A Study of Unconventional Gas Accumulation in Dannevirke Series (Paleogene) Rocks, Canterbury Basin, New Zealand

Abstract

This thesis aims to assess the potential of unconventional gas accumulation of Danevirke aged (65-43 Ma) mudrock of the Canterbury Basin, South Island, New Zealand. Unconventional hydrocarbon resources contained in low-porosity, low-permeability rocks are potentially a large source of natural gas. Recent developments throughout the United States and increasingly so in Australia, signify a shift in exploration efforts from conventional natural gas targets towards unconventional shale gas plays and basin centred gas systems. Despite extensive international progress made in this field of exploration, little is known about New Zealand unconventional hydrocarbon systems.

The Canterbury Basin is approximaty 360,000km² in area and is located approximately between 44°S and 46°S. The deepest part of the basin is located offshore and is known as the Clipper Sub-Basin, which exhibits economic basement depths of 6500m. The Clipper Sub-Basin is a late Cretaceous syn-rift horst and graben feature which trends north east-south west and is bound basinward by the Benreoch High and landward by the Canterbury Bight High. Dannevirke aged transgressive rocks overlay these structures and intermittently exhibit gas-charged intervals in low porosity facies.

Elevated gas concentrations are recorded in four exploration wells in the Clipper Sub-Basin from gas chromatograph readings (up to 2 .7/00.4%). These high-gas zones correspond to intervals of elevated quartz (up to 72wt%), whereas non-gaseous intervals corresponded to quartz values as low as 30wt%. Scanning electron microscopy results do not reveal biogenic silica populations in the cutting samples examined. High silica is related to diagenetic silica transformations of mica, various clay minerals, pyrite and silica transformations. Although no visible porosity is observed in thin sections, FMI wireline analysis illustrate natural fractures predominately occur in siliceous intervals, where resistive fractures can account up to one fracture per 10m of stratigraphic thickness. These fissile or laminated brittle lithologies are likely hydrocarbon conduits or accumulation intervals for wet gas. RockEval pyrolysis results indicate the siliceous mudrocks are organic le-n, comprising an immature gas-prone source rock which averages 1.5% total organic carbon.

Findings made in this research are compared to the. Whangai Formation, considered in this study to be a comparable shale gas system and also to the Monterey Formation of the United States which is a known basin centred gas system. Dannevirke aged sediments found in the Clipper Sub-Basin appear to constitute the requisites of a near-to-source, direct type., basin centred gas system. Implications of this study open up the possibility that New Zealand's widespread Paleocene-Eocene mudrocks are capable of natural gas accumulation and therefore viable natural gas exploration targets in New Zealand.