The global energy sector from a subsurface perspective
Caption to headline image: Biostratigraphers working on an offshore rig in the Northern North Sea. Micropalaeontology analysis like this allows important calibration during drilling operations to compare the pre-drill geoprognosis versus observed stratigraphic markers to safely reach the desired target and maximise reservoir contact. Palaeoenvironmental indicators can also improve understanding of the depositional system across the reservoir interval which can improve estimates of net to gross and other parameters that impact STOOIP calculations in exploration wells. Photo: Halfdan Carstens
From determining how rocks correlate to reconstructing palaeoenvironments and palaeotemperatures, which is relevant to the petroleum geologist, fossils are of great significance to science and industry.
From determining how rocks correlate to reconstructing palaeoenvironments and palaeotemperatures, which is relevant to the petroleum geologist, fossils are of great significance to science and industry
While there are lots of different methods for dating rocks, fossils (through applied biostratigraphy) provide the ultimate foundation for the correlation of sedimentary rocks, leveraging zone fossils, bioevents and fossil assemblages in both outcrops and the subsurface (in wells) to determine the relative ages of rocks and their stratigraphic organisation.
Some example standard fossil groups that are commonly used for correlation in different geological periods. Thanks for Dr David Ray for support in preparing this image.
Different groups of fossils are used for different periods of geological time, for example graptolites in the Silurian, conodonts in the Permian and ammonites in the Jurassic (see time scale). Obviously, zone fossils can only be used in the environments where they existed, which is why we have different ammonite zone fossils for the Boreal and Tethyan realms. Palynomorphs and other fossils are required to constrain time-equivalent non-marine stratigraphy.
In addition to biozones, we also use bioevents such as First Appearance Datum (FAD) and Last Appearance Datum (LAD), which are observations in any given succession of when a species first occurs or disappears. FADs and LADs are often shown on range charts and may bracket the entire biozone, or a shorter interval of time in that biozone depending on species radiation patterns and persistence of the right environmental conditions at that specific location. In addition to LADs, extinction events where whole clades of organisms became extinct are important stratigraphic markers.
When Things Get Complicated
Where diagnostic zone fossils are absent, fossil assemblages can be used to try and determine a more precise understanding of age for correlation than any individual genus or species may be able to designate alone.
Sounds simple? Think again – biostratigraphic interpretation can be very complex and subject to lots of important nuances. Nevertheless, the importance of fossils for understanding the age relationships of rocks remains fundamental to our ability to correlate and map rocks, from William Smith’s first geological map to present day sequence stratigraphic studies of the subsurface. Applied biostratigraphy enables time-equivalent successions to be correlated, highlighting sometimes non-obvious relationships compared with lithostratigraphic approaches. As a result of this, fossils are commonly used in subsurface drilling programmes leveraging a technique called ‘biosteering’.
Biosteering typically requires a pilot well to determine the occurrence, abundance and ratios of different microfossil species to be used. An experienced micropalaeontologist is then required at the rig site of new wells to monitor the taxa encountered and ensure the target zone is reached, and any horizontal wellbores stay in-zone. This is particularly valuable in extended reach lateral drilling.
If the fossil content of the layers above and below the target reservoir (white) is different, biostratigraphers can help geosteer a well back into the reservoir when crossing a fault with unknown throw. Source image: Aubrey Whymark – Geosteerer.com
Complex social systems
Assemblages of fossils can give us insights into ecological systems (through coexistence of different taxa) and we can glean a lot of information on the food chain and dietary habits through anatomical studies and observations from coprolites and preserved stomach contents in exceptional fossils.
Beyond basic ecological information, exceptional fossils allow mode of life, or behavioural traits to be observed (checkout the excellent book ‘Locked In Time’ by Dean Lomax). Lomax highlights examples of fossils that demonstrate reproduction, evidence for parental care, social behaviour, sense of community, migratory patterns and much more.
Left: The famous ‘Berlin’ specimen of Archaeopteryx lithographica. This is a photograph of the original fossil, a specimen that is displayed in the Museum für Naturkunde in Berlin. Source: H.Raab (User:Vesta) under licence CC BY-SA 3.0. Right: Compsognathus. This is a cast of a fossil held at the Bavarian State Institute for Paleontology and Historical Geology acquired by Joseph Oberndorfer in Bavaria, Germany, in 1859. Source: Matthias Kabel under licence CC BY 2.5. Comparative anatomical studies on fossils like these led Thomas Henry Huxley to propose that birds evolved from dinosaurs. Although Archaeopteryx is now not considered directly ancestral to birds, Huxleys interpretation of the link between birds and dinosaurs is of course now widely accepted.
From such specimens we know for example that complex social systems existed in dinosaurs over 190 million years ago. For instance, an important fossil site in southern Patagonia shows convincing evidence for herd behaviour with age segregation in a population of the sauropodomorph Mussaurus patagonicus. Fossils of eggs and hatchlings, juveniles, and adults (individuals and pairs) were all grouped separately at the locality!
One example of life mode I particularly like is the evidence for K-selected viviparity of plesiosaurians. Plesiosaurians (like ichthyosaurs) are known to have given birth to live young, most likely to a single progeny of large size following a long gestation period. The young have been interpreted to stay close to the mother, probably for some considerable time after birth, with pod behaviour a bit like modern whales. Having had the good fortune to find two partial Colymbosaurus megadeirus skeletons for both a mature adult and juvenile in very close proximity to each other (just a few metres) this example really resonates for me (see photo). The key difference between these two specimens is that on the adult, the neural bones are fused to the vertebral centra, while on the juvenile they are not.
An example range chart for some terrestrial cephalopods. From the classic work ‘Cours élémentaire de Paléontologie et de Géologie Stratigraphiques’ by Alcide D’orbigny. Photo: James Etienne.
Testing Regional Geological Models
The radiative trends of species (proven by fossil distribution) provide lots of information on palaeogeography, including how, and when, landmasses were connected, and the presence of seaways and connections between marine basins. Indeed, the occurrence of both Mesosaurus and Glossopteris in both South Africa and South America led Alfred Wegener to propose the theory of continental drift. While continental drift was rightfully not accepted as the correct mechanism, the juxtaposition of these continents we now well understand through plate tectonics where geodynamic models can explain how identical fossils can be found on either side of the South Atlantic Ocean.
Since plate tectonics governs the organisation of continental and oceanic crust through time, fossils are thus incredibly useful tools in testing, validating or refuting different geodynamic models. The connection between marine basins (so-called ‘oceanic gateways’) is particularly important for understanding thermohaline circulation patterns and their impact on surface heat distribution on earth in the past.
During the Eocene, the Eurasian Basin (precursor to the modern Arctic Ocean) was a relatively isolated water body with a stratified column and a fresh water nepheloid lid. We know this because of the basin-wide occurrence of a fossilised freshwater fern called Azolla (See also GeoExPro 2016/02). Surface drainage from the surrounding North American and Eurasian landmasses poured into the Eurasian Basin allowing Azolla to bloom, drawing down vast volumes of nitrogen and carbon dioxide from the atmosphere. In fact, this palaeoecological event is thought to have been a contributing factor to global cooling and the late Cenozoic ice ages.
Side by side – two partial plesiosaurian skeletons collected over a period of more than 2 years. On the left, an adult Colymbosaurus – the neural arches are fused to vertebral centra. On the right, the partial vertebral column of a juvenile found within a few metres of the other specimen, at the same stratigraphic level. Were they directly related? These specimens are currently on display in the Abingdon Museum. Photo: James Etienne.
Reconstructing Past Climates
Since most organisms occupy ecological niches where they have adapted to their environment, fossils and fossil assemblages can provide a lot of insights into palaeoenvironmental conditions. For example, the coiling direction of some foraminifera (clockwise versus counter clockwise) can determine the likely range of water temperatures in which they lived. Some marine fauna (particularly fish) have strong preferences for the range of salinity they can tolerate (stenohaline organisms) which can be good indicators of palaeosalinity. Groups such as sponges, large bivalves (e.g. rudists), large benthic foraminifera and green-blue algae are good tropical marine indicators, while others such as some species of brachiopods and crinoid dominated bioherms can be indicators of cooler water conditions.
In the terrestrial environment, spore, pollen and plant macrofossils can provide information on likely air temperature, humidity and precipitation. Stomatal indices from fossil leaves can be used to imply atmospheric gas composition, and invertebrate mammal populations, the degree of hypsodonty (teeth adapted for grassland) can even be used as an aridity indicator. The observations above are known as proxy indicators for the environment, but fossils can also provide the raw material for direct geochemical studies such as stable carbon and oxygen isotope analysis on mollusc shells. Such analyses can provide information on seawater composition and palaeotemperature and are typically referenced to the Peedee belemnite.
The Peedee belemnite (or PDB) is actually Belemnitella americana – a belemnite found in exposures of the Peedee Formation along the banks of the Peedee River in North and South Carolina. Since the original material is no longer available (the original study by H.C. Urey was published in 1951), you will see V-PDB (Vienna-PDB) a lot in the literature, which references another standard NBS-19 relative to PDB (although that is now also exhausted and has been replaced byIAEA-603 which is itself calibrated relative to V-PDB). A similar approach can be taken with tooth enamel of vertebrate fossils and has even been used to interpret changes in hominin diet over time.
Burial Modelling and Thermal Maturity Analysis
In addition to all the insights mentioned above, some fossil material can even help provide proxy information on the burial history of the rocks in which they were deposited. This is really important in basin and petroleum systems modelling. With an understanding of how geothermal heatflow has evolved overtime, it is possible to infer the likely burial depth of petroleum source rocks.
The kerogen type can provide an indication of the original source material and even a prediction of the likely hydrocarbon phase. Type I kerogen of lacustrine algal origin is typically oil prone, as is type II marine kerogen, although oil sourced from these kerogen types can be cracked to gas at sufficiently high temperatures. Type III kerogen, typically derived from land plants, tends to be gas prone, although there are some exceptions where tropical plants with thick waxy cuticles can produce liquid hydrocarbons.
Thermal Maturity
Vitrinite reflectance is a method used to determine coal rank, but is more generally used as a method to assess thermal maturity achieved during burial in basin and petroleum systems modelling. Determined under a microscope, vitrinite (a type of maceral or woody tissue typically derived from vascular plants) is subject to an incident light source and the amount of reflectance from the vitrinite surface is measured.
Since vitrinite reflectance increases with thermal maturity, it can be a good indicator of the likely depth of burial of sedimentary rocks. Sudden jumps in vitrinite reflectance profiles measured along well bores can also provide proxy evidence for cryptic unconformities. Of course, the interpretation requires care and attention, especially as vitrinite is durable and can be reworked into younger rocks that have not been subjected to as much burial.
Alteration Indices
Developed in the 1970’s by a group of researchers from the USGS, the Conodont Alteration Index (or CAI for short) is a colour-based scale for the thermal alteration of the apatite that composes conodont fossils. Applicable in rocks of Palaeozoic and Triassic age, the CAI is a visual microscope-based method for determining the approximate likely maximum temperature range the fossils were subjected to on burial. It has many uses – not only for evaluating burial modelling in hydrocarbon exploration, but also for metamorphic and structural geology studies. Thermal alteration indices have also been developed for other fossil groups including graptolites and acritarchs (organic-walled microfossils).
Versatility
In this article, I have mentioned just a handful of examples of the ways in which fossils can be used to develop insights into the geological past. Whether used for their distinctive physical appearance to correlate rocks, in cladistics for determining evolutionary trends, as physical material for geochemical studies or for palaeobiological analysis, fossils allow a forensic analysis of sedimentary rocks and a window into the geological past.
