Mudstone compaction and its influence on overpressure generation, elucidated by a 3D case study in the North Sea

Abstract

Mudstones are one of the least permeable rocks in most sedimentary sequences. Accordingly they can act as seals for fluid flow leading to abnormal overpressures. Nevertheless, mudstone compaction and related permeability and porosity decrease are not adequately described in current basin modelling software, because only mechanical compaction is taken into account. In reality, however, clay minerals undergo severe chemical diagenesis which certainly influences petrophysical properties and compaction. In this context a mathematical approach which has been originally developed in soil mechanics has been adapted to basin modelling. The underlying mathematical equations are carefully explained in the text. In the basic equation the compression coefficient is a function of void ratio and effective stress. Using these equations, overpressure can be predicted by using petroleum systems modelling techniques. This is shown for a real 3D case study in the North Sea, in which strong overpressure occurs. A compaction model for mudstones that depends strongly on the clay content of the individual stratigraphic units is used for the calibration of porosities in the 3D case study. In addition, a chemical compaction model that reduces porosities by using a kinetic reaction is used for the deeper part of the basin where mechanical compaction processes are less important. The pressure generation process depends strongly on permeability and compressibility of the porous medium. Therefore, the use of mudstone compaction and permeability models is sufficient to produce pore overpressures. In the case studied, abnormal overpressures are generated during burial together with the petroleum generation process. The mechanical and chemical compaction mechanisms ensure that the pressures are preserved in the deeper part of the basin.

Permeability anisotropy and resistivity anisotropy of mechanically compressed mudrocks

“…These formations develop permeability anisotropy with increasing stress and decreasing porosity. Changing mudrock porosity, and hence permeability, in a sedimentary basin directly affects fluid migration, consolidation rates and overpressure generation (see Broichhausen et al, 2005, Bethke, 1989. Basin models are often used to simulate the evolution of sedimentary basins with time and compression.…”

Factors controlling the thermo-mechanical deformation of oil shales: Implications for compaction of mudstones and exploitation

“…In view of the limitations cited above, there seems to be a consensus that compaction is essentially mechanical at shallow depth (o2 km) for basins with low geothermal gradients (25-35 1C/km) while chemical compaction dominates below this depth (Bjørlykke and Høeg, 1997;Nyga˚rd et al, 2004;Broichhausen et al, 2005). The transition from mechanical to chemical compaction depends on several parameters such as the mineralogy and petrophysical properties of the sediment, the depth of burial, as well as the degree of dissolution and precipitation that its minerals experience with changes in temperature.…”

“…The compaction of mudstones during burial and subsidence has major implications on fluid generation and transport in sedimentary systems. It has therefore been of wide interest in studies on the evolution and understanding of dynamic processes in sedimentary basins (Terzaghi, 1925;Athy, 1930;Weller, 1959;Perrier and Quiblier, 1974;Bayer and Wetzel, 1989;Broichhausen et al, 2005). Compaction proceeds with increasing stress and temperature in nature.…”

Compaction-formed platy limestone from the Middle Cambrian Zhangxia Formation (Western Hills, China): towards a new classification for bedded limestone

“…Nevertheless, the question at which overburden pressure and temperature the boundary between mechanical and chemical compaction should be drawn is still open to debate as discussed by Heidug and Leroy (1994), Zhang and Spiers (2005), Lehner (2009) and also earlier documented by Park and Schot (1968), Rezak and Lavoie (1993) and underlined by the introduction of the term ''chemomechanical process'' (Lehner 2009). Many calculations and modelings were carried out to elucidate the loss of porosity and fluid during compaction (Meyers a Hill 1983;Ortovela et al 1993;Broichhausen et al 2005), and the time and depth of formation of stylolites (Audet 1995). Even self-organization (Railsback 1998) and the fractal structure of stylolites was analyzed (Drummond and Sexton 1998).…”