Sedimentological and diagenetic controls on the reservoir quality of marginal marine sandstones buried to moderate depths and temperatures: Brent Province, UK North Sea

Abstract

A study of the Brent Group sandstones, in well 211/13-A33 in the Thistle Field, northern North Sea, UK, from a sequence of shoreface, foreshore, delta plain and finally barrier shoreline depositional environments, was undertaken with the aim of contrasting the roles of depositional and diagenetic processes on reservoir quality. These subarkosic and sublitharenite sandstones were buried to ~9000 ft TVDSS and have experienced a maximum burial temperature of about 95 °C. This study utilised wireline and core analysis data, with core samples studied using petrography, XRD, SEM-CL, and SEM-EDS. The dominant diagenetic cements are kaolinite and siderite, with minor quantities of pyrite, chlorite, illite, K-feldspar, calcite and quartz cement. A common sequence of diagenetic events occurred in sandstones from all four depositional environments. The dominant reservoir quality controlling factors are depth-controlled mechanical compaction and variable kaolinite growth, with grain size being an important secondary control. The shoreface sandstones have the highest degree of compactional porosity-loss because they have the greatest quantity of detrital ductile grains. Quartz cement is a minor phase in all formations because these sandstones have only just exceeded the temperature threshold for quartz cement growth. Siderite is found in all formations but with the greatest quantity found in the shoreface and delta plain sandstones, since these environments were most enriched in detrital iron minerals such as biotite and chlorite. The shoreface sandstones host stratigraphically-localised, pore-occluding calcite cement that has compartmentalised the reservoir. Overall, the foreshore and delta plain sandstones have the best reservoir quality because they have (i) few ductile grains resulting in low degrees of compaction, (ii) low quantities of pore-filling cement and (iii) they tend to be medium-grained. In contrast, the shoreface sandstones and barrier shoreline sandstones have poorer reservoir quality as they have (i) moderate to large quantities of ductile grains, (ii) moderate quantities of pore-filling cement and (iii) they tend to be fine- or very fine-grained. Detrital sand mineralogy and grain size vary as a function of depositional environment and these have subsequently played a major role in linking diagenesis and reservoir quality to depositional environment.