title: Sandstone reservoir quality: assessing the negative and positive controls: a study of the eodiagenetic Corallian sandstones of the Weald Basin
Creators: name: Barshep, Dinfa Vincent
Publisher: name: University of Liverpool Repository
Year: 2024

description: Shallow marine sediments are at the transient boundary between continent and ocean and are affected by an interplay of mechanisms that control sediment deposition including tectonism, relative sea level rise and climate. Consequently, shallow marine environments present varied sedimentological, mineralogical, and stratigraphic properties which makes reservoir quality prediction challenging. Assessment of diagenetic controls and their interlinking relationships is important for effective reservoir quality prediction hence in this study, shallow marine Corallian sandstone reservoir rocks from Palmers Wood and Bletchingley fields were investigated to understand the controls on porosity and permeability (reservoir quality). This study involves wireline analysis, core analysis, sedimentary core log analysis, optical petrography, SEM-EDS analysis and geochemical analysis techniques to understand the provenance, depositional and diagenetic controls on reservoir quality. The results show that these shallow marine sandstones have low evidence of compaction but low to moderate porosity for their relatively shallow burial depths of less than 1700m during their maximum burial. Their porosity ranges from 0.8 to 30% (average of 12.6%) and permeability range from 0.01 to 887 mD (average of 31 mD). The Corallian sandstones also show a unique composition of medium to coarse grained siliciclastic sandstones with abundant bioclasts and Fe-ooids deposited during a tectonic active phase undergoing sea level rise. Provenance analysis revealed a mixed mafic-felsic provenance which supplied sediments extensively enriched in quartz and Fe by sedimentological processes especially hydrological sorting. Tectonism influenced water depth, controlled the quantity of detrital bioclasts and the resulting calcite cement and clay matrix distribution. Early bioclast-derived calcite cement retarded compaction and together with calcite from organically derived CO2, is the dominant cause of porosity loss. Other cements include berthierine, pyrite, siderite, dolomite, apatite, illite, kaolinite and quartz. The reservoir quality of the Corallian sandstones have no significant depositional controls; reservoir quality was reduced by cementation and pore-filling clays, locally preserved by authigenic quartz-inhibiting berthierine grain coats and enhanced by detrital grain dissolution as well as cement dissolution. Reservoir quality in the Corallian sandstones can therefore be predicted by considering abundance of calcite cement, pore-filling clay and Fe-clay coats. This research presents a potential first sandstones reservoir quality study in the Palmers Wood and Bletchingley which are relatively poorly studied hence paucity of data to predict reservoir quality away from wells. This study can be used to understand eodiagenetic conditioning of sediments for mesodiagenetic processes such as chlorite grain coats formation from berthierine as these sandstones straddle the conditions between surface controls on eodiagenesis and mesodiagenesis.