A Seismic Investigation into the Thermal and Physical Properties of the Hydrate Bearing Pukeroro Ridge, Hikurangi Margin, New Zealand

Abstract

Understanding the subsurface dynamics of the world’s oceans has become more important over time due to current climate issues, hazard evaluations and an ever-decreasing abundance of resources. Fluid and heat flow processes operating beneath oceans around the world are some of the most significant processes involved with many of the research sectors focused on studying the ocean floors.

Pukeroro Ridge, located east of the lower North Island of New Zealand, recently has been surveyed a number of times, including by a conventional petroleum industry seismic survey in 2009 and a high-resolution seismic survey combined with a heat flow study in 2015. The variety of data types collected in a structurally complex area affected by dynamic subsurface flow provides us with a unique opportunity to (1) investigate the subsurface dynamics of an extensive gas hydrate accumulation, and also (2) evaluate a number of analytical techniques and processes that can be applied in such settings.

Throughout this thesis, high–resolution and conventional seismic data are compared, and a combination of the data sets is used to better characterise subsurface features across Pukeroro Ridge. The bottom simulating reflections (BSR) identified in the seismic data, in combination with the heat flow results, are used to better understand thermal conductivity variations in the area. Additionally, the effectiveness of using the BSR as an isotherm from which we can estimate heat flow along seismic lines, without heat flow measurements, is evaluated.

The high-resolution data were found to have a 2 - 3 m vertical resolution in the 800 m immediately beneath the subsurface. The data identified fine scale bedding and any discontinuities in the reflections, particularly the BSR. The conventional seismic data had a lower resolution; they were able to image features with a vertical length scale of about 10 m. Discontinuities found in the high-resolution data were not evident in the lower resolution data and reflections appeared more continuous and uniform. In contrast, the lower frequencies of the conventional seismic data allowed for increased penetration, enabling imaging at depths of over 3500 m beneath the seafloor. The high-resolution data lost resolution in some localised regions; this was determined to be because of the short offsets of the data limiting imaging power in region of dipping strata.

The seismic data identified two main lithological units: a bedded unit and a non-bedded unit, of which the constituents were narrowed down to a siltstone/mudstone of Pliocene – Pleistocene age, based on previous research. The seismic data also identified several reverse faults dipping 40-45°W. A section of localised fluid flow was identified based on an amplitude reversal, which is consistent with an appropriate geological interpretation of the feature. Beneath this reversal, a strong BSR was identified which became an integral part of further research.

The method of estimating the thermal conductivity in the subsurface based on using the BSR as an isotherm was deemed to be sufficiently accurate. The results were reasonable based on the lithological units identified. The method highlighted an increase in thermal conductivity associated with increasing pressure as a result of depth. This increase was greater than what would be expected as a result of purely hydrostatic pressure. Thermal conductivities spiked up to 2.1 W/m/K in concurrence with heat flow anomalies, of up to 56 mW/m^2. These highs also coincided with the fluid flow features previously identified during the seismic interpretations. The thermal conductivities also suggest regional fluid migration.

Finally, although the evaluation of the heat flow estimation method was partially successful, it also highlighted several methodological issues. The method appeared effective for stable continuous areas with widespread, uniform lithological units and minimal fluid movement. The method also required a reasonable understanding of the thermal conductivities in the region. The testing highlighted several scenarios where the estimations had a higher level of uncertainty, such as those with localised fluid features or areas with poorly constrained thermal conductivities.

Overall, the research made effective use of the substantial quantity and variability of data available in this unique locality. While the findings allowed for important correlations and a greater understanding, they would benefit from further research, including drilling, which would enable calibration of the thermal conductivity measurements. More heat flow transects would also allow for comparison and investigations into the accuracy of the heat flow estimations undertaken.