Structural interpretation of borehole image logs from onshore southern East Coast Basin wells

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Griffin, A.G. 2018 Structural interpretation of borehole image logs from onshore southern East Coast Basin wells. Lower Hutt, N.Z.: GNS Science. GNS Science report 2017/47. 85 p.; doi: 10.21420/G2X642

Abstract: Three stratigraphic holes (Orui-1A, Ranui-2 and Te Mai-2), drilled in southeast Wairarapa to assist petroleum exploration in the onshore southern East Coast Basin, had acoustic and resistivity borehole image logs, conventional cores and wireline log data acquired through significant parts of the wells. The image logs have been interpreted, and where possible, calibrated to core observations. The interpretations provide an insight into the subsurfacestructure and stress, and to a lesser extent, the regional stress fields, and reservoir and seal integrity of this part of southern onshore East Coast Basin. The Orui-1A and Te Mai-2 wells were imaged using a borehole televiewer imaging tool (BHTV), which records the contrast in the acoustic amplitude and the travel time of the signal. The dominant fracture types identified in the processed images from these wells are those of low acoustic amplitude and those that have a visible travel time signal on the image logs. Poorimage quality, particularly in Te Mai-2, meant that fewer features (both fractures and in situ stress) could be identified in the imaged interval, resulting in low confidence analysis. A resistivity microimaging tool was used to acquire image logs in Ranui-2. Low resistive (i.e.conductive) fractures are more numerous in the well. The resistivity-based images acquired in Ranui-2 are of better quality than the acoustic images of Orui-1A and Te Mai-2, making it easier to identify in situ stress features in the imaged interval. Fracture strike directions vary between formations and between wells. The Wanstead, Waipawa and Whangai formations were all encountered by more than one well, allowing comparisons to be made between the formations in the wells. Fracture strike orientations are similar in these formations (NNE–SSW to N–S, NE–SW, and NE–SW in the Wanstead, Waipawa, and Whangai formations respectively), particularly in the low resistivity and low acoustic amplitude fractures, despite the wells being separated by faults, or located in different structural blocks.Borehole breakouts were the dominant in-situ stress feature identified in the wells to establish the minimum horizontal stress direction (Shmin ). The maximum horizontal stress direction (SHmax ) is inferred to be at 90° to Shmin. There is a N–S SHmax direction in Orui-1A, while insufficient data in Te Mai-2 provides a low confidence NE–SW SHmax . Ranui-2 has a NW–SE-trending SHmax for the entire well, however when looking at the individual formations, the in-situ stress features change down the well, with SHmax changing from W–E to NW–SE. The identified fractures and in situ stress features in the imaged intervals from these wells provide an insight into the subsurface manifestations of deformation with depth in the southern Wairarapa area. The information gained from these features can be used in studies relating to fault seals and fracture permeability, and subsurface fluid flow between potential source to reservoir rocks. (auth)