Impact of kaolinite and iron oxide cements on resistivity and quality of low resistivity pay sandstones

Low resistivity pay (LRP) zones have been identified within many siliciclastic reservoirs around the world, and have gained significant attention due to their ability to hold substantial hydrocarbon volumes. Previous studies have focused on investigating the role of highly conductive minerals, microporosity, bound water, and bed thickness in reducing resistivity within LRP zones. However, the impact of kaolinite and iron oxides in influencing the resistivity of LRP zones remains poorly understood. To address this, we examined outcrops cemented by pore-filling kaolinite and iron oxides, which serve as analogues to subsurface reservoirs. This study combined field description with laboratory analyses, including petrography, mineralogy, porosity, permeability, resistivity, and nuclear magnetic resonance to investigate the occurrence of kaolinite and iron oxide cements and their impact on resistivity and reservoir quality. The study observed various iron-bearing structures including concretions, fracture infills, layers, ferricrete, and liesegang bands. These structures are cemented by microporous pore-filling kaolinite (up to 18%; average: 6.6 %) as well as goethite and hematite (up to 21%; average: 3.5%). Observations from this study indicate that both depositional processes, such as grain size, and diagenetic pore-filling cements, significantly influenced the resistivity and reservoir quality of sandstones. Fine-grained sandstones containing substantial amounts of microporous kaolinite (>15%) and iron oxides (>17%) cements show very low resistivity values (<= 0.2 Omega m), as well as low porosity (<10%) and very low permeability (<1 mD). Conversely, medium to coarse-grained sandstones with lower concentrations of kaolinite (<15%) and iron oxides (<5%) cements exhibited higher resistivity values (2-40 Omega m; average: 8 Omega m) and displayed higher porosity (average: 22%) and permeability (average: 1300 mD). This study underscores the importance of understanding and characterizing the presence and distribution of iron oxides and kaolinite cements, along with other factors like microporosity and grain size within LRP zones. Such understanding is crucial for accurate resistivity measurements, reservoir quality prediction, and hydrocarbon estimation in sandstone reservoirs worldwide.

Automated summary

This study examined the influence of kaolinite and iron oxides on the resistivity and reservoir quality of low resistivity pay zones. The presence of these cementing minerals was found to significantly affect the resistivity and permeability of sandstone reservoirs, with fine-grained sandstones containing higher concentrations of kaolinite and iron oxides exhibiting lower resistivity and permeability values.