Impact of silica diagenesis on the porosity of fine‐grained strata: An analysis of Cenozoic mudstones from the North Sea

Abstract

Silica diagenesis has the potential to drastically change the physical and fluid flow properties of its host strata and therefore plays a key role in the development of sedimentary basins. The specific processes involved in silica diagenesis are, however, still poorly explained by existing models. This knowledge gap is addressed by investigating the effect of silica diagenesis on the porosity of Cenozoic mudstones of the North Viking Graben, northern North Sea through a multiple linear regression analysis. First, we identify and quantify the mineralogy of these rocks by scanning electron microscopy and X‐ray diffraction, respectively. Mineral contents and host rock porosity data inferred from wireline data of two exploration wells are then analyzed by multiple linear regressions. This robust statistical analysis reveals that biogenic opal‐A is a significant control and authigenic opal‐CT is a minor influence on the porosity of these rocks. These results suggest that the initial porosity of siliceous mudstones increases with biogenic opal‐A production during deposition and that the porosity reduction during opal‐A/CT transformation results from opal‐A dissolution. These findings advance our understanding of compaction, dewatering, and lithification of siliceous sediments and rocks. Moreover, this study provides a recipe for the derivation of the key controls (e.g., composition) on a rock property (e.g., porosity) that can be applied to a variety of problems in rock physics.

The Influence of Magmatic Intrusions on Diagenetic Processes and Stress Accumulation

“…It has been proposed that diagenetic processes may affect the fluid flow properties in the subsurface (e.g., [14,47]), a statement which is confirmed by the results in this study. This is the case, not just due to how diagenesis modifies rocks porosity and permeability, but also to how these diagenetic alterations change the rock's physical properties.…”

“…Earlier studies of magmatic intrusions and their effect on porosity evolution and diagenesis of reservoir rocks have come to contrasting conclusions (e.g., [14,15,[122][123][124]). The diagenetic processes of sandstone reservoirs are likely to be controlled by the initial composition of the sand, original pore water composition, content of neighboring lithologies, burial and thermal history, and timing of cementation relative to accumulation of petroleum [17,47,82]. These criteria can largely explain the contrasting conclusions that are put forward in the referenced literature.…”

“…Diagenetic transformations are reported from sedimentary basins worldwide (e.g., [26][27][28][29][30][31][32][33][34]). The Monterey Formation in California, USA, has been extensively studied with regard to the transformation of opal A to opal CT (e.g., [35][36][37][38][39]), but diagenetic alterations have also been observed several places in wells and on seismic data from the Norwegian Continental Shelf (e.g., [24,[40][41][42][43][44][45][46][47]). All of the latter studies report observed transitions of either opal A to opal CT or smectite to illite in the Vøring Basin, offshore mid-Norway.…”

Arrested versus active silica diagenesis reaction boundaries—A review of seismic diagnostic criteria

“…This transformation comprises a complex set of dissolution–reprecipitation reactions (Kastner et al, 1977; Leeder, 1982; Stein & Kirkpatrick, 1976; Williams & Crerar, 1985; Williams et al, 1985; Wrona, Jakson, et al, 2017), which lead to marked changes in the host sediment petrophysics, particularly a porosity drop (Chaika & Dvorkin, 1997, 2000; Isaacs, 1981; Meadows & Davies, 2007, 2009; Nobes et al, 1992; Tada, 1991; Weller & Behl, 2015). These petrophysical variations are essentially due to extensive dissolution of opal‐A and lesser impacts from precipitation of pore‐filling opal‐CT (Varkouhi, Cartwright, et al, 2020, Varkouhi, Tosca, et al, 2020; Wrona, Jackson, et al, 2017; Wrona, Magee, et al, 2017; Wrona, Taylor, et al, 2017). A marked reduction in the content of biosilica frustules following dissolution significantly reduces the sediment stability, which makes its framework susceptible to abrupt collapse, sharp reduction in intergranular and intragranular porosities, and an appreciable interstitial‐water expulsion (Varkouhi, Cartwright, et al, 2020, Varkouhi, Tosca, et al, 2020).…”

Kinematics of Polygonal Fault Systems: Observations from the Northern North Sea

“…It is thus worth noting that the transformation from opal-A to opal-CT probably migrated upward through the lower Hordaland Group, initiating in the Middle or Late Eocene and continuing, potentially, to the present (Wrona et al, 2017a). Apart from the opal-A/CT transformation, the Hordaland Group shows no other significant compositional and diagenetic changes affecting physical rock properties (Wrona et al, 2017b).…”