The impact of provenance and diagenetic reaction gradients on CO2 fluid-reservoir interactions (CASP.RFP.1)

Uncertainty on the suitability of sandstone reservoirs for CO2 storage may be reduced with a better understanding of how feldspars influence reservoir response to CO2 injection. Sub-basinal variations in feldspar composition may be an important yet hitherto overlooked factor. This is because feldspar reactivity to CO2 fluid is strongly dependent on its composition, which is in turn a function of provenance and the severity of subsequent burial diagenesis. Moreover, feldspar dissolution during burial diagenesis and the generation of secondary porosity and clays is also compositionally driven and fundamentally changes rock texture and reactivity to CO2.

Figure 1. Map showing the location of the investigated Forties Sandstone Member and its axial and lateral turbidite systems with contrasting provenance.

Disentangling the relative role and reactive significance of these provenance and burial diagenesis drivers is explored via a case study from the Central North Sea (Fig. 1). Major element chemistry and Pb isotopic signatures of the feldspars have identified granularity within the Eocene Forties Sandstone Member (Fig. 2).

Figure 2. Plot showing the increasing proportions of albite with depth from the Forties Sandstone Member. The two parallel trends highlight sub-basinal provenance-related compositional variation.

The expected feldspar content, composition and its influence on the reactivity of Forties Sandstone reservoirs at planned storage sites (Fig. 1) may be deduced from the identified regional trends. More generally, reservoir models for injection sites tens of kilometres distant from well control may be amended to –more accurately account for feldspar driven provenance and burial effects.

Research programme

A suite of Forties Sandstone Member samples collected from North Sea exploration wellbore materials will be investigated (Fig. 1). The samples come from diverse locations and depths to facilitate the identification of provenance and burial effects.

The drivers for the varied sandstone composition will be investigated using several complementary techniques. A statistical evaluation of the data will be contextualised within a literature-derived sedimentological facies model. From this, a framework for the shifts in feldspar composition as a result of different provenance and degrees of burial will be generated.

Provenance interpretations will be facilitated by investigating samples collected from within the sediment source region resulting from fieldwork in Scotland and legacy CASP expeditions.

The SEM-derived compositional texture will be assessed in order to qualitatively evaluate the degree that the variation in feldspar chemistry and abundance, driven both by provenance and burial diagenesis may alter reservoir-CO2 interactions.

Deliverables

  • Data release and associated presentation for client companies – Forties Sandstone Member samples. Datasets will include:  petrography, SEM, feldspar Pb, conventional heavy mineral, detrital zircon U-Pb geochronology and detrital apatite combined U-Pb geochronology and trace element geochemistry data. An accompanying explanatory PowerPoint presentation will contain plots of these data with initial interpretations.
  • Data release and associated presentation for client companies – Sediment source region samples. Datasets will include: SEM, feldspar Pb and zircon U-Pb data. An accompanying explanatory PowerPoint presentation will illustrate how these data inform the provenance of the Forties Sandstone Member.
  • Geotechnical report 1 – A generic model for evaluating the effect provenance and diagenesis play on CO2 interaction with arkosic sandstone reservoirs will be formulated from the case study data. The model will be designed to assist risk evaluation where well control and injection site are tens of kilometres distant and the reservoir interval has undergone different degrees of burial.
  • Geotechnical report 2 – Containing a full description of the provenance and diagenesis variation identified in the investigated Forties Sandstone Member samples and SEM-derived elemental maps and interpreted mineralogy illustrating their effect on compositional texture and thus reservoir-CO2 interactions. The expected reservoir composition at the target CO2 injection sites will be presented along with a qualitative assessment of reservoir behaviour difference between well control sites and proposed injection sites.

Project duration

The project has a planned duration of 18 months.

Contact: Michael Flowerdew for further information about this research and licensing options.

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