How to describe and interpret clastic core

After writing “Describing and Interpreting Carbonate Core” I received several requests for a similar post for clastic reservoirs. Before I begin I should clarify that this article should not be used as a guideline for mudrock (“shale”) cores.Although more straight forward than carbonate rock, siliciclastic cores present challenges of their own. In my last post I mentioned how the lack of sedimentary structures and ichnofossils in most carbonate cores can be daunting. In clastics you get the full spectrum – so you better be prepared to deal with it systematically.This is my approach (there may be better ways to do it, but since I am the author you’re sort of stuck with mine for now):

1. Product

The rock you have before you was transported as grains then deposited. In the form of grain-size, sorting, and sedimentary structures it contains data on flow velocity and distance from the source. Every sedimentary structure (or lack thereof) counts. Learn the relationship between oscillatory flow vs unidirectional and combined flow to better understand these products. If you’re working on deepwater systems it would be worth taking the time to familiarize yourself with the diversity of sediment gravity flows.To document what you see you’ll need to learn how to use a hand lens correctly, which means you cant hold it up like you’re admiring a butterfly. You need to get up-close-and-personal with the rock. You will need a grain-size chart on the core’s slabbed face to compare grain-sizes. There is no such thing as “eye-balling it” in sedimentology. Besides grain-size you’ll need sorting too. Now this whole grain-size and sorting business you probably learned in college, so what’s the big “hoo-ha”?Well maybe you didn’t know this but ripples only develop in certain grain sizes, and dunes wont form when the sediment gets too coarse. Subtle variations in grain sizes can be critical and sorting can provide all kinds of information. Over time you’ll know what grain size range certain sedimentary structures tend to be more common in, like Hummocky Cross Stratification in fine sand.As important as lithofacies are, their contacts are just as important. Make sure you document whether the contact was gradational, sharp-regular, or sharp-irregular.

Tidal processes dominated deposition of this cat-back dune in the Cretaceous Frewens Sandstone of the Powder River Basin, USA

2. Process

For every sedimentary structure ask yourself: what hydrodynamic conditions would be needed to form this feature in this particular grain-size and in this order of events?

The fun doesn’t stop once deposition has ceased. In the marine realm (and often on land as well) this is when critters of all kinds will churn the sediment, to dwell, to seek food and sometimes to escape the unwelcome sand or mud dumped on them. This is the complicated world of ichnology and I am no expert. Fortunately, I have had the honor of learning from some of the best, and if you’re serious about describing core I suggest you do too.

Our ichnology maestro Dr. Sadeque showing how its done during a workshop on the Muddy “J Sandstone”

3. Context

To me context comprises of all supporting evidence that may prove or disprove an interpretation. This could be facies associations in core, log signatures and even seismic data.

In simple terms, just because you see vertical burrows in “clean” sandstone doesn’t mean it’s a beach deposit. Every piece of the puzzle needs to come together. In certain cases core just isn’t enough. For example it’s ridiculously difficult to distinguish tide-dominated deltas from estuaries from core alone. You need clear demonstration of progradation to differentiate one from the other. This often comes from integrating logs or seismic data.

Armored mud-ball from the deepwater sandstones of the Cretaceous Lysing Formation, Offshore Mid-Norway

4. Interpretation

This for me is the most fun part because after working hard towards an interpretation I like to play devil’s advocate and see how many ways I can shoot my own model down. This is science, not religion and you can’t hold on to your beliefs. You’re going to be wrong often, so instead of waiting for someone else in a different company looking at your data to come up with an alternate interpretation, why not go through the exercise yourself?

As a consultant I will often provide my clients with multiple cross-sections using the same set of core data, with the stipulation that one is more likely than the other, and acquiring additional data will reveal which of the two is more feasible.

Coarse-grained inversely graded debrite analogs for the Granite Wash of the Anadarko Basin

To conclude remember that your core description is not going to end up as a poster on your wall. This is a product others will use for decades perhaps. Make it user-friendly. If you’re working for a fancy consulting firm that loves adding a million curves for isotopes etc, ask yourself, can the average asset team geologist even use these data? Provide what’s useful and avoid the bells and whistles. Software packages can make your life easier but will the final user of your product be able to orient themselves to their core using your graphic representation?

Now go have fun and put some pictures on linked-in. If you would like a step-by-step methodology for describing core, check out knowledgette.

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How to describe and interpret clastic core

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