Structural and sedimentological interpretation of well data from the Wairoa area, North Island, New Zealand

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Lawrence, M.J.F. 2018 Structural and sedimentological interpretation of well data from the Wairoa area, North Island, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2018/28. 76 p. + 16 enclosures; doi: 10.21420/G23W81

Abstract: Seventeen wells were drilled in the Wairoa area of northern Hawke’s Bay, to investigate hydrocarbon potential. In four of these wells, drilled on the Kauhauroa Anticline (Kauhauroa- 2, Kauhauroa-5, Waitahora-1) and the Tuhara Anticline (Tuhara-1A), borehole image data was acquired. The Schlumberger Oil Based Mud Imaging tool (OBMITM) was used in well Waitahora-1, while the Schlumberger Fullbore Formation MicroImager tool (FMITM) was acquired in the other three wells. These data suites provided an opportunity to generate an integrated interpretation of the subsurface geology of the area. Therefore, the aim of this study was to generate full sedimentological and structural interpretations of the borehole image data from the four imaged wells with a view to improving the understanding of the subsurface geology of the area. Ultimately, the aim is to set up a series of onshore subsurface analogues that can be used in the interpretation of seismic data in Hawkes Bay where there is limited or no well data, and potentially provide a basis for any other future work. Some preliminary work was also done investigating extending borehole image-based interpretations to wells in the Wairoa area that have no image data.

The raw borehole image data was loaded into a Landmark RecallTM Version 5.4 database. Data quality was assessed, then processed using standard methods to generate false colour images. The quality of processed images was assessed continually during interpretation.

Structural dip magnitudes range from moderate (circa 20º, Tuhara-1A, Waitahora-1) to high (locally >30º, Kauhauroa-2, Kauhauroa-5). Structural dip is to the NW-NE, except in well Kauhauroa-2 where it is to the ESE-SE. These orientations are consistent with regional structure. Observed fractures vary from rare to common throughout the detrital successions, with fracture densities seldom exceeding 1 fracture/metre. Fracturing is more intense in the Kauhauroa Limestone than in detrital formations, but many of these fractures are limited to within individual beds. As a result, fractures in limestones cannot be matched across bed boundaries or correlated around the borehole, giving the Kauhauroa Limestone a ‘pseudobrecciated’ appearance. The dominant strike orientation of fractures that can be correlated around the borehole is broadly NE-SW, with subordinate strike directions of E-W to ENE-WSW, and broadly NW-SE. Fracture dip directions are dominantly to the WNW-N, with subordinate directions in the range NE-S. Identified fracture clusters have been interpreted to reflect local sub-seismic structures that are of local significance, but their orientations indicate they are generally related to the regional structuration. In-situ stress features indicate Shmin is oriented to the NW-SE to NNW-SSE and SHmax is oriented NE-SW to ENE-WSW, which matches previous studies.

Image facies analysis has resulted in 20 distinct image facies being recognised in wells Kauhauroa-2, Kauhauroa-5, and Tuhara-1A in which the FMITM tool was run. The image facies reflect three broad lithologies: 1) marine pelagic to hemipelagic mudstones deposited in bathyal settings, 2) bathyal turbidite sandstones, siltstones, and heterolithics, and 3) nontropical shelf limestones. The distribution of image facies, wireline log responses, sandstone bed thickness, and also sediment dispersal patterns, where practical, were used to identify a series of sedimentary packages in each well. Only a few packages were recognised in the thick mudstone successions (Pindari and Tangihau mudstones, Wheao Formation), but many more were recognised in the Tunanui Formation. In the Tunanui Formation, the packages are thought to relate to changes in depositional setting such as overlapping fan lobes or levee deposits, and a shifting locus of deposition based on slope gradients. Unfortunately, there is no direct correlation of identified sedimentary packages between wells. Depositional packages were also recognised in the limestones in well Kauhauroa-2, which has the most continuous GNS Science Report 2018/28 v Kauhauroa Limestone succession. The depositional packages occur within three broad units that become increasingly argillaceous, stratigraphically upwards. The uppermost unit appears to have been redeposited. All the broad units and their sedimentary packages probably reflect scales of depositional control. Unfortunately, the quality of the OBMITM image data in well Waitahora-1 has precluded direct comparison of image facies with the other three wells.

Net to gross was determined in detrital successions but was only significant (i.e., >0.1) in the Tunanui Formation, where it varied both laterally and stratigraphically. Maximum net to gross was 0.47 in sedimentary packages K5l in well Kauhauroa-5, and T1K in well Tuhara-1A. Sediment dispersal information was derived primarily from the Tunanui Formation. Sediment dispersal data are of low confidence due to comparatively low dip magnitudes involved. Dispersal data either showed no preferred orientation, or predominantly northerly trends. Softsediment deformation data provided a low confidence interpretation of a E-W striking paleoslope.

An electrofacies scheme was developed as a means of extending high resolution borehole image interpretations to wells where borehole images were not acquired. Despite image facies being distinct and identifiable, the conventional wireline log data ranges for each image facies displayed considerable overlap. As a result, individual image facies could not be unequivocally differentiated using conventional wireline log cross-plots. In the current preliminary form, extending the electrofacies scheme to unimaged wells would be imprecise, and would not provide good indications of equivalent facies. Therefore, more work is required before the scheme can provide meaningful results. (auth)