Sub-Garn Sands Blog : Part 1 – Determining Core And Sample Selection

Sub-Garn Sands Blog Info Box

Building upon the knowledge we’ve recently gained on the relationships and controls between lithology, facies, provenance, diagenesis, depth and reservoir quality within the Jurassic Garn Formation along the Halten Terrace (2021), this new study will extend our understanding into what PetroStrat defines the ‘Sub-Garn Sand Complex’ (Ile, Tofte and Tilje formations).

We’ll be sharing highlights of our integrated sedimentological and stratigraphic workflow at key project milestones with this series of 6 blog posts.

Other articles in this blog series, and related content;

Sampling Selection

Ahead of our Reservoir Geologists visiting the NPD Geobank, it was important to first review existing datasets (accessed using DISKOS) to ensure that we’d be capturing a representative variation of Ile, Tofte and Tilje formation sands within our forthcoming reservoir quality analytical programme (petrography, SEM imagery and XRD analysis). As with the completed Garn study (2021), a key aim will be to explore the relationships between lithology, facies, provenance, diagenesis, location, depth and reservoir quality – ultimately to provide geological context on porosity & permeability trends that are much better than may be expected in deeper (>4,000m) sections of the Sub-Garn. For consistency with the Garn study, 1 sample will be analysed for every 10m of core described (i.e., 275 samples across the 2.75km of core).

Our process first involves extracting numerical porosity-permeability data from conventional core analysis (CCA) PDF reports and plotting this relative to formation (as defined biostratigraphically, by PetroStrat) and depth in Excel charts (see Figure 1). We recognised within the Garn study the importance of pore-lining chlorite in helping to preserve primary porosity (resisting mechanical compaction and preventing the nucleation of quartz-cement overgrowths) and so for the Sub-Garn we’ll be sampling high-permeability outliers from each formation at varying depths for every well, to observe the role of authigenic cements and grain size in forming reservoir sands. Our sample selection was also aided through reference to core photos (Figure 2), with which we generated a preliminary facies identification that enables us to incorporate a variation in facies within the 275 samples. Grain density data in CCA reports was reviewed to help us avoid sampling around highly cemented plugs, and reference to composite logs (particularly gamma & density neutron) was important to understand how representative the cored sections of these 3 sands are in the 28 wells, relative to their total thickness.

PetroStrat thanks companies sponsoring this project for their contributions in advising upon our sample selection process.

Stratigraphic Database and Biostratigraphic Framework

A consistent stratigraphic framework is an essential component in any well based geological study. To help ensure that appropriate, and age-equivalent sections of the ‘Sub-Garn Sands Complex’ (Ile, Tofte and Tilje formations) were consistently included, described, sampled, interpreted and correlated – the project team drew upon the PetroStrat’s recently updated & extended Norwegian Sea Stratigraphic Database of 213 wells.

PetroStrat stratigraphers utilised microfossil distribution over the Aalenian – Pliensbachian interval to assign sequence and lithostratigraphic interpretations for each of the wells. This followed a process of reviewing all publicly available biostratigraphic data and identifying specific marker and influx events that are characteristically associated with particular regional marker horizons. Distribution of these biostratigraphic events within the individual wells provides the basis for the stratigraphic framework applied in the Sub-Garn database.

Key to biostratigraphic correlation of wells in Sub-Garn reservoir study :

  • Ile Formation: J50.1 sequence (light grey)
  • Ror and Tofte Formations: J40 sequence (dark grey)
  • Tilje Formation: J30.3-J30.2 sequence (yellow)

Our stratigraphic review highlighted instances where the lithostratigraphic formation picks published on the NPD Factpages website are open to re-interpretation based on the observed biostratigraphic marker events. This has significant implications for the proposed thickness and distribution of the sands of the Ile, Tofte and Tilje formations, within the study area when compared to previous stratigraphic interpretations.

The lithostratigraphic interpretation for each well includes both the updated PetroStrat interpretation and original operator interpretation (represented by the NPD Factpage picks), with discrepancies highlighted pink in the ‘Group/Formation Deviation’ column. Our summary chart for 6507/7-8 (Figure 2) offers an interesting example, where the original interpretation had a relatively thick Ile Formation, yet the revised PetroStrat interpretation re-assigns the majority of this interval into the Garn and Not formations.

Utilising the updated regional stratigraphic understanding for all 28 wells in this Sub-Garn study has facilitated a coherent approach to the core description and petrographic sampling program. This ensures our analytical datasets will consistently capture key horizons of interest within the revised lithostratigraphic units, and will ultimately allow the meaningful mapping of our sedimentological observations, to allow insights on regional controls such as a variation in sediment provenance being linked to diagenesis through the Jurassic.

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