Sedimentology and depositional environments of Murihiku Supergroup sediments exposed in the Southland Syncline, New Zealand: Implications for reservoir potential in the Great South Basin

Abstract

<p>A considerable amount is known about the biostratigraphy and organic geochemistry of the Murihiku Supergroup sediments exposed in coastal outcrops of the Southland Syncline, New Zealand. Much less work has been undertaken on the sedimentology of these strata, or understanding their depositional environments and depositional trends through time. What these implications are for reservoir prospectivity in the adjacent Great South Basin, has also had little study focused on it. This thesis addresses these issues by undertaking outcrop-based sedimentological and facies interpretations of these rocks, thin-section based petrographic composition and provenance analysis, augmented by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), as well as porosity and permeability measurements from outcrop core plugs. Petroleum industry seismic data has additionally enabled seismic facies mapping of Murihiku rocks in the offshore Great South Basin. Outcrop observations point to a progressive change in depositional setting, from shelf / upper slope settings during the Late Triassic, to base of slope turbidite deposition in the Early Jurassic. This transgression is followed by regression into fluvial settings in the youngest outcropping Murihiku rocks in the study of Middle Jurassic age. Petrographically the sandstones are feldspathic and lithic arenites and feldspathic and lithic wackes. Provenance suggests derivation from an evolving, intermediate arc that was becoming more siliceous through Late Triassic and Middle Jurassic time. Diagenesis is characterised by early calcite and chlorite precipitation which have almost completely destroyed any primary porosity. Any secondary micro porosity has subsequently been infilled through dissolution of framework grains and zeolitization. SEM and core plug porosity and permeability measurements corroborate the diagenetic changes observed petrographically, with only fluvial facies of Middle Jurassic (Upper Temaikan) age showing any measureable porosity or permeability. As a result, reservoir potential for the Late Triassic to Middle Jurassic, Murihiku Supergroup rocks analysed in this study is low. Younger Murihiku sandstones which are postulated to occur offshore in the Great South Basin are likely to be less influenced by burial diagenesis. As shown from North Island occurrences, these younger successions hold some potential.The reservoir potential for these youngest portions of the Murihiku succession therefore remains positive, both in the Great South Basin, as well as other frontier areas of Zealandia, and continue to provide an exploration target for the petroleum industry.</p>