The influence of silica on carbonate diagenesis in chalk – Ekofisk Formation, Danish Central Graben

Kasper H. Blinkenberg, Kresten Anderskouv, Theis I. Sølling, Khalid Al-Ramadan, Lars Stemmerik

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Abstrakt

Upper Cretaceous–Danian chalk forms a range of complex diagenetic fabrics despite its commonly white and monotonous appearance in outcrop and core. This reflects the interaction between processes linked to composition, depositional facies, texture, sedimentation rates, bioturbation, mineralogy, burial and hydrocarbon charging history. Such fabrics have a substantial impact on reservoir quality, either maintaining or reducing porosity and permeability through diagenetic processes. The potential influence of silica, a key component in the chalk, on the diagenesis of the chalk matrix is still poorly understood and forms the topic of this study. A new approach integrating qualitative methods such as micro-computed tomography (μCT) and scanning electron microscopy (SEM) with methods like X-ray diffraction (XRD) analyses supplemented with contact angle measurements, is used to characterise chalk fabrics and investigate the impact of silica on wettability. The study is focusing on a relatively narrow stratigraphic interval, the Danian Ekofisk Formation reservoir in a single well from the Danish North Sea, the Sif-1X, to avoid uncertainties associated with regional variations in burial history, depositional environment, age and hydrocarbon charging history. The Ekofisk chalk in Sif-1X is characterised by two fabrics distinguished based on variations in calcite microcrystal texture, silica content, silicification types and clay content. They are interpreted to represent two different diagenetic pathways. The Ekofisk Fabric I is silica-poor and has the highest porosity and permeability. It is interpreted to reflect a dual-step diagenetic pathway of initial mechanical compaction, during early burial, followed by later, limited pore-filling cementation associated with pressure dissolution. The Ekofisk Fabric II occurs in intervals with more than 5% silica and displays overall inferior reservoir quality compared to the Ekofisk Fabric I. It is interpreted to be the result of local calcite dissolution and cementation related to release of silica-rich fluids during the stabilisation of metastable opal phases into α-quartz, and the progressive stabilisation of silica ultimately led to pervasive silicification of the fine-grained chalk and cementation of the calcite microcrystals. We demonstrate, based on multi-scale data integration, that silica not only affects the chalk and other fine-grained carbonates through pervasive silicification but potentially also has a significant control on the carbonate system, leading to changes in the calcite microcrystal textures. This adds to the complexity of the porosity and permeability evolution in silica-rich chalk and other mixed silica-carbonate systems.

OriginalsprogEngelsk
Artikelnummer105846
Antal sider13
TidsskriftSedimentary Geology
Vol/bind415
DOI
StatusUdgivet - apr. 2021

Programområde

  • Programområde 5: Natur og klima
  • Programområde 3: Energiressourcer

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