Analysis of fluid flow systems and links to potential gas hydrate deposits in the northern Taranaki Basin, New Zealand

Abstract

In this study we provide evidence for gas hydrates in the Taranaki Basin occurring off the continental margin on the West Coast of New Zealand. Modelling of the gas hydrate stability zone (GHSZ) suggest that pressure and temperature conditions for hydrate formations exist at water depths of 500 m and greater, which corresponds to the mid-continental slope and deeper regions of the basin. Interpretation of high-resolution 2D and 3D seismic data collected in the basin reveal the presence of large deep seated faulting, polygonal faulting, seismic chimneys/pipes, mounds, pockmarks and high amplitude anomalies here interpreted as free gas. These features are linked to evidence suggesting upward migrating fluids (liquids and gas) from Late Cretaceous deposits into the shallow sediments where hydrate deposits may be expected. Subsequently, the observation of patchy, polarity-reversed, high amplitude reflections (HARs) at the base and in the regional GHSZ are interpreted as bottom simulating reflectors (BSRs) indicating the presence of free gas below a layer of hydrates, which exist at the base of gas hydrate stability (BGHS). These reflections appear predominantly on the down-dip side of canyon incisions that are commonly located on the continental slope. It is proposed here that these lower the extent of the BGHS and consequently trap up dip migrating fluids, which may then facilitate the formation of gas hydrates in these regions. The patchy nature of these reflections is believed to be characterised by areas of focussed fluid flow into heterogeneous deposits in the shallow sediments. This suggests that sources of gas may be derived from thermogenic origins associated with the Late Cretaceous source rocks in the basin. The lack of any strong HARs observed in the deeper regions of the basin suggests that upward migration in these areas may be trapped well below the BGHS and subsequently migrate laterally into the continental slope regions. However, a number of moderate amplitude reflections in the GHSZ are observed suggest that more diffuse gas emissions here may be at play, resulting in possibly weak BSR reflections. The proposed presence of hydrates is likely to affect slope stability in the shelf margin as drilling for oil and gas moves further out into the basin. Furthermore, the dissociation of hydrates may be a contributing factor of greenhouse gas emissions. However, production of gases from hydrates as a by-catch during conventional gas extraction may open up new frontier exploration in the basin.