A geochemical approach to reservoir characterisation: Assessing the origin and distribution of porous zones in feldspathic sandstones, Aotea Basin, New Zealand

Abstract

Three wells have been drilled in the frontier Aotea Basin targeting a Cretaceous reservoir interval. Feldspar is prolific, occurring as detrital grains of plagioclase, K-feldspar, and the dominant mineral within plutonic and volcanic lithic fragments. In sandstone lithofacies these feldspathic grains have undergone variable diagenetic reaction, resulting in locally significant secondary porosity, kaolinite and/or carbonate. In most cases detrital plagioclase has undergone more reaction than K-feldspar. Quartz overgrowths, authigenic kaolinite and carbonate have reduced porosity, impaired permeability, and decreased reservoir potential of the sandstones, while feldspar dissolution has locally increased pore volume and pore size, thereby improving reservoir quality. Pore size distribution quantified by image analysis demonstrates a strong link between plagioclase reactions, porosity enhancement, and effective porosity. In this study we show how regular downhole sampling coupled with laboratory analyses (geochemical and/or automated mineralogical techniques) can provide a method for predicting porous zones in the feldspathic sandstones. Downhole geochemistry shows that sandstone intervals impoverished in Na, Ca, ± Mg or Fe can represent high permeability “sweet” zones, characterised by enhanced plagioclase dissolution (low Na) associated with minimal carbonate cement (low Ca, Mg, Fe). Kaolinite-rich horizons are more difficult to identify from elemental data due to in situ reprecipitation of Al and Si (as kaolinite); however, these intervals are captured by downhole mineralogy data. A regular downhole sampling and analysis approach is recommended for future reservoir characterisation studies in emerging industries where there is a need to understand the distribution of reactive minerals and/or where pore space is a commodity.