Variations in sandstone diagenesis with depth, time, and space, onshore Taranaki wells, New Zealand

Abstract

Study of diagenesis in cores from Kapuni Field, Waihapa‐2, Kaimiro‐2, and New Plymouth‐2 shows many expected features—quartz and feldspar dissolution and precipitation, clay mineral precipitation, and carbonate dissolution and precipitation. Local variations may overprint broader trends; early quartz overgrowth in parts of the basin is not recognisable in Kapuni wells, and feldspars may be etched differentially. In the Kapuni Group, early siderite precipitated from pore waters similar to those in which the host sediment was deposited, whereas late calcite was probably deposited from fresh water. In the Moki Formation, some calcite may be similarly late, but variations in pH modified deposition. Cathodoluminescence of new growth of calcite and quartz shows considerable variety, and some luminescent and non‐luminescent quartz have grown contemporaneously. Most stable isotope data show regular trends. Petroleum has migrated within the Taranaki Basin at least twice: once during early diagenesis of the Kapuni Group, and later after diagenetic mineral growth had occurred. Deep sandstones of the Farewell Formation in the Kapuni Field, though now virtually impermeable, probably passed through a period of diagenetically modified porosity that allowed migration of early hydrocarbons.

Keywords:

• diagenesis
• Taranaki
• Moki Formation
• Kapuni Group
• clay minerals
• quartz overgrowth
• feldspar overgrowth
• dissolution
• heavy minerals
• carbonates
• cathodoluminescence
• stable isotopes