INFLUENCE OF LITHOFACIES AND DIAGENESIS ON NORWEGIAN NORTH-SEA CHALK RESERVOIRS

The depositional mechanism of chalk is a kev influence in the chalks u ltimate reservoir quality. Classically, the depositional mechanism is interpreted from core descriptions, Where core data are lacking, dipme ter and borehole imagery logs have proven useful in making lithofacies assessments. Criteria for recognition of three chalk categories are e stablished. Category III chalks correspond to those chalks that have b een deposited by gravity flows or slumping and tend to have the best r eservoir parameters. Category I chalks are most often affiliated with pelagic deposition and tend to have the poorest reservoir parameters. Category II chalks are intermediate between I and III. Anomalously hig h primary porosities have been maintained in Norwegian North Sea chalk s a here the effects of mechanical and chemical compaction have been l imited. The diagenetic pathway of a chalk reflects changes brought abo ut by mechanical and chemical compaction. Five factors most heavily in fluence the diagenetic pathway: (1) burial depth. (2) chalk type, (3) overpressuring, (4) presence of hydrocarbons, and (5) original grain s ize. Assessments of the sedimentological model, diagenetic pathway, an d resultant reservoir quality are provided in case studies of Edda, To r, and Eldfisk fields. Because the distribution of chalk is largely in dependent of existing structures, most field's have a component of str atigraphic/diagenetic trapping. Each case study shows unique examples of how petrophysical and reservoir engineering data can be incorporate d in assessments of chalk type and the diagenetic pathway and how they may affect reservoir parameters and productivity.