biostratigraphic program design and management • interpretation and integration

Biostratigraphic Sampling Guidelines

Below are some general guidelines for acquiring suitable biostratigraphic samples from wellsite, outcrop or sample repositories.
As with many other types of samples, the key issues are:

  1. Suitable sample material
  2. Quantity of sample material
    1. Amount needed for different fossil types
  3. Sample interval
  4. Composite sample thickness

Summary table for sampling quidelines:
The table below summarizes the important points of biostratigraphic sampling requirements. Items are expanded upon in the text following this table.

Analytical Objective Fossil Groups Sample Quantity (grams) Sample Interval Sample Types

Sequence stratigraphy
Depositional systems
Paleoenvironments

All available fossil types: Palynology
Forams
Nannofossils
150-200 gm unwashed ditch
>1" SWC
50 gm washed ditch
>4 samples per cycle
10' (3m) cuttings interval
uniform spacing prevents cycle skipping
Unwashed ditch
SWC
Core
Washed ditch
Age of sediment packages and unconformities All available fossil types: Palynology
Forams
Nannofossils (L.Mz-Cz)
150-200 gm unwashed ditch
>1" SWC
50 gm washed ditch
Tighter spacing around lithologic transitions SWC
Core
Unwashed ditch
Washed ditch
Paleoclimate reconstruction
(Most effective Olig.-Recent)
Marine: Forams, Palynology
Nonmarine: Palynology, Diatoms, Ostracods
Ditch:
100 gm Forams
50 gm Palynology
>1" SWC
Tightly spaced sample interval required to generate proxy curves (10'-30') Unwashed ditch
Washed ditch
Broad paleoenvironment mapping Marine: Forams
Nonmarine: Palynology
Ditch:
100 gm Forams
50 gm Palynology
1" SWC
Resolution proportonal to sample interval (30'-120') Any type
Regional age correlations Most applicable fossil type:
Palynology
Forams
Nannofossils
Any available Resolution proportonal to sample interval (30'-90') Any type

Suitable sample material: ^top

Suitable sample material may come from several sources.
The type of sample should be clearly labeled on the sample bag.

NOTE: Any sample that is free of contamination and can be stratigraphically positioned is suitable for analysis.

Preferred sample types: ^back to- Suitable sample material

    Cuttings, or ditch, samples are good if they are collected on a frequent enough scale to address the exploration problems. The drill bit and circulation perform a mechanical running average on the lithologies penetrated. This has a positive effect for biostratigraphic analysis on a small scale, but can blur the recovered information if the sample rate is too large.

    Unwashed cuttings samples are preferred over washed and dried cuttings samples. Washed samples may have had much of the fine fraction washed away at the rig site. Washed samples are often dried in a frying pan over a hot plate at the well site (not recommended). The excessive heat can destroy or alter organic fossils.

    Sidewall cores are good for quality control of the cuttings and bracketing stratigraphic horizons. Tightly spaced SWCs are good for detailed small scale studies.
    Conventional core is usually only available over short reservoir sections and is useful for addressing issues on a finer scale.

Unwashed cuttings: ^back to- Suitable sample material

Unwashed cuttings samples collected at the smallest practical sample interval are preferred for well studies. Roughly one pound of most lithologies will provide enough sample material for the analysis of several different fossil types. Samples can always be composited to larger sample intervals later.

Washed and dried cuttings: ^back to- Suitable sample material

Washed and dried cuttings will work, but often the fine grain material that contains most of the fossils is washed away. Air dried samples are fine, but often drying times are reduced by "frying" the samples in a skillet on a hot plate. This heating practice can "cook" organic fossils, alter the geochemistry of the sample and alter many mineral phases. Do not allow sample-catchers to "cook" samples. If they must, note it on the sample bag.

Sidewall cores: ^back to- Suitable sample material

Sidewall cores are good for quality control of cuttings samples, especially concerning downhole caving. However, sidewall cores alone often do not provide adequate coverage or sampling of the section to answer all types of biostratigraphic questions.

Conventional core: ^back to- Suitable sample material

This is great source of samples. However, the thickness of available conventional core material is usually very limited and often restricted to reservoir facies that may not be as fossiliferous as finer grain facies.
When sampling conventional core, consider compositing a material from an interval of the core, rather than spot sampling.

Miscellaneous circulation drilling: ^back to- Suitable sample material

Drilling mud and mud additives are often a source of biostratigraphic contamination. Many mud additives are raw or slightly processed fossil-bearing rock material such as lignite, limestone or bentonite.

Quantity of sample material needed: ^top

    The size of unwashed sample needed is approximately 2 cups (one pound wet) unwashed cuttings sample. Detailed information on the required sample size for different types of sample type is given below in several equivalent measurement systems.

Unwashed cuttings:

Ideal sample size:
Roughly one pound of wet sediment (dry equivalent measures below)
1-2 cups, 8-16 oz, 250-500 grams, 250-500 ml, 0.25-0.5 liter

Recommended minimum sample size:
0.25 pound wet (dry equivalent measures below)
0.5 cup, 4 oz., 115 grams, 120 ml, 0.135 liter

Absolute minimum sample size:
Whatever you can get. In some cases, nannofossils and palynology can be successful with very small sample sizes.

Sidewall cores, conventional core and washed cuttings:

These sample types are "cleaner" so less material is required
2-4 tablespoons
1-2 oz.
30-60 grams
30-60 ml
Washed cuttings and sidewall cores typically fall into the absolute minimum due to the small size collected after washing. Washed cuttings are not preferred, but acceptable if unwashed cuttings samples are not available.

Influence of lithology on amount of material required

    Lithology affects the amount of sample needed for different fossil types.
    For example, fine grain carbonaceous clastics are best for palynology.
    Calcareous shales and fine to medium grain muddy limestones are best for foraminifera and nannofossils.
    In general more sample material is needed as:
    - grain size increases
    - weathering (oxidation or acidification) of the lithology increases
    - recrystallization of material increases

Amount of sample needed by fossil type:

    Palynology:
    A few grams of fine grain carbonaceous sediment can yield several hundred thousand palynological specimens in some cases.

    Micropaleontology:
    As little as 40 grams of some marine sediments.

    Nannofossils:
    As little as 1 gram of marine clay or silt.

    Siliceous microfossils:
    As little as 5 grams.

Fossil type Composition Age range Area of application Ave. amount of sample needed
Palynology Organic walled: pollen, spores, algal cysts, fungal spores Recent-Late Precambrian Marine and
Nonmarine
50-75 gm
golfball sized
Foraminifera Calcium carbonate + composite Recent-Cambrian Marine 100 gm
baseball sized
Nannofossils Calcium carbonate Recent-Late Jurassic Marine, low latitude 5 gm
pea sized
Diatoms Silica Recent-Mesozoic Marine/nonmarine
destroyed by burial below 5000'
5 gm
pea sized
Radiolaria Silica Recent-Ordovician Marine 50 gm
golf ball sized
Ostracods Calcium carbonate Recent-Cambrian Marine and
Nonmarine
100 gm
baseball sized
Conodonts Calcium phosphate Triassic to Cambrian Marine 500 gm
softball sized
Macrofossils and Trace fossils Variable Recent-Late Precambrian

Marine and
Nonmarine (rarely)

Core or outcrop


Sample interval: ^top

    Samples must be acquired and analysed with an interval fine enough to answer the geologic question.
    For detailed studies where the biostratigraphy is to be applied to sequence stratigraphy, paleoclimate, or high resolution chronostratigraphy, the finer the sample interval the better the potential results.
    Acquiring samples is inexpensive! Get as many samples with the smallest sample interval possible. Sample analyses can always skip every other sample or samples can be composited into a larger sample interval.

Determining sample interval: ^top

    If logs or seismic sections are available, the frequency of sequence stratigraphic "packages" or intervals can be estimated. Think of sequence stratigraphy as a sine wave. Under-sampling a curve will result in the reconstruction of a curve with a different wave length and frequency. To reveal the most basic nature of stratigraphic variations, each interval must have at least one sample. In practice, more than one sample is needed to characterize each sequence stratigraphic package. A minimum of 4 samples is needed to define a cycle or interval (see Ledbetter and Ellwood, 1976. Geology, v. 4 no. 5: 303-304). Samples should be taken at 10 m (30 ft) intervals in sections that require detailed biostratigraphy for application to sequence stratigraphy, paleoenvironments, and chronostratigraphy. Important seismic and log boundaries should be bracketed. Thirty (30) meter (90-100 ft composite) samples may be adequate for analyses in sections where the lower resolution information may be adequate for gross chronostratigraphy applied to geohistory modeling.

Composite samples: ^top

    Thirty meter 100 ft) composite samples can be made by combining equal parts of each 10 meter (30 ft) sample. We are assuming that samples were originally collected over 30 foot intervals. Obviously, if samples were larger or smaller the resulting composite sample interval will change.

Targeting specific horizons: ^top

    Questions about specific lithologic units can only be answered accurately if the unit is sampled. Again, several samples should be taken to ensure that fossil recovery is adequate to characterize the age and paleoenvironment of deposition of the unit.

NOTE: It is preferable to collect the original sample interval and composite the samples in our lab before we process for microfossils.

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