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

WARRENDER, J. M., Conoco (UK) Ltd., Aberdeen, Scotland, and D. A. SPEARS, University of Sheffield, Sheffield, England

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. Proven zones of unswept oil have been detected below doggers in some wells, isolating them from overlying higher permeability Etive Formation sands, long since flushed by the advancing water-injection front. Thus, the existence of doggers in Murchison field is potentially important in delaying water breakthrough which would otherwise occur by down-coning of water from the Etive Formation.

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 profile sand bodies, characterized by low detrital clay and mica contents, low bioturbation, and high pre-cementation porosities and permeabilities. Several coarsening-upward sequences have been recognized, related to distinct phases of vertical shoreline aggradation. 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 th ee significant, laterally continuous calcite-cemented zones occur, the most extensive of which has a lateral extent of at least 5 km. All significant doggers 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.

AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)