Calcite Cemented Layers, Their Characterization and Use in Improving Reservoir Recovery from Murchison Field, Northern North Sea

Abstract

Murchison field is a mature development forming part of the Brent oil province in the East Shetland basin, northern North Sea. The productive reservoir consists of coastal deltaic sands of the Middle Jurassic Brent Group and is produced by a line-drive waterflood mechanism. The Rannoch Formation in Murchison, one of the field's most prolific producing zones, is characterized by the presence of calcite-cemented zones, known locally as doggers, which occlude all porosity and behave as vertical barriers to fluid flow. Geochemical-mineralogical analysis of the carbonate cements allied to sedimentological studies of host sandstone indicate that they occur as continuously cemented layers with relatively homogeneous compositions. Dogger occurrence is generally confined to the upper parts of individual coarsening-upward confined to the upper parts of individual coarsening-upward profile and sand bodies, characterized by low detrital clay and mica contents, low bioturbation, and high pre-cementation porosites and permeabilities. The carbonate bands are thus thought to represent zones of maximum pore water flow through high permeability conduits during burial diagenesis. Wireline correlation supported by core description indicates that at least three significant, laterally continuous calcite-cemented zones occur, the most extensive of which has a lateral extent of at least 5 km. All significant doggersmore » have been mapped geologically and incorporated into the full-field reservoir simulation model. Based on this new understanding, a revised infill drilling and perforation strategy has been devised for the Rannoch Formation, aimed at improving recovery from this complex reservoir zone.« less