Modelling of porosity evolution and mechanical compaction of calcareous sediments

Abstract

A continuum mechanics model for the gravitational compaction of sediments is derived by assuming that the sediments are normally pressured and in a one‐dimensional state of stress. Sediment strength is characterized in terms of effective stress laws adopted from soil mechanics. The model is a relatively simple mathematical formula that gives the porosity as a function of burial depth. The shape of the porosity profile is controlled by two mechanical parameters, the compression index and the void ratio at an effective stress of 100 kPa. The model was verified by analysing the porosity—depth data of oozes and chalk from the Ontong Java Plateau, gathered during Leg 130 of the Ocean Drilling Program. The mechanical parameters of the sediments were estimated using a least‐squares method to fit the theoretical profile to the porosity data. The theoretical profile described accurately the ooze porosity data over depth ranges of 100 m or more. However, over smaller length‐scales of 10–50 m there were systematic deviations between the theoretical porosity values and the ooze porosity data. The porosity deviations correlated with variations in the mean grain size of the sediments, due in part to changes in the foraminifera abundance. In the case of the oozes, the estimated mechanical parameters were consistent with published values obtained from one‐dimensional compression tests. In contrast, the estimated mechanical properties for the chalks differed from published values. The chalk porosities were lower than could be explained by mechanical compaction. This explanation is supported by the compressional (P‐wave) velocity data. In the chalk sections, the P‐wave velocity increases more rapidly with burial depth than it does in the ooze sections, suggesting that sediment elastic properties are increasing due to interparticle binding.

Rock physics interpolation used for velocity modeling of chalks: Ontong Java Plateau example

“…These high values of the critical porosity are mainly related to the ooze spherical structure, which is similar to that of sandstones [22], and to their foraminifera content [6]. Positive correlations between foraminifera content and porosity, mean grain size and velocity have already been explained by other authors [3,4,6]. This is further confirmed by scanning electron microscopy images that show intragrain porosity within foraminifera tests [23].…”

“…The relationship between P-wave velocity, porosity, and burial depth has been discussed widely by other authors (e.g. [3][4][5][6][7][8]). …”

“…Audet [4] used a theoretical compaction curve based on soil mechanics theory to explain the observed trends in porosity and velocity versus depth for the normally pressured oozes and chalks in the Ontong Java Plateau, while Japsen *Address correspondence to this author at the Department of Earth Science, University of Bergen, N-5007 Bergen, Norway; Tel: +47-48350591; Fax: +47-55583660; E-mails: mohammad.saberi@geo.uib.no, reza_zeydan@yahoo.com [5] investigated such relationships for overpressured chalk anomalies in the North Sea. However, Fabricius [6] related diagenesis (burial effects) of chalk sediments to elastic Pwave modulus by construction of a series of isoframe curves, where isoframe values can be related to the Biot coefficient [8].…”

Experimental method for estimation of compaction in the Oxfordian bedded limestones of the southern Kraków-Częstochowa Upland, Southern Poland

“…Precisely, about 100 kPa of load correspond to a 20-metres-thick overburden (cf. Audet 1995). It was assumed that the accumulating layer is subjected to a minimum load X min = 0.01 kPa (X min >0) which gradually increases with the weight of accreting overburden.…”

Structural evidence for the nature of hiatal gaps in the Upper Cretaceous to Holocene succession recovered from the Eratosthenes Seamount

“…Shortly after the geometry of burrows was first used to study deformation (Crimes, 1975), the different response of carbonate sediments to stress was demonstrated (Byers and Stasko, 1978). Grain size and carbonate content have also been shown to have a profound effect on shortening behavior (e.g., Gaillard and Jautée, 1987;Audet, 1995). At Eratosthenes, both the carbonate and organic contents change significantly from burrow fill to surrounding material.…”