Porosity in chalk – roles of elastic strain and plastic strain

Abstract

Chalks originate as Cretaceous to Recent pelagic or hemipelagic calcareous ooze, which indurate via burial diagenesis to chalk and limestone. Because they accumulate in pelagic settings with high environmental continuity, chalks may form thick formations and even groups. For this reason, and because chalks have a simple mineralogy (low magnesium calcite, silica and clays), they are ideal for the study of diagenetic processes including the depth-related decrease of porosity. It is the aim of this study to illustrate how the evaluation of in situ elastic strain can help in understanding these processes including the interplay between stress-controlled diagenetic processes and processes furthered by thermal energy. Petrophysical core and well data can be used for analyses of how porosity reduction via pore collapse and pressure dissolution is related to in situ elastic strain. The data in question are: depth, density of overburden, pore pressure, ultrasonic P-wave velocity and dry density/porosity. The analysis reveals that the transition from ooze to chalk is associated with high elastic strain and consequent pressure dissolution at calcite-particle contacts causing contact cementation. The transition from chalk to limestone is also associated with high elastic strain, especially at clay-calcite interphases causing development of stylolites via pressure dissolution, and consequent pore-filling cementation. Following each transformation the elastic strain drops rapidly. The observation of this diagenesis-related pattern in elastic strain of the sedimentary rock is novel and should not only be helpful in understanding the porosity development in sedimentary basins, but also add basic scientific insight.

The history of calcite diagenesis and origin of exceptionally negative oxygen isotope values in chalks of the Niobrara Formation, Denver Basin, USA

“…where ϕ i is the initial porosity at zero burial depth, − a is an exponential curvature factor, depth' the depth over which porosity decreases by approximately 1/ e , and θ the empirically derived rotation factor. The fitted curve yields an initial porosity of 57% at zero depth, a reasonable value for a chalk ooze within many tens of metres of the sea floor (Fabricius, 2007, 2020; Garrison, 1981; Mallon & Swarbrick, 2002). Fluid is lost from the system as pore volume decreases.…”

“…By analogy with chalks in general, small amounts of calcite cementation (Garrison, 1981; Schlanger & Douglas, 1974; Tagliavento et al, 2021) and recrystallisation (Börre & Fabricius, 1998; Fabricius, 2007; Neugebauer, 1974; Scholle, 1977) may have occurred concurrently with mechanical compaction, but the presence of early cements have not been documented in the Niobrara. However, rhombohedral terminations atop coccolithophore prisms in the Niobrara chalks do become more equant and larger with burial (Figure 6A,B), which is a phenomena indicative of early recrystallisation (Fabricius, 2020).…”

“…floor (Fabricius, 2007(Fabricius, , 2020Garrison, 1981;Mallon & Swarbrick, 2002). Fluid is lost from the system as pore volume decreases.…”