Sub-Garn Sands Blog : Part 4 – Facies Assignments Using Core Descriptions

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;

Our core descriptions at the NPD Geobank facilities in Stavanger, are being undertaken to consistently define depositional settings across the Tilje, Tofte and Ile formations through ~2.7km of core in 30 study wells. Dr. John Cater, Paddy Clarke, Hannah Torrance, Mirko Looser and Jamie Scorgie are completing this work through visits in November 2022 and January/February 2023. A key aim is to differentiate deltaic settings, and then cross-reference sedimentary facies against authigenic and diagenetic mineralogy based on petrography, to help us understand the causal relationships and controls on reservoir quality.

Core description – Interrogating the sedimentological record

When describing core, particularly at a detailed 1:50 scale and in large regional studies with teams of geoscientists contributing, it’s important to consistently document all visible sedimentary features and not to simply note the facies. Facies may be re-interpreted at a later date, when our understanding of the depositional evolution of a study area changes. With a detailed record of technical observations, it’s possible to confidently compare or correlate facies between wells, and to re-evaluate facies interpretations. It is therefore important that a focused team, trained in the same way, undertake our core descriptions based on a principle of first generating disciplined descriptions, with interpretations that can later be amended. Key observations that we record are palaeo-salinity and the dominant sedimentary transport processes, which we use to define a facies classification.

Sub Garn Sands Blog Part 4 Facies Assignments Using Core Descriptions PetroStrat Staff At NPD Geobank Stavanger
Sub Garn Sands Blog Part 4 Facies Assignments Using Core Descriptions PetroStrat Staff At NPD Geobank Stavanger

To recognise salinity of the environment of deposition (ranging from freshwater to fully marine) we use trace fossils, as evidence of the organisms adapted to live in that niche environment. For example, bivalves have a shell that can open and close when conditions become intolerable and hence, can occupy a setting with variable freshwater influxes from rivers, that other organisms cannot. Trace fossil assemblages and specifically our index-trace fossils (deemed to be high-confidence indicators of freshwater or marine settings, through studying modern environments) are recognised by PetroStrat as being characteristic of different zones within a delta, or away from a delta where there’s less freshwater input.

We also use sedimentary transport processes, which we see evidence of in sedimentary structures – a key facet of process-based sedimentology. For example, in deposits from delta-front setting, we observe evidence of wave action (reworking sediment delivered by rivers), bioturbation (can obliterate sedimentary structures) and current action (tractional, uni-directional or bi-directional tidal currents). This allows us to differentiate wave-dominated, tide-dominated and bioturbation-dominated areas. A combination of sedimentary transport processes and trace fossils enable us to assign palaeoenvironments.

Interpreting lithofacies and environmental facies

At the interpretation stage, we assign both a lithofacies description (effectively a rock type), alongside a general environmental setting (facies). For example, we assign a proximal delta front facies, on the basis of salinity tolerant trace fossils and a general lack of silt, recording higher energy than more distal locations.

Finally, our hand-drawn graphical core logs are made graphically consistent by our professional draftsperson, redrawn using Illustrator software, before these image files are incorporated within WellCAD charts alongside wireline logs and CCA datasets.

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