Carbonate cementation in Upper Eocene clastic reservoir rocks from the North Alpine Foreland Basin (Austria)

Abstract

A strong relationship between carbonate precipitation and microbial gas generation is evident for the Upper Eocene reservoir rocks of the North Alpine Foreland Basin. To achieve a better understanding of this relationship, 40 samples of limnic to shallow marine, gas-, oil-and water-bearing sandstones were studied to determine mineralogy and diagenetic history. The specific mineral parageneses were used to reconstruct changes in the hydrogeochemical conditions over time. Thus, authigenic mineral phases within reservoir rocks are an important archive for the reconstruction of pore fluid composition changes.The eogenetic pore space evolution of investigated Eocene sandstones is influenced by their primary mineralogy, which is strongly controlled by (i) depositional environment, (ii) detrital input and (iii) transport distances. Thus, a low compositional maturity is associated with high feldspar and high clay mineral content. Authigenic clay minerals, formed during several stages of diagenesis, play an important role for reservoir quality, due to pore space reduction.During eogenesis, authigenic micritic and sparitic carbonate phases are precipitated, which decreases the pore space. These eogenetic carbonate cements exhibit isotope values of about δ C: -5.9 to +2.2‰ and O: -8.3 to -4.3‰ [VPDB]. Some of these sam- 18ples indicate a trend towards lighter δ O values (-17.2‰), which is attributed to meteoric flush.Within the Eocene sandstones, two types of strongly cemented zones with low permeabilities can be differentiated: (i) extraordi- 13nary light δ C (-28.4‰) carbonates, which formed due to degradation of organic matter at the stage of advanced sulfate reduc- 13tion and (ii) heavy δ C (δ C: +8.7‰), which precipitated at the fermentation zone. Within the reservoir sandstones telogenesis is characterized by mineral destabilization (e.g. carbonate and feldspar corrosion) and kaolinite precipitation. The formation of authigenic kaolinite booklets resulted into a decrease in porosity.

Multiphase carbonate cementation in the Miocene Pétervására Sandstone (North Hungary): implications for basinal fluid flow and burial history

“…Accordingly, only a minor amount of hydrocarbons migrated from underlying shales after thermal maturation. Grundtner et al (2017) reported similar isotopic values in calcite cement of Upper Eocene sandstone from the North Alpine Foreland Basin (Austria) and concluded that cementation occurred during advanced stage of sulphate reduction. The possible source of Mg in this calcite can be the ankerite which was replaced by these calcite phases.…”

Petroleum Systems in the Austrian Sector of the North Alpine Foreland Basin: An Overview

“…Shallow-marine sandstones of the Ampfing Formation are laterally more continuous than the channel sandstones of the Voitsdorf Formation and the Cerithian Beds and have a net thickness of 10 m. Most sandstones show moderate sorting and are classified as lithic arkoses, but litharenites, subarkoses and sublitharenites are also observed in the northern part of the North Alpine Foreland Basin (Fig. 5) (Grundtner et al, 2017). Rare glauconite and the presence of foraminifera support a marine depositional environment.…”

“…Whereas carbonate cement is typically rare in non-marine to brackish sandstones, the amount of carbonate cement in sandstones of the marine Ampfing Formation often exceeds >5 vol%. Early diagenetic carbonate precipitation, promoted by alkaline conditions, prevented further compaction but the carbonate was was later partly dissolved (Grundtner et al, 2017). The isotopic composition of the carbonate cement indicates the non-marine formation of carbonate minerals in the Voitsdorf Formation and the Cerithian Beds, and a meteoric flush in the laterally-continuous Ampfing Formation.…”

“…1c), four sandstone intervals with low lateral continuity have a net thickness of 2.3 m (Nachtmann, 1989). The sandstones are poorly sorted and can be classified as lithic arkoses with 40 vol% quartz and 18 vol% feldspar on average (Grundtner et al, 2017). Lithic arkoses with a net thickness of up to 6.1 m also occur in the Cerithian Beds at the Sat field (Nachtmann, 1989).…”

“…Single well-cemented layers, up to 50 cm thick, may occur locally in all units. They formed either during advanced stages of sulphate reduction or are associated with fermentation (methanogenesis) (Grundtner et al, 2017).…”