Biostratigraphy is making use of fossils to aid in stratigraphic determination, correlation etc. It is unrestricted to the kinds of fossils found in rocks which can of course range from the microscopic to the gigantic (there is a formally defined biozone in the Late Cretaceous of North America defined by a genus of dinosaur!). Common kinds of fossils familiar to most geoscientists as biostratigraphic tools include ammonites, graptolites, trilobites, mammals and, of course, microfossils.

In the hydrocarbon industry, although some primary information comes from outcrop studies, the majority of data collected is derived from drilled boreholes. The drilling process destroys rock (to a great extent ) and by extension any identifiable-by-sight fossils contained within them. Fortunately very-small sized fossils (“microfossils”) are largely immune to the drilling process and remain intact within rock fragments in ditch-cuttings samples. It is these fossils upon which this booklet, and the bulk of this course, are based. However, many of the principles covered can also be applied to larger fossil types.

Biostratigraphers normally enter the profession based primarily on a personal choice of what microfossil type(s) they would like to study. The topic is too vast for a single person to master every aspect of every fossil group of even the complete stratigraphic range of a particular fossil group. This may also be influenced by their teachers and perhaps an interest in a particular geologic time period or geographical region. Training at undergraduate level is insufficient and industrial practitioners of biostratigraphy have at least Masters or Doctoral level specialist qualifications, followed by a period of training “on the job”. In fact, biostratigraphic specialists tend to be very specialised indeed with some being experts on, for example, Neogene benthonic forams from the Gulf of Mexico and having relatively little knowledge/experience with microfossils outside these parameters.

The method of sample processing also has a surprisingly strong influence on the particular fossil group(s) a scientist will study. Each of the three sub-disciplines of micropaleontology (in the loose sense) have their own distinctive preparation techniques and each will favour the recovery of microfossils within that particular sub-discipline. For example, traditional micropaleontological preparation techniques are such that foraminifera (all types), ostracods, diatoms, radiolarian and conodonts are all potentially recoverable from samples using the same processing/analytical workflow. Also, palynological preparations do not exclude any of the four main palynomorph fossil types. The only factors that will exclude individual fossil types from samples will be the age of the rock, the environment of deposition and the type of lithology. See below for further discussion on this topic.

In an applied situation, biostratigraphers will often need to turn their hand to working with any fossils they may find in their various prepared samples. A “micropaleontologist”, for example, may be a foraminifera specialist but he or she will be expected to also try and identify species of other microfossil types found in the same sample (e.g. ostracods or radiolaria). Should more expertise be required then another specialist may be assigned or contracted in.