Porosity development in deep sandstones, Taranaki Basin, New Zealand

Abstract

Reservoir quality in the Taranaki Basin is controlled by both depositional facies and diagenesis. High initial porosities and permeabilities in manyfluvial and littoral sandstones directed fluid flow such that these facies underwater widespread cementation by silica, carbonate and authigenic clays, and later dissolution of carbonates. Current commercial production of hydrocarbons in mainly from secondary, dissolution porosity in sandstones at depths of 1750–3600 m. Recent studies suggest that oil may not be released from terrestrial source rocks in western New Zealand until depths of 5.5 km or more. Knowledge of porosity development at such depths is therefore essential for understanding migration pathways and the controls on hydrocarbon accumulation. Organic species and/or carbon dioxide evolved from kerogen before liquid hydrocarbons are important to the formation of secondary porosity in sandstones by carbonate dissolution. Further, hydrocarbon migration should follow soon after the evolution of these, in order that re-cementation does not occur. High resolution nuclear magnetic resonance (NMR) spectroscopy shows that in western New Zealand basins, the evolution of solvents continues to at least 4 km and may persist almost until the expulsion of liquid hydrocarbons.