Influence of diagenetic features on petrophysical properties of fine-grained rocks of Oligocene strata in the Lower Indus Basin, Pakistan

Abstract

Abstract Nari Formation is considered as one of the most important oil and gas exploration targets. These fine-grained tight sandstone reservoirs face enormous challenges due to their extremely low matrix porosity and permeability. Hence, in this regard, the study was carried out to collect the high-quality data on petrophysical properties along with mineralogy and microstructural characteristics and diagenesis. The experiments performed includes the petrographic study and scanning electron microscopy, and X-ray diffraction analyses. Besides, the measurement of petrophysical properties was carried out to assess the likely influence of the reservoir quality. The petrographic analysis shows predominantly fine- to medium-grained grey samples along with calcite, clay, lithic fragments and iron oxides. Further, the thin-section observations revealed that the quartz is a principal mineral component in all the analysed samples ranging from 52.2 to 92.9%. The bulk volume of clay minerals that range from 5.3 to 16.1% of. The porosity and permeability measured range from 5.08 to 18.56% (average 7.22%) and from 0.0152 to 377 mD (average 0.25 mD), respectively. The main diagenetic processes that affected the sandstones of Nari Formation are mechanical compaction, grain deformation, cementation and quartz dissolution and have played a significant role in influencing the quality of the reservoir rock. Overall, it appears that the primary petrophysical properties (porosity and permeability) were decreased due to the mechanical compaction, lithification, cementation, and framework grain dissolution. Based on the integrated mineralogical, microstructural analysis, and the laboratory-based petrophysical properties, the samples exhibited poor porosity, permeability, and moderate clay content, which indicate that the Nari Formation is a poor quality reservoir.