MOGE, M. J. C., TOTAL Centre Scientifique et Technique, St. Remy les Chevreuse, France

ABSTRACT: Diagenetic Heterogeneities in Mid-Jurassic Sandstone Reservoirs in the North Sea

Carbonate-cemented intervals are common in the mid-Jurassic deltaic and shoreline sandstone reservoirs of the North sea and become a major problem when they are laterally extensive, as they affect vertical communication, create different fluid flow units, and reduce net pay. In a vertical section through a reservoir, these cemented zones commonly make up 110% of the total thickness.

Cores and well logs from eight wells in three oil fields (Bruce and Lyell in the UK sector, and Veslefrikk in the Norvegian sector) have been studied in order to understand the diagenetic processes responsible for these carbonate cements, and to relate them to sedimentological and stratigraphic patterns within the deposit.

The carbonates occur as nodules, concretions or stratiform layers 0.5 to 5m thick. They comprise poikilotopic calcite formed during three successive phases of cementation, ranging from early to late in the burial history. Each phase of cementation can be characterized by oxygen and carbon isotopic signature, as well as by cathodolumiunescence patterns. By studying the occurrence of vertical variations of the 87Sr/86Sr ratio in the formation water above and below the carbonate-cemented horizons it has been possible to identify those cemented horizons which are laterally extensive, and form internal barriers within the reservoir.

These carbonate barriers generally coincide with sedimentological discontinuities such as flooding surfaces or the abrupt appearance of storm beds. This relationship between diagenetic barriers and sedimentological/stratigraphic events furnishes practical guidelines to predict their occurrence in this type of reservoir.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.