Abstract
In-situ δ18O measured in the quartz overgrowths help identify temperature and fluid origin variations responsible for cementation of the pore network (matrix and fracture) in the Buntsandstein Gp. sandstone reservoirs within the Upper Rhine Graben. The overgrowths record two types of the evolution of δ18O: 1) a monotonous decrease of the δ18Oovergrowth interpreted as linked to an increasing burial temperature and 2) random fluctuations, interpreted as pointing out the injection of allochthonous fluids in faulted areas, on the cementation processes of the pore network (both intergranular and fracture planes). Fluids causing the quartz cementation are either autochthonous buffered in 18O from clay illitisation; or allochthonous fluids of meteoric origin with δ18O below − 5%. These allochthonous fluids are in thermal disequilibrium with the host sandstone. The measured signal of δ18Oovergrowth measured from samples and calculated curves testing hypothetic δ18Ofluid are compared to T–t evolution during burial. This modelling proposes the initiation of quartz cementation during the Jurassic and is validated by the in-situ 40Ar/39Ar dating results obtained on the feldspar overgrowths predating quartz overgrowths. A similar diagenetic history is recorded on the graben shoulders and in the buried parts of the basin. Here, the beginning of the pore network cementation predates the structuration in blocks of the basin before the Cenozoic graben opening.
Highlights
Cementation is a crucial feature of sandstone reservoir quality and was the topic of numerous studies (Houseknecht 1984; McBride 1989; Lundegard 1992; Bjørlykke and Egeberg 1993) and is still the focus of recent studies (Fischer et al 2013; Bjørlykke 2014; El-Khatri et al 2015; Henares et al 2016; Yuan et al 2017; Schmidt et al 2020)
The nature and temperature of fluids flowing in faults are strongly impacting the properties and mineralogy of rock volumes in the vicinity of faults, especially the permeability distribution in petroleum and geothermal reservoirs, which may result in reservoir compartmentalisation (Labaume and Moretti 2001; Brockamp and Clauer 2005; Faulkner et al 2010; Aretz et al 2015; Navelot et al 2018)
Quartz overgrowths are present in a large amount in the intergranular spaces, mainly developed on the largest detrital grains, whose surface is not coated by smectite or illite
Summary
Cementation is a crucial feature of sandstone reservoir quality and was the topic of numerous studies (Houseknecht 1984; McBride 1989; Lundegard 1992; Bjørlykke and Egeberg 1993) and is still the focus of recent studies (Fischer et al 2013; Bjørlykke 2014; El-Khatri et al 2015; Henares et al 2016; Yuan et al 2017; Schmidt et al 2020). Triggered by fluid flow and compaction, the development of quartz overgrowths may reduce the initial porosity and impact permeability (Worden and Morad 2000; Götte et al 2013; Therkelsen 2016). It is crucial to understand the pathways, timing, and origin(s) of the fluid(s) responsible for quartz cementation to improve diagenetic heterogeneity prediction in reservoirs. The nature and temperature of fluids flowing in faults are strongly impacting the properties and mineralogy of rock volumes in the vicinity of faults, especially the permeability distribution in petroleum and geothermal reservoirs, which may result in reservoir compartmentalisation (Labaume and Moretti 2001; Brockamp and Clauer 2005; Faulkner et al 2010; Aretz et al 2015; Navelot et al 2018)
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