Mineral Alteration in a Low Enthalpy Hydrocarbon Well, Taranaki, New Zealand

Abstract

Mineral alteration in the Eocene Mangahewa Formation sub-arkoses of Well Mn-2 is a product of millions of years of water-rock interaction in a rock-dominated environment. The thermal peak, at about 150?C, occurred 4Ma ago. Since then temperatures have fallen to about 120-130?C. Temperature, pressure, permeability/porosity , degradation of organic material, the composition of porewaters and gases and time induce alteration and deposition of new minerals in this low enthalpy hydrocarbon well. Kaolinite, illite-smectite, pyrite and carbonates may have initially formed under near-equilibrium conditions but subsequent coarsening of crystals or ion diffusion within or among crystals may be kinetically-induced. In contrast, the growth of quartz during the diagenetic history of the Mangahewa Formation may be entirely kinetic. Healed fractures and overgrowths in quartz indicate that earlier solutions were enriched in Cu and Zn while the later ones have high contents of Ca, K, Ni, C1 and Ge.