Chemical compaction of mudrocks in the presence of overpressure

Abstract

In sedimentary basins, compaction disequilibrium generates overpressure during rapid burial of fine-grained sediments in the mechanical compaction regime, at temperatures below ~70°C. Mudstones behave differently at greater depths in the chemical compaction regime, at temperatures above ~100°C, where evidence suggests that porosity reduction with increasing depth and temperature continues independently of effective stress up to high values of overpressure. We offer an explanation for this behaviour. The horizontal alignment of clay mineral grains is enhanced during clay diagenesis, creating sub-horizontal, flat pores. Because of their flexibility, the flat pores tend to close even under low values of normal effective stress acting across them. Thus, chemical compaction can proceed unless the net expulsion of pore water from the mudstones is inhibited sufficiently for the flat pores to be held open, which necessarily requires the pore pressure to approach the lithostatic stress. In the Lower Kutai Basin, density log reversals are encountered in mudstones in the chemical compaction regime at depths of 3–4 km, where the pore pressure is close to the lithostatic stress. We attribute these reversals to the inhibition of dewatering during clay diagenesis at shallower depths, when the pore pressure was already close to lithostatic stress. Porosity was preserved by the very high pore pressure holding the flat pores open while the mudstone matrix was being cemented by the products of clay diagenesis. We coin the term ‘chemical undercompaction’ for this process.

Regional variation in Cretaceous mudstone compaction trends across Haltenbanken, offshore mid-Norway

“…At higher temperatures, starting around 120 °C, kaolinite transforms to illite provided that potassium is still available (Giorgetti et al 2000;Nadeau et al 2002). By the onset of the chemical compaction stage, mudstones are sufficiently lithified such that they would not yield by mechanical compaction during ongoing burial, even if the pore pressure were hydrostatic (Goulty et al 2012).…”

“…However, it is necessary that excess porewater escapes in order for compaction to proceed. At present, there is no validated method for modelling overpressure generation in mudstones undergoing chemical compaction where pore-fluid escape is inhibited, leaving room for speculation about the process (Goulty et al 2012). Hansen (1996a) determined a porosity-depth trend for hydrostatically pressured Cretaceous and Tertiary mudrocks on the Norwegian Shelf, but he cautioned against using his trend at depths greater than 2600 m below seafloor.…”

A Diagenesis Model for Geomechanical Simulations: Formulation and Implications for Pore Pressure and Development of Geological Structures

“…Previous researchers have established 70 °C as the threshold temperature in conceptual chemical compaction models (Bjørlykke & Høeg, ; Bjørlykke, ; Dutta, ). Other researchers, however, suggested that at temperatures of about 65 °C smectite might already start to transform to illite in mudstones (Goulty et al, ) but that chemical compaction (porosity loss due to chemical reactions) does not start until temperatures of about 100 °C depending on the mineralogical composition and thermal histories of the sediments (Goulty et al, ). On the other hand, quartz cementation and chemical compaction in sandstones are often estimated to occur at temperatures above 80 °C (Bjørlykke et al, ; Walderhaug & Bjørkum, ) although it has recently been suggested that quartz cementation might start at 60 to 70 °C (Harwood et al, ).…”

“…Second, we have considered that the change in permeability for a given diagenetic porosity loss is the same as the change that would produce an equivalent mechanical‐induced porosity loss (we assume preservation of the porosity‐permeability relationship). In reality, however, this might not be the case, as diagenetic processes might enhance grain orientation and involve textural changes in sediments that could change the porosity‐permeability relationships (Bjørlykke, ; Goulty et al, ). Finally, other processes in addition to mechanical and nonmechanical compaction, which are not taken into account in our models, might have contributed to the overpressure observed in the UK Central Graben (e.g., fluid expansion mechanisms, buoyancy due to oil/gas column, gas generation, and lateral transfer).…”

Two-step Wireline Log Analysis of Overpressure in the Bekapai Field, Lower Kutai Basin, Indonesia

“…In previous articles (Ramdhan & Goulty 2010, 2011, we have suggested that overpressure has been generated by unloading processes, especially gas generation, because mudstone density trends continue to increase downwards through the pressure ramp and the depth to top of hard overpressure coincides with the vitrinite reflectance threshold for gas generation. Furthermore, we suggested that the density reversal observed in the deepest well below the sharp pressure ramp was a consequence of 'chemical undercompaction', a process in which porosity was preserved by very high pore pressure holding pores open while the mudstone matrix was being cemented by the products of clay diagenesis (Goulty et al 2012). We no longer think that chemical undercompaction is the correct explanation because it has become clear that diagenetically altered siliciclastic mudstones continue to compact mechanically in response to increasing effective stress .…”