A compaction front in North Sea chalk

Abstract

[1] North Sea chalk from 18 wells shows a pronounced porosity drop, from ∼20% to less than 10% over a compaction front of less than 300 m. The position of the compaction front is independent of stratigraphic position, temperature, and actual depth, but closely tied to an effective stress (load stress minus fluid pressure) of ∼17 MPa. These observations require a strongly nonlinear rheology with a marked increase in compaction rate at a specific effective stress. Grain-scale observations demonstrate that the compaction front coincides with marked grain coarsening and recrystallization of fossils and fossil fragments. We propose that this nonlinear rheology is caused by stress-driven failure of the larger pores and the associated generation of reactive surface area by subcritical crack propagation away from these pores. Before the onset of this instability, compaction by pressure solution is slowed down by the inhibitory effect of organic compounds associated with the fossils. Although the compaction mechanism is mainly by pressure solution, the rheological response to burial may still be dominantly plastic and controlled by the (fracturing controlled) rate of exposure of reactive surface area. The nonlinear compaction of chalk has significant implications for the evolution of petroleum systems in the central North Sea, both with respect to sea-floor subsidence above hydrocarbon-producing chalk reservoirs and for the formation of low-porosity pressure seals within the chalk.

Compaction of North-Sea Chalk by Pore-Failure and Pressure Solution in a Producing Reservoir

“…However, the pore size distribution is somewhat uncertain. In the following we assume a Weibull distribution ( [38], see subfigure in Figure 8) with 2.2 µm as mean pore size [20] that was fitted to SEM image data of chalk from wells in the same formation. We calculate microscopic fracture density as a function of effective stress (see Figure 8) with these input parameters.…”

“…FIGURE 10 | Strain rate calculated assuming a Weibull distribution of pore size, using a range of mean pore size and effective stress values and dissolution rates calculated from the theoretical model. A line representing the modeled mean pore size for Ekofisk chalk (following Japsen et al [20]) and average strain rate for this period is also plotted on the figure. The contour line crosses the line corresponding to Ekofisk chalk.…”

“…A line representing the modeled mean pore size for Ekofisk chalk (following Japsen et al [20]) and average strain rate for this period is also plotted on the figure. The contour line crosses the line corresponding to Ekofisk chalk.…”

“…This threshold is defined by linear elastic fracture mechanics: r max = γ E/σ 2 e where E is the Young's modulus of the rock and γ is the interfacial energy of the rock-pore fluid surface [20].…”

“…Except for the effective stress we use constants obtained from literature (see Table 1). For pore size distribution p(r) we use a Weibull-type distribution with a shape parameter 1.5 and mean pore size of r mean = 2.2 µm as found in Japsen et al [20]. Effective stress is calculated using the relation…”