Prediction of Mechanical Compaction in Mudstones and Its Implication for Basin Modeling

Nazmul Haque Mondol, Knut Bjørlykke, Jens Jahren, Øyvind Marcussen, and Chris Peltonen
Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, N-0316, Oslo, Norway
(e-mail: [email protected])

Mechanical compaction of mudstones depends on many factors. Mudstone compaction trends vary within wide limits because of their heterogeneity in terms of grain size and mineralogical composition. Mudstones contain a variety of clay minerals (smectite, illite, chlorite, kaolinite etc.) and sand, silt and clay-sized particles which can be composed of carbonates, siliciclastic, volcaniclastic and organic materials. This study investigates the mechanical compaction of a series of reconstituted mudstones and compares results with natural mudstone compaction trends found in well log data in the Mesozoic and Cenozoic sediments from the North Sea area and the Vøring Basin, offshore Norway. Experiments were conducted under controlled pore pressure and increasing effective stress up to 50 MPa corresponding to about 4.5 to 5 km burial depth in normally compacted sediments. Laboratory investigation shows that the mechanical compaction of mudstones varies within broad limits as a function of clay minerals, sand, silt and clay-sized particles of other minerals present and also pore fluid composition and pore pressure. Smectite-rich samples compact much less and have higher porosity and lower P- and S-wave velocities compared to kaolinite-rich samples at the same effective stress. The P- and S-wave velocities increase with increasing sand and silt content in the clay mixtures. The higher P- and S-wave velocities are observed in samples with certain amount of sands and silts mixed in with clay. Thus, treating mudstone as a uniform rock type ignoring factors like clay mineralogy, grain size, and the amount of other mineral present, pore fluids and pore pressure will introduce significant errors in estimation of compaction trends. The simplified mudstone compaction trends (porosity/density/velocity-depth) used in basin modeling and exhumation studies will therefore potentially introduce large errors in the modeling. Experimental compaction trends agree well with well log data. By comparing the experimental trends with well log data and published compaction curves, it can be possible to estimate porosity/density/velocity versus depth trends in mudstones for a wide range of shale lithologies. Using these trends will improve basin modeling for shallow burial depths dominated by mechanical compaction.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands