Chemical and mechanical processes during burial diagenesis of chalk: an interpretation based on specific surface data of deep‐sea sediments

Abstract

Burial diagenesis of chalk is a combination of mechanical compaction and chemical recrystallization as well as cementation. We have predicted the characteristic trends in specific surface resulting from these processes. The specific surface is normally measured by nitrogen adsorption but is here measured by image analysis of scanning electron micrographs. This method concentrates on the micritic matrix alone. Deep‐sea sediments are ideally suited to the study of burial diagenesis because they accumulate in a relatively conservative tectonic setting. We used material from the Ontong Java Plateau in the Pacific, where a > 1 km thick package of chalk facies sediments accumulated from the Cretaceous to the present. In the upper 200–300 m the sediment is unconsolidated carbonate ooze, throughout this depth interval compaction is the principal porosity reducing agent, but recrystallization has an equal or larger influence on the textural development. In the chalk interval below, compaction is not the only porosity reducing agent but it has a larger influence on texture than concurrent recrystallization. Below 850 m grain‐bridging cementation becomes important resulting in a lithified limestone below 1100 m. This interpretation is based on specific surface data alone, and modifies current diagenetic models.

Stylolites, porosity, depositional texture, and silicates in chalk facies sediments. Ontong Java Plateau – Gorm and Tyra fields, North Sea

“…Image analysis was done on backscatter electron micrographs of impregnated polished samples also studied by Borre & Fabricius (1998), to measure nannofossil matrix porosity, and amounts of large particles (e.g. foraminifers) and pores by use of the filtering method described by Borre (1997;Figs 2 and 3).…”

“…Recrystallization involves local simultaneous dissolution of carbonate particles and overgrowth on carbonate particles, where no net transport of Ca 2+ and CO 2À 3 is required and no porosity change necessarily is involved, whereas pore-filling cementation is a result of diffusion and net precipitation of material derived, e.g. from dissolution at stylolites (Borre & Fabricius, 1998).…”

Discriminating cool‐water from warm‐water carbonates and their diagenetic environments using element geochemistry: The Oligocene Tikorangi Formation (Taranaki Basin) and the dolomite effect

“…This study uses the term deep burial diagenetic realm when burial depths were sufficient for the onset of chemical compaction as a result of increasing overburden pressure. For carbonate deposits this typically requires a few 100 m of burial (e.g., Hood & Nelson 1996;Nicolaides & Wallace 1997;Borre & Fabricius 1998). By this stage, pore fluid chemistry is usually influenced by increased temperatures and pressures, and possibly salinities, in a strongly reducing environment, involving marine-modified or connate fluids.…”

Rock Physics Interpolation Used for Velocity Modeling of Chalks: Ontong Java Plateau Example

“…This depth interval is commonly referred to as the ooze interval [16]. In this interval, mechanical compaction forms a more rigid rock frame [17] and we may have contact cementation [6].…”

“…This progressive diagenesis (ooze-chalk-limestone) has been addressed by many authors and includes changes in the pore geometry [10,19] as well as the reduction in porosity (e.g. [2,16,17,20,21]). Saberi et al [10] showed how we can incorporate the effect of these changes into rock physics characterization of the chalks (Fig.…”