Diagenesis and reservoir quality of deeply buried Upper Jurassic sandstones in the Central Graben

Abstract

This study deals with the diagenesis and reservoir quality of deeply buried (>4 km) Upper Jurassic sandstones from the Central Graben, North Sea. Petrophysical and petrographical analysis has been done on the cored interval from well 1/3-9S. Quartz cementation is mostly considered as the most important porosity-destroying process in deeply buried quartz rich sandstones. Hence it is important to recognize the factors that may hinder the quartz precipitation and preserve the reservoir quality (porosity and permeability) to below the depths, usually recognized as economic basement. Micro-quartz and illite coating has been observed in the study area. Grain-coats are believed to be the important porosity preserving mechanisms for deeply buried (>4 km) sandstones in North-Sea. High porosity (>10%) and low porosity (<10%) zones have been marked in deeply buried Upper Jurassic sediments. It has been observed that micro-quartz and illite coatings are present in all of the samples. Presence of illite coating around the detrital grains is sparse, while micro-quartz coats are extensively observed around the grains. These observations indicate that the presence of micro-quartz coating is significantly important in preserving the porosity. Moreover SEM analysis shows that in high porosity zones higher amount of micro-quartz coating has been observed as compare to the low porosity zones. Micro-quartz grain coating is common in the Upper Jurassic sediments of North Sea and is generated by the transformation of siliceous sponge spicules (Rhaxella Perforata). Overall high IGVs (30 – 38%) have been observed in this study. High IGVs and well preserved porosity indicates that mechanical compaction is not very significant in the cored interval. It has been observed that samples with higher carbonate cement (≥ 6%) exhibits comparatively higher IGVs as compare to the samples without or lower carbonate cementation. This early carbonate cement, well sorting and fine grain size of the particles helped to reduce the effect of mechanical compaction and preserve the high IGVs and porosity.