Sampling for paleontology
| Exploring for Oil and Gas Traps | |
| |
| Series | Treatise in Petroleum Geology |
|---|---|
| Part | Predicting the occurrence of oil and gas traps |
| Chapter | Applied paleontology |
| Author | Robert L. Fleisher, H. Richard Lane |
| Link | Web page |
| Store | AAPG Store |
The quality of the interpretations made by a paleontologist can be strongly influenced by the quality of the samples and their preparation. Many factors, therefore, are affected by the actions of the sample collector and processor. Controlling those factors can have a significant impact on the utility and application of paleontologic data.
Drill cuttings
Most paleontologic analyses for the petroleum industry are performed on drill cuttings (fragments of rock cut by the drill bit and circulated to the surface by the mud system). Often, a sample of the drill cuttings is collected for each joint of drill pipe added to the drill string (i.e., every length::30 ft, or 9 m) and represents an amalgamated sample of the rocks penetrated in that interval.
Advantages of using cuttings
Although the use of cuttings is less precise than conventional or sidewall cores, this method of sampling can yield surprisingly robust paleontologic results at a reasonable cost. Biostratigraphic analyses can be performed on cuttings at the well site, providing significant economic benefit in picking core points, casing points (which may prevent blowouts), and stopping points (to prevent unnecessary drilling below the objective section).
Disadvantages of using cuttings
Certain shortcomings are inherent in analyses based upon drill cuttings:
- Samples can contain a mix of the different lithologies penetrated.
- Downhole contamination (caving from rocks above the sample interval) can occur.
- Stratigraphic resolution can be no finer than the sampling interval.
- Lost-circulation material added to the mud system can so dilute the system that little or no formation material is contained in the “sample.”
Sampling program
To obtain good samples for paleontologic studies, the following points in the sampling program should be considered.
- Begin in the planning stage and work with the engineer/geologist in charge. Clearly define the responsibility for sample collection, and specify the importance of accurate collection and labeling. Rig personnel/mud loggers have been known to gather many samples at one time but to label them as having come from different depths.
- Plan the sampling program to include fine-grained rocks because lithologies conducive to the recovery of microfossils are normally not reservoir facies.
- Specify depths (beginning and end) at which cuttings should be collected.
- Specify frequency of cuttings collection. Commonly a sample is collected every length::10 ft (3 m) and composited into 30-ft (9-m) intervals for each joint of drill pipe added to the drill string (i.e., every length::30 ft, or 9 m). However, the geologist can specify a different interval if necessary.
- Clearly label samples with waterproof marker as to whether they have been “lagged” (i.e., depths corrected for circulation time).
- Clearly define the responsibility for sample collection and specify the importance of accurate collection and labeling.
- Adjust sampling size to the fossil group being used. The following chart gives rough approximations of the quantity needed.
| Fossil Group | Quantity |
|---|---|
| Nannofossils | pea to teaspoon size |
| Foraminifera | 1–2 cups |
| Palynomorphs | 1–2 cups |
| Conodonts | 5 cups |
Pitfalls to avoid
To obtain good samples for paleontologic studies, several pitfalls should be avoided when collecting samples:
- Avoid reusing drilling mud from other wells to preclude contamination with microfossils from the previous drill site.
- Remember that drilling muds made from naturally occurring materials may include microfossils (e.g., pollen and spores in lignite, dinoflagellates in bentonites). If unrecognized, these can distort the biostratigraphic signal.
- Monitor the mechanical effects of different bit types. In particular, diamond and composite (polycrystalline diamond composite, or PDC) bits can metamorphose the cuttings, destroying the fossils contained in them.
Laboratory techniques
Finally, workers must use proper handling techniques in the laboratory to obtain good samples for paleontologic studies:
- Use the least destructive technique for processing samples. This can be accomplished by mechanical (detergent, Varsol®, blender, etc.) and chemical (H2O2, various acids, bases, etc.) techniques. Overprocessing damages or destroys fossils, reducing the usefulness of the sample. Indurated rocks may require thin section.
- Carefully monitor the laboratory sample processing techniques that extract microfossils from the sediment. To obtain high-quality residues, try the following:
- Stain sieves between samples with methyl blue.
- Clean utensils between samples.
- Clearly label containers.
- Eliminate contamination from sediment “popping” during boiling by appropriate vessel spacing.
For specific processing techniques, see Kummel and Raup.[1] or Feldmann et al.[2]
Other sample types
Despite the emphasis on cuttings, other sample types are used for paleontologic analysis. Cores are particularly useful in carbonate plays. Analysis of sidewall cores (especially of nonreservoir rock) can reliably identify lowest occurrence horizons because they avoid sampling caved material. The rise of international exploration has led to increased analysis of outcrop material; outcrop samples should be unweathered and “clean” to avoid contamination and leaching. Megafossils, which can be collected in many outcrop sections, may define the known local stratigraphic framework.
Sample preparation
A wide variety of techniques is available for preparing samples for paleontologic analysis. Most of these involve methods of freeing fossils from sediment and concentrating them for ease of examination. The choice of preparation process depends upon the lithology and the type of fossil; the essentially mechanical techniques used to extract foraminifera from clastic sediments are totally different from the chemical methods used to free paly-nomorphs or to extract conodonts from limestone. Kummel and Raup[1]) and Feldmann et al.[2]) describe many of the preparation techniques in detail.
See also
References
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