Improved reservoir characterisation of a Chilean tight sandstone reservoir

This dissertation investigates the petrophysical characterization of a tight gas greensand, Zona Glauconitica (ZG), from the Magallanes basin in Tierra del Fuego island, Chile. The sandstones are mineralogically immature probably reflecting a source area that is rich in volcanics with a low rainfall. This has partly led to the samples containing a significant amount of iron-bearing glauconite and/or chlorite. These clays impact reservoir quality and pose challenges during petrophysical evaluation because they lead to a complex microstructure containing significant microporosity and impact various properties. Glauconite and chlorite have a moderate cation-exchange capacity (CEC), adding a second conductive water layer and decreasing the rock resistivity. Therefore, the classical Archie’s (1942) model is unsuitable, and shaly-sand water saturation models are preferred.
Analysis of core and well-logs was used to study ZG’s electrical, elastic and flow properties. The reservoir is divided into three distinct petrofacies (PRT1 – 3). PRT1 contains most of the producible gas; it lacks glauconite, has a bimodal pore size distribution, low permeability (0.01 – 1mD), moderate to high porosity (22 – 27%v/v), and low specific surface area (~3.4 m2/g). It has the highest iron (14 %wt) and CEC (72 meq/100g), which provide a photoelectric factor signature to quantify the clay volume. PRT1 has the best reservoir quality due to the presence of secondary porosity and the suppression of quartz cementation by the grain-coating chlorite. PRT 2-3 has ultra-low permeability (< 0.01 mD), low to high porosity (11 – 29%v/v), unimodal PSD of nanometric size, high specific surface area, moderate iron content and CEC values. Indicators of good reservoir quality are a Vp/Vs ratio> 1.75 and Log FZI> -1.25 using the Flow Zone Indicator method. The Indonesian and modified Simandoux models best fit ZG’s water saturation profile, as it is a freshwater reservoir (12,000 NaCl) with significant clay mineral electrical contribution.