Estimating horizontal stresses for mudrocks under one-dimensional compression

- We measured the horizontal-to-vertical effective stress ratio (K0) of mudrocks.
- Laboratory tests were performed over the stress range of 0.1 MPa-100 MPa.
- K0 generally increases with effective stress level during normal compression.
- Smectite-rich mudrocks display a more rapid increase in K0 with stress.
- High horizontal stresses can develop at depth solely due to mechanical compression.

We examine horizontal stresses in mudrocks for the case of one-dimensional mechanical compression through laboratory measurements of the horizontal to vertical effective stress ratio, i.e. K0. Previous empirical relationships which predict horizontal stresses as a function of depth or overburden stress are site specific and cannot easily be generalized. The K0 values of various resedimented mudrocks from a diverse range of geologic origins have been investigated for effective stresses ranging from 0.1 to 100 MPa. The value of K0 during normal compression (K0NC) varies systematically as a function of both stress level and mudrock composition. K0NC generally increases with stress level and values as high as 0.90 have been measured at effective stresses approaching 100 MPa. This finding implies that high horizontal stresses can develop at depth solely due to normal mechanical compression and that tectonic stresses, creep, or geologic unloading are not necessary. Correlations have been developed which allow K0NC to be estimated from a mudrock's liquid limit, an easily and inexpensively measured property which reflects both the quantity and type of clay minerals present in a mudrock. High liquid limit, smectite-rich mudrocks display a more rapid increase in K0NC with increasing stress compared to more silty, low liquid limit mudrocks. K0 measurements made on laboratory resedimented specimens compare well with measurements made on intact core samples as well as in situ tests. Tests carried out to examine K0 for overconsolidated materials show that K0 during unloading can be well approximated by a power-law function originally proposed by Schmidt (1966). An original equation is proposed to describe K0 during reloading. Our experimental results reveal sediment behavior at high stress levels that has not been previously observed and provide useful information on how horizontal stresses evolve within basins.