2 deepdev		97	DITF	776	n/a	2	2	6	L	754	797	728	1210
Ranui-2 shallow	−40.9423/175.9178	89	BO	779	n/a	18	59	14	L	639	919	627	918
Ranui-2 shallow		97	DITF	776	n/a	2	2	6	L	754	798	627	918
Ranui-2 deep		142	BO	1055	n/a	23	117	11	L	926	1192	918	1210
Ranui-2 deep		n/a	DITF	n/a	n/a	n/a	0	n/a	L	0	0	918	1210

All the information required for the World Stress Map (WSM) quality ranking is included in the table. Azimuth: Interpreted orientation of S_Hmax using circular statistics. Number: The amount of recognised feature pairs (breakouts (BO) or DITFs) in a single well or a section of the well. S.D.: Standard deviation calculated according to the circular statistics of bi-polar data by Mardia (1972) with a weighting depending on the length (short: L) of the feature. Length: the added length of the fractured borehole sections. Top and Bottom: The depth of the uppermost and lowermost stress indicator found in the borehole. Depth: The mean between top and bottom stress indicator. Top log and Bottom log: top and bottom of the imaged interval. Location: East Coast Basin. Date of the tool run = 7 February 2012. Ranui-2 shlwdev = above c. 728 m TVDss, Ranui-2 deepdev = below c. 728 m TVDss (depth at which well deviation becomes ≥10°). Ranui-2 shallow = above c. 918 m TVDss, Ranui-2 deep = below c. 918 m TVDss (change in S_Hmax azimuth downhole).

When the S_Hmax is classified as being either above or below circa 940 m md, based on the change in deviation, two distinct trends can be seen, although there is a general W–E orientation in both intervals (Figure 4C and D, Table 2). The vertical interval 843–940 m md has a mean S_Hmax azimuth of 100° (±13°, combined length = 24 m), although it has few data points (n = 5, Figure 4C). Below 940 m, where the borehole starts to deviate ≥10°, the mean S_Hmax azimuth is 134° (±27°, n = 36, combined length = 152 m, Figure 4D). The two DITFs identified in the imaged interval have been assigned an E-quality due to the small sample size (Table 2), and have not been used to determine the S_Hmax orientation in this study.

Few onshore orientational data exist in and around Ranui-2 in the WSM. This study's mean S_Hmax azimuth from breakouts, (128° ± 29°, n = 41) is similar to that obtained from nearby earthquake focal mechanism (S_Hmax 115°–119°) and offshore well (Tawatawa-1 breakouts mean S_Hmax = 111° ± 6°, n = 48) data (Heidbach et al. 2016b). The Ranui-2 breakouts in the WSM have had their orientation corrected based on the wellbore deviation. These breakouts have a mean S_Hmax azimuth of 114° (±20°, n = 78), and have been assigned a B-quality ranking. The DITFs have a mean S_Hmax azimuth of 100° (±8°, n = 3), and are of D-quality (Heidbach et al. 2016b).

Confidence in determined S_Hmax orientation from borehole breakouts

The deviation from vertical in Ranui-2 potentially introduces error into the determination of in-situ horizontal stress field orientations from observations of induced features on a borehole surface. Determining the three principal stress magnitudes was not part of this study, however a range of estimated magnitudes were used in the Landmark Recall^TM Sigma module for a range of given depths, to test whether the vertical stress affected the borehole breakout azimuths observed in the deviated section of the well (see online supplementary material, including Table S2). The resulting compressional failure model (Figure 5) shows that the borehole breakouts should have a S_Hmax strike azimuth of ∼50°–60° even with a borehole deviation of 25°, which is similar to that observed in the imaged interval (Figure 4A). This suggests that in Ranui-2, the vertical stress has little or no effect on the borehole breakout azimuths. Hence, high confidence can be placed on the in-situ stress orientations obtained from the borehole breakouts in the FMI imaged interval.

Figure 5. Compressional failure plot for Ranui-2, modelled using the Landmark Recall^TM Sigma module. Input parameters are given in Table 2 in the supplementary material. The plot shows that for a borehole with 25° deviation, borehole breakouts should have a strike azimuth of ∼50°–60°. The observed borehole breakout azimuths from the imaged interval in Ranui-2 strike at ∼55°. The compressional failure plot confirms that the borehole deviation is not affecting the borehole breakouts in Ranui-2. Plot is in Schmidt lower hemisphere.

S_Hmax versus depth

When the S_Hmax azimuths obtained in this study are plotted against true vertical depth subsea (TVDss), there is a sharp change in the mean S_Hmax azimuth at c. 918 m TVDss (c.1130 m md), near the top of the Tangaruhe Formation (Figure 6, Table 2). Above c. 918 m TVDss, the mean S_Hmax azimuth is 089° (±14°) (n = 18), with an overall length of 59 m, whereas below this depth, the mean S_Hmax azimuth is 142° (±11°, n = 23), with an overall length of 117 m.

Figure 6. Depth partitioning of the mean S_Hmax azimuth direction in borehole Ranui-2. Borehole breakouts (BO) above c. 918 m TVDss (c. 1130 m md) indicate an azimuth of 089°–269° (blue), while below c. 918 m TVDss, the borehole breakouts show a 142°–322° azimuth direction (purple). The black arrow indicates the mean S_Hmax azimuth obtained from the DITFs identified in the interval above c. 918 m TVDss (n = 2). S_Hmax is calculated using the World Stress Map circular statistics calculations in Heidbach (2016a). Triangles are derived from borehole breakouts, while squares depict the DITFs identified in the imaged interval of the borehole. Errors on S_Hmax azimuth are 1 standard deviation, calculated using circular statistics, borehole breakouts (BO) weighted by the BO number, quality ranking from the World Stress Map. nBO = number of BO; nDITF = number of drilling-induced tensile fractures. The black bar on the right indicates the depths of the imaged interval in borehole Ranui-2, in metres TVDss.

The borehole starts to deviate greater than 10° at c. 940 m (c. 728 m TVDss), which is above the depth at which the mean S_Hmax azimuth changes direction. This suggests that the rotation in the stress field (change in mean S_Hmax azimuth) has not affected the change in deviation in the borehole. The hole azimuth of the borehole itself starts to rotate at c. 900 m md (c. 688 m TVDss, ECEV III Ltd 2012), with the mean azimuth 095° rotating towards 119° below this depth.

Despite a c.230 m depth difference between the change in well deviation at 728 m TVDss, and the sharp change in mean S_Hmax azimuth downhole at 918 m TVDss, the mean S_Hmax azimuthal directions calculated from breakouts are generally similar to each other down the well (Figure 4C and D, Figure 6, Table 2). The intervals above 728 m TVDss (change in well deviation, ‘shlwdev’ in Table 2) and 918 m TVDss (change in mean S_Hmax downhole, ‘shallow’ in Table 2) have mean S_Hmax azimuths of 100° (±13°) and 089° (±14°), with overall lengths of 24 and 59 m, respectively. Below these changes in depth, the mean S_Hmax azimuths are 134° (±27°, change in well deviation, ‘deepdev’ in Table 2) and 142° (±11°, change in mean S_Hmax downhole, ‘deep’ in Table 2), with overall lengths of 152 and 117 m, respectively. The higher standard deviations suggest higher variability of the S_Hmax direction.

Fractures

A total of 592 fractures were identified in the imaged interval in Ranui-2, with the majority (98%, n = 578) being of low resistivity (conductive) (Table 1). Partial (i.e. discontinuous) low-resistivity fractures (Figure 7A) are the most numerous (n = 500, or 84%), with ‘full’ continuous low-resistivity fractures being the next dominant (n = 78, 13%). The remainder of the fractures are non-conductive (high resistivity) (Table 1). In Ranui-2, low resistivity (conductive) fractures are appear dark on the image. These fractures are conventionally assumed to be ‘open’ at the borehole well and hydraulically conductive (Barton et al. 1995). The resistive fractures appear bright on the image. The mean fracture strike orientation for the entire imaged interval is 045° (NE–SW), with the low-resistivity fracture strike direction also following this trend (Figure 7A). This NE–SW strike orientation is similar to that reported in ECEV III Ltd (2012), which also shows a subset of the low resistivity fractures (minor conductive fractures, with both continuous and discontinuous traces, and with narrow apertures) having an E–W strike orientation. In this study, fracture apertures were not measured, however there is a minor E–W strike orientation in the low resistivity fractures in Figure 7A, which may equate to that seen in ECEV III Ltd (2012). There is no obvious dominant fracture strike orientation for the high-resistivity fractures identified in this study (Figure 7B).

Figure 7. Rose histograms showing proportionally scaled fracture strike orientations, and dip magnitudes and directions for the fractures identified in the entire imaged interval in Ranui-2. A, Low resistivity fractures are in dark grey (full sinusoids, n = 78) and black (partial sinusoids, n = 500). B, High resistivity fractures are in white (full sinusoids, n = 4) and pale grey (partial sinusoids, n = 10).

Fracture strike orientations vary down the well, and change from a W–E to NW–SE direction at the top of the imaged interval, to a more NE–SW direction below c. 920 m md (Figure 8). This depth is close to the depth at which the well becomes deviated >10° (c. 940 m md, 728 m TVDss), and occurs near the top of the Whangai Formation. Further down the well, in the interval 1060–1120 m md, there is a wide spread in fracture strike orientation direction (Figure 8).

Figure 8. Rose histograms showing the proportionally scaled fracture strike orientations, and dip magnitudes and directions for the four fracture sets identified in Ranui-2, based on general fracture strike orientation. The formations identified in the imaged interval are shown in the middle, with fracture set (i) corresponding to the Karamu Sandstone, fracture sets (ii) and (iii) to the Whangai Formation, and fracture set (iv) comprising of the Tangaruhe and Springhill formations. The mean S_Hmax azimuth directions from the borehole breakouts (after circular statistics have been applied, but not corrected for well deviation) above and below c. 1130 m md, are shown on the right. The fractures in fracture set (i) strike in a similar W–E direction to the S_Hmax above 1130 m, while fracture sets (ii) and (iv) strike oblique to the NW–SE S_Hmax azimuth direction below 1130 m.

The open (low resistivity) fractures can be classified into four major sets (i): 70°–120°N, set (ii): 30°–60°N, set (iii): 340°–050°N, and set (iv): 20°–60°N (Figure 8). None of the fracture sets follow the mean S_Hmax azimuth directions identified above and below c. 1130 m md (089° and 142°, respectively), although fracture set (i) is broadly parallel to the NW–SE mean S_Hmax azimuth direction for the interval above c. 1130 m md (Figure 8).

The fracture sets were compared against the mean S_Hmax direction in the well, to see if there is any connection between the fracture strike orientation, particularly in the low resistivity fractures, and present day in-situ stress in the borehole. Rajabi et al. (2010) suggests that there are two main hypotheses about the relation of in-situ stress and open fractures: (1) ‘stress-sensitive fractures’, in which fractures are hydraulically conductive due to being suitably oriented for failure (shear or tensile) in the present day stress tensor (Barton et al. 1995; Sibson 1996; Finkbeiner et al. 1997), and (2) ‘stress-insensitive fractures’, in which fractures are ‘propped open’ due to partial mineralisation or from fracture wall asperities and thus may be oriented in any direction and have no relation to the present-day stress tensor (Laubach et al. 2004). Of the four fracture sets identified in the imaged interval in Ranui-2, only fracture set (i) may be genetically linked to the present-day stress orientation, as it is generally parallel to the mean S_Hmax azimuth of the well (Figure 4B and Figure 8). Fracture sets (ii) and (iv) are oblique to the mean S_Hmax azimuth in Ranui-2, suggesting that both stress-sensitive and stress-insensitive fracture sets occur, and may have significant influence on hydraulic conductivity (Figure 4B and Figure 8).

Discussion

Stress on pre-existing faults and fractures affects which fractures will be open to fluid flow (e.g. Barton et al. 1995; Finkbeiner et al. 1997; Rajabi et al. 2010). Understanding fluid flow is of fundamental importance in conventional and unconventional petroleum reservoirs as it provides evidence of where and how fluids have migrated e.g. from source to reservoir rocks (Allis et al. 1997; Finkbeiner et al. 1997; Moretti 1998; Aydin 2000; Cartwright et al. 2007; Webster et al. 2011; Ilg et al. 2012). It is also important in relation to seal rocks (‘cap-rocks’) as fracture permeability can affect seal capacity (Cartwright et al. 2007). Fluids typically prefer to flow along fractures that are oriented parallel or sub-parallel to the present-day S_Hmax, or maximum compressive stress (Barton et al. 1995; Rajabi et al. 2010). This is because such fractures have the lowest normal stress across them, and therefore provide the least resistance to flow, however this may not always be the case. Laubach et al. (2004) show that while open fractures can have any strike relative to S_Hmax, chemical processes such as cement precipitation while fractures are opening can cause resistance to fracture closure, making the fractures insensitive for changes in effective stress. Processes such as these can affect permeability of the rock. The relationship between S_Hmax and permeability in Ranui-2 was not assessed as part of this study, and although there may be a correlation, further study is needed.

S_Hmax versus Depth

Using the length-weighted method, there is a distinct change in the S_Hmax azimuth direction at c. 1130 m md (918 m TVDss), from 89° ± 14° to 142° ± 11° (Figure 8, Table 2). To determine if there was any change in S_Hmax azimuth direction as a result of the well deviation, the S_Hmax azimuths were calculated, again using the length-weighted method, for above and below c. 940 m md (728 m TVDss), where the well becomes deviated >10°. Table 2 shows that for these intervals, the S_Hmax azimuth deviation above c. 940 m is 100° ± 13°, while below c. 940 m md, it is 134° ± 27°.

These rotations in S_Hmax azimuth within Ranui-2 are ‘fourth order of stresses’ as they occur at wellbore (1–1000 m) scale (Rajabi et al. 2017a, 2017b). On a worldwide scale, these highly localised stress features have been associated with geological features such as lithological changes, faults, and fractures (e.g. Mount and Suppe 1987; Reynolds et al. 2005; Heidbach et al. 2007; Rajabi et al. 2017a, 2017b), although in this study, no faults were observed on the image at c. 940 m and c. 1130 m md.

The change from a general W–E direction to a NW–SE direction in present day mean S_Hmax azimuth at c. 1130 m md (918 m TVDss) occurs near the top of the Late Cretaceous Tangaruhe Formation interval (1122.50–1171.40 m md, 910.50–959.40 m TVDss; ECEV III Ltd 2012) in Ranui-2. This change in azimuth may be related to a change in lithology, such as a change in grain size, clay content, or rock strength (Bruno and Winterstein 1994). The Tangaruhe Formation is a weakly to moderately indurated, interbedded fine sandstone and mudstone of probable turbidite origin (ECEV III Ltd 2012). Underlying this, is the Springhill Formation, a moderately indurated mudstone-dominated formation, with thick sandstone beds (ECEV III Ltd 2012). Overlying the Tangaruhe Formation is the Whangai Formation, typically a massive to weakly bedded siliceous siltstone of Late Cretaceous to Paleocene age (ECEV III Ltd 2012). It is possible that the comparatively more-indurated nature of the older Tangaruhe and Springhill formations have affected the breakout direction in the borehole.

The mean S_Hmax azimuth associated with the change in borehole deviation at c. 940 m md, to >10°, occurs within the Whangai Formation (901.40–1122.50 m md, 689.40–910.50 m TVDss) in Ranui-2. There is little change in the siltstone-dominated lithology of the formation down the well (ECEV III Ltd 2012), suggesting that the change in the mean S_Hmax azimuth direction related to the change in deviation at c. 940 m md, is not related to lithology.

Fractures and S_Hmax

Integrating core with the image log provides a means of ‘ground-truthing’ the image interpretation (Luthi 1990; Genter et al. 1997; Went and Fisher 1999). Most of the fractures in Ranui-2 have been interpreted as being open, based on their low resistivity (conductive) and dark appearance on the image log. Core was obtained over three intervals in the borehole, between 921–927 m md, 1059–1061.07 m md, and 1377.10–1383.10 m md (ECEV III Ltd 2012). Analysis of core #1, comprising of siltstone, shows most of the visible fractures are open, are not filled by drilling mud or other sediments, and show no sign of healing mineralisation (ECEV III Ltd 2012). Core #2 and most of core #3 are too fractured to be able to determine whether the fractures were open or filled, however photographs of the slabbed core in ECEV III Ltd (2012) suggest that the fractures are open. As the majority of fractures in the borehole have a NE–SW strike, and are oblique to the present day S_Hmax (Figure 8), it is possible that other factors such as material anisotropy (e.g. stratigraphic layering of different lithologies) or local stresses and strains (Bruno and Winterstein 1994) within or around the borehole are affecting the fractures, although this was not assessed during this study.

Above c. 940 m md (728 m TVDss), where the borehole is vertical, some of the W–E-trending fractures would probably allow fluid flow through the rock, as the fractures are (a) likely to be open, based on image log interpretation and core analysis, and (b) parallel to the mean W–E S_Hmax azimuth direction above c. 1130 m md, possibly allowing fracture permeability (Figure 8) (Barton et al. 1995, 2009; Finkbeiner et al. 1997).

By comparing fracture set (i) with the mean S_Hmax azimuth orientation interval above 1130 m md, it appears that most of the fractures are parallel with the mean present-day S_Hmax orientation. Fracture sets (ii) and (iv) are perpendicular to both (above and below c. 1130 m md) present-day mean S_Hmax orientations (Figure 8), which may suggest possible restricted fluid flow, even though the fractures have been interpreted as being open. For fracture set (iii), there is no dominant fracture strike direction, although the full low resistivity fractures display a minor N–S orientation. This N–S fracture orientation is perpendicular to the mean S_Hmax azimuth obtained for the interval above c. 1130 m md, again suggesting possible restricted fluid flow through the rock. Laubach et al. (2004) found that narrow cement bridges within otherwise open fractures parallel or sub-parallel to S_Hmax, could account for their resistance to closure, however analysis of the Ranui-2 core does not support this theory (ECEV III Ltd 2012). Another possibility is that the nearby active strike-slip Carterton fault is affecting the present day in-situ stress occurring in Ranui-2.

The fractures identified within the individual formations were also compared to the mean S_Hmax of the borehole, to see if there are any relationships. Fractures within the middle Miocene Karamu Sandstone have a mean W–E to NW–SE strike orientation, similar to the mean present day S_Hmax of the borehole (128° ± 29°, Figure 9). This would suggest that the fractures in the Karamu Sandstone are open, and maybe good conduits for fluid flow. The sandstone lies in the vertical part of the borehole, so the in-situ stress is unlikely to be affected by any deviation effects.

Figure 9. Proportionally scaled rose diagrams showing the present-day mean S_Hmax azimuth direction and fracture strike orientations for the fractures identified in the formations in the imaged interval in Ranui-2. The mean S_Hmax was determined from borehole breakouts identified from the imaged interval in borehole Ranui-2, using the circular statistical method of Heidbach (2016a). Low resistivity fractures are in dark grey (full sinusoids) and black (partial sinusoids). High resistivity fractures are in white (full sinusoids) and pale grey (partial sinusoids).

In contrast, fractures within the underlying Late Cretaceous Whangai, and mid-Cretaceous Tangaruhe and Springhill formations all display a general NE–SW strike orientation (Figure 9), which is almost perpendicular to both the overall mean present day S_Hmax for the well, and the mean present day S_Hmax of 142° (±11°) below c. 1130 m md (918 m TVDss; Figures 8 and 9). While most of the fractures are thought to be open due to their dark, low-resistive appearance on the image logs, they are not parallel to the S_Hmax. It is possible that obliqueness to S_Hmax (Laubach et al. 2004), possible clay smear in the fractures (although this was not evident in what little core was obtained), mineralisation within the fractures (again, not evident in the core) (Laubach et al. 2004), or that the fractures are the result of episodic changes in fluid pressure in excess of hydrostatic (Laubach 1988) may inhibit fluid flow.

In-situ stress and regional tectonics

Patterns in present-day stress in sedimentary basins can provide valuable information on the sources of stress (Heidbach et al. 2007; Rajabi et al. 2017b).

The East Coast Basin lies within the active Hikurangi subduction margin, where deformation has included shortening, extension, vertical-axis rotation, and strike-slip faulting in the upper plate, all of which occurred over the last ∼24 Myr (Nicol et al. 2002, 2007; Wallace et al. 2004; Townend et al. 2012). There is clear evidence that in southeast North Island the present-day plate margin strain partitions into a western strike-slip dominated zone and an eastern zone of compressional deformation, and that the organisation of such partitioning has varied through the Neogene (Cashman et al. 1992; Beanland et al. 1998; Nicol et al. 2002, 2007). This may account for the differences in fracture and in-situ stress directions in the Ranui-2 borehole.

The location of Ranui-2 borehole is within a zone of strike-slip dominated faulting, as shown by the nearby active strike-slip Carterton Fault (Lee and Begg 2002). The Carterton Fault is one of few faults in the onshore East Coast Basin that are oriented ENE–WSW, parallel to the present-day relative plate motion vector, and oblique to other faults in the basin, which are margin parallel, and align with the NE–SW-trending North Island Dextral Fault Belt (Beanland et al. 1998; Lee and Begg 2002; Schermer et al. 2004). Few studies are published on the origin and timing of the Carterton Fault, which Carne et al. (2011) suggest is a dextral splay fault that has bifurcated off the northern section of the Wairarapa Fault. It is possible that the Carterton Fault may be younger than other faults in the basin, since it is parallel to the relative plate motion vector (Carne et al. 2011; Litchfield et al. 2014). The Carterton Fault may also account for the differences in fracture and present day in-situ stress directions in the well, due to its associated strain patterns in an otherwise complex structural zone, affecting the borehole (Cashman et al. 1992; Nicol et al. 2002, 2007). In a regional sense, the 128° S_Hmax direction from borehole breakouts in Ranui-2 is oblique to the strike of the Carterton Fault (Figure 10). It is unlikely that the fault is affecting S_Hmax in the upper vertical (<10°) part of the borehole (down to c. 940 m md, c. 728 m TVDss). However, lower down in the borehole, where deviation is >10°, it is possible that either the vertical stress is influencing the locations of the induced features on the borehole wall, and/or nearby strike-slip faults e.g. Carterton, Masterton, and/or Wairarapa faults, are affecting the present day in-situ stress in the lower deviated section of the borehole, due to the complex nature of the nearby North Island Dextral Fault Belt (Darby and Beanland 1992; Beanland et al. 1998; Nicol et al. 2002; Wallace et al. 2004; Carne et al. 2011; Litchfield et al. 2014).

Figure 10. Rose histogram showing the proportionally scaled fracture strike orientations for the fractures above and below c. 918 m TVDss (c. 1130 m md) in borehole Ranui-2, based on the change in S_Hmax azimuths determined from the borehole breakouts. Note the greater spread in strike orientations for the fractures above c. 918 m TVDss. The in-situ stress directions for the same depth intervals are shown by the black arrows on the map. The active and inactive mapped faults in the Wairarapa area from Lee and Begg (2002). The inset map shows the S_Hmax direction from focal mechanisms and plate motion data (yellow arrows) from Townend et al. (2012), and the current geodetic direction (blue arrows) of the onshore East Coast Basin from Wallace et al. (2004).

The mean S_Hmax azimuth from Ranui-2 is generally consistent with the present-day S_Hmax direction for this sector of the Hikurangi margin, as obtained from the earthquake focal mechanisms and plate motion data of Townsend et al. (2012), shown in Heidbach et al. (2016b) (Figure 10), and parallel to the regional stress orientation (Townsend et al. 2012; Heidbach et al. 2016b). Existing stress data in the WSM shows that further north from Ranui-2, the S_Hmax azimuths obtained from focal mechanisms (e.g. Townend et al. 2012) start to rotate, and the stress regimes change from an unknown stress regime at Ranui-2, to normal- and thrust-faulted regimes to the north-west and north, respectively (Heidbach et al. 2016b). This difference in the change in mean S_Hmax azimuth direction from the southern parts to the northern parts of the basin is most likely due to the basin-wide complex plate tectonics occurring in the East Coast Basin (Nicol et al. 2002, 2007; Wallace et al. 2004; Townend et al. 2012).

Conclusions

This study highlights the utility of subsurface image log data sets in constraining subsurface in-situ stress conditions, and presenting present-day stress (S_Hmax azimuth orientation) data for part of the onshore southern East Coast Basin. Borehole breakouts identified in the image logs acquired in Ranui-2 have been used to determine the subsurface mean present day S_Hmax orientation. The data show that there are two mean S_Hmax orientations, above and below c. 1130 m md (c. 918 m TVDss), at 089° (±14°) and 142° (±11°), respectively. Fracture sets identified in the imaged interval show different fracture strike directions, with fracture set (i) equating to most of the middle Miocene interval, striking W–E to NW–SE, whereas fracture sets (ii) and (iv), comprising most of the Late and mid-Cretaceous intervals, have a general NE–SW fracture strike orientation. The overall mean present-day S_Hmax orientation for Ranui-2 fits with the regional S_Hmax orientation derived from earthquake focal mechanisms and plate motion data for the onshore southern East Coast Basin.

In this study, it was have found that stress directions in Ranui-2 change above and below c. 1130 m md (918 m TVDss). This depth is just below the change in deviation (from vertical to 25°) in the borehole. This change in stress direction could be related to:

1. Changes in lithology, bedding, structure, etc within the borehole, which can manifest the stress orientation in different ways e.g. changes in stratigraphic layering can reorient the principal stress direction if the rock is anisotropic,

2. Influence of surface faults which affect the in-situ stress orientation locally within the borehole.

The southern onshore East Coast Basin has experienced a complex deformation history as part of the active Hikurangi subduction margin, which may account for the difference in fracture strike directions and present-day S_Hmax orientations in Ranui-2. This study illustrates the local conditions and effects in the borehole, whereas first motion studies (e.g. Townend et al. 2012) reflect larger scale effects in a more regional setting. Further study is needed to account for the deviation effects, if any, in borehole Ranui-2, including determining the remaining stress tensor components (S_v, S_Hmax and S_hmin magnitudes).

Acknowledgements

This study was undertaken as part of the Petroleum Basins Research Programme via Strategic Science Investment Funding from the New Zealand Government (Ministry of Business, Innovation and Employment) to GNS Science. East Coast Energy Ventures are thanked for running the imaging tool in Ranui-2, and making the data publicly available and commenting on an early version of the manuscript. Mark Lawrence, Cécile Massiot, and Kyle Bland (GNS Science) are thanked for their comments and discussion. Cécile is also thanked for her assistance with the Landmark Recall^TM Sigma module. Greg Browne and Mark Lawrence (GNS Science) are thanked for reviewing the manuscript. Thanks also go to Mojtaba Rajabi and David McNamara, who critically reviewed the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author.

ORCID

Angela G. Griffin http://orcid.org/0000-0002-1406-6209

Additional information

Funding

This study was undertaken as part of the Petroleum Basins Research Programme via Strategic Science Investment Funding from the New Zealand Government (Ministry of Business, Innovation and Employment) to GNS Science.