Search

Compaction and evolution of rock properties of Cenozoic mudstones are investigated in the Quadrant 15, offshore Norway. The study area is a normally subsided basin located in the southern Viking Graben of the Norwegian North Sea. A suite of well log data from 41 exploration and appraisal wells is utilised to study the compaction behaviour of Cenozoic mudstones. The detailed petrophysical analyses suggest the occurrence of high quantities of smectite in the Eocene and Oligocene sediments, in general agreement with previous publications. The transition from mechanical to chemical compaction takes place between 70-90oC corresponding to depth 2-2.5 km. The geothermal gradient is higher in the south of the study area but, temperature alone is not sufficient to explain chemical diagenesis in the area. Initial mineralogy seems to be as important as temperature with respect to cementation. The degree of chemical compaction and cementation reflects the initial smectite content and the availability of potassium for the reaction of smectite to illite and quartz to take place. The overall results of this study contribute to delineate compaction processes controlled by geological processes (e.g. provenance and depositional environments) in a highly prosperous region in terms of hydrocarbon exploration and production.

Petropgrahic evidences of two different quartz cement morphologies have been obtained from Late Cretaceous mudstones, offshore Norway. 1) Fine-grained micro-pore-filling quartz found as discrete grains or short chains or clusters of inter-connected crystals interpreted to be part of larger micro-quartz crystal networks typically interlocking with aggregates of micro-quartz and clay crystals found close to and below 2500 m burial (~80-85 oC). 2) Thin sheet-like quartz platelets mainly oriented normal to the overburden. The quartz platelets originate as flakes (~90-100 oC) that evolve into well-developed patchy continuous quartz cement identified at 4300 m/150 oC. The quartz cement is likely sourced by silica released from the dissolution of smectite and precipitation of illite-smectite/illite (and kaolinite to illite) due to elevated silica super-saturation in the system during burial. The observed inter-connected micro-quartz crystals are associated with a sudden Vp-velocity increase reflecting an increase in the mudrock stiffening interpreted to be related to a critical amount of micro-quartz networks and aggregate formation. The quartz platelets will during increased burial act as stiffening agent reinforcing and further cementing together the earlier formed micro-quartz networks and aggregates.

SUMMARY This study investigates the effects of grain size on petrophysical and acoustic properties of experimentally compacted synthetic mudstones. Four brine-saturated kaolinite aggregates sorted by the grain size were compacted in the laboratory under vertical effective stress up to 50 MPa. Results show that the kaolinite aggregates compacted differently and have atypical acoustic and petrophysical properties as a function of grain size. The maximum compaction was observed in a mixture which contained mixed grain sizes (composite aggregates) whereas the minimum compaction was found in the mixture containing the smallest grain size (