To get good biostratigraphic results, you need to start with a good set of samples. But there are several factors to consider before heading off with a sample bag and a marker pen.
Probably the first thing to consider is, what are the issues or questions that need resolving? What are you hoping to get from the biostratigraphic data? Once you know that, then you can start looking at where to take your samples. For example, if you want information about specific lithologic units, then you need to ensure those units are adequately sampled.
You also need to consider the lithology. Palynomorphs (spores, pollen, dinocysts, etc) are microscopic and settle more readily in low energy environments. As such, it is always best to target dark-coloured mudstones or siltstones first for palynology. But don’t let sandstones discourage you; unless the sands are very clean, you should still have good microfossil recovery. In some cases, palynological results from sandy lithologies are even better than the encompassing mudstones. A general rule of thumb is that as grain size increases, the volume of material collected for sampling should likewise increase.
Coals can also be worth sampling, too. When it comes to dating coals or even coal seam correlations, it is the lithology directly above or below a coal that is generally sampled, while the coal itself is avoided. But we often find that a good suite of palynomorphs can be extracted directly from coals, thereby allowing far more accurate age determinations and correlations. So just as with sandstones, don’t be put off by shiny black coals!
And what about the actual sample type – cuttings, sidewall core, conventional core? Is one better than the other?
Cuttings samples are perhaps the most common type of sample we get. They are a valuable source of data, providing a continuous signal through each well, and that is especially useful over key intervals. Caving chips can sometimes be incorporated within cuttings samples, and evidence of this can be seen in the palynological assemblages. When this occurs, we may get a mixed assemblage within a sample, but this rarely affects the zonal interpretation as the majority of events defining any given zone are based on first down-hole appearances of key species.
As cuttings samples are usually taken over 10m, 5m or 3m intervals, you may see multiple ages in just a few samples when you have a condensed section. In these cases, the best samples are sidewall and conventional cores as they allow you to target very specific horizons. This is the same when sampling through sandstones, as sidewall and conventional cores allow you to target the mud/silt horizons within the sandstone, and in thermally mature areas, such horizons can often yield better preserved palynomorphs. Conventional cores can also be extremely useful for integrating high resolution palynological data with sedimentological and ichnological observations. This powerful tool allows us to accurately constrain depositional models.
What all this means is that for palynological sampling, cuttings, sidewall and conventional core are all a great source of data, each with its own advantages. By using certain sample types for specific intervals within a well, you can obtain more accurate age constraints.
Our staff are happy to help advise on sampling programs for all new wells or review projects so you get the best value for money, but even once the samples have arrived at our office, there are still additional steps we can take to ensure we produce the best palynological slides and therefore the best datasets possible. And this is where having an on-site laboratory really becomes beneficial.
For example, as part of our standard processing methodology, our palynologists will check all slides prior to the oxidation step. This allows us to determine what, if any, oxidation is needed, as this varies from sample to sample. But it also gives us the chance to check the overall quality of the slides because when it comes to processing, one size does not fit all.
Often, the standard methodology will need to be modified to better suit the type of sediment being processed; some samples will have excess oil from the drilling mud that needs to be removed, while others might contain excessive amorphous organic material or other small particulate matter. Particularly sandy samples might need longer dissolution to extract the organics, or a coal sample might need longer in a different acid. We can also make additional slides of a particular sample, or even have that sample reprocessed, if it turns out to be important for refining a zonal pick.
As you can see from this summary, there are several factors to bear in mind when it comes to collecting samples for palynological analysis, so if you have any questions or would like further advice, please contact us at [email protected].