Research paperCompaction and rock properties of Mesozoic and Cenozoic mudstones and shales, northern North Sea

- Rock physics depth trends help to infer the variations of rock physical properties.
- Hordaland Group contains less smectite in southern part of the study area.
- Occurrence of overpressure was detected within the smectite-rich mudstones.
- Lack of potassium source hinders the chemical diagenesis despite high temperatures.

In this study, rock physical properties and their evolution resulting from compaction processes are investigated for Mesozoic and Cenozoic mudstones and shales located in southern Viking Graben and adjacent areas within the Norwegian North Sea. The studied sediments are deposited within a progressively-subsided sedimentary basin with no major experience of exhumation events. A suite of well log data from 43 exploration wells was utilized to study the compaction behaviour of the Mesozoic and Cenozoic mudstone and shale intervals. The gamma ray log-derived shale volume (Vsh) was used to define different lithofacies and discriminate between the studied mudstones and shales.The rock properties as a function of burial depth were plotted for the identified mudstone and shale intervals. The trends could be divided into a mechanical compaction part and a chemical compaction part depending on the prevalent processes controlling the rock properties with burial depth. The transition from mechanical compaction domain to the zone of dominant chemical compaction takes place between 70 and 90 °C corresponding to a depth of 2-2.5 km.The onset of chemical compaction and cementation occurs in the same sediment found at the same depth range almost throughout the study area in spite of variable geothermal gradient indicating a lithological control on the development of chemical compaction. The degree of chemical compaction and cementation reflects the initial smectite content and the availability of potassium for the smectite to illite and quartz reaction to take place. This study contributes to the understanding of compaction processes in fine-grained siliciclastic sediments delineating the controlling factors in a region which can be regarded a natural laboratory to study compaction mechanisms due to being a subsiding basin with the extensive availability of pertrophysical data generated by hydrocarbon exploration and production activities in the area.