Modern Advanced Sedimentological Core and Outcrop Studies
Bioturbation of sediments is synonymous with bad news for many geologists when they are describing new cores and outcrops, as sedimentary structures can be partly or even completely disrupted by biogenic activity. Nevertheless, trace fossils can provide invaluable information on the environmental conditions that prevailed during or shortly after sediment deposition. As such, recognition and interpretation of trace fossils and their associations, together with analysis of sedimentary and diagenetic structures, can be usefully integrated into modern advanced sedimentological core and outcrop studies.
What Are Ichnofacies?
The early successful use of trace fossils was greatly advanced by Adolf Seilacher in the 1960s, who introduced the concept of ‘ichnofacies’ as a way to split the marine realm into four main zones covering nearshore environments to the deep basin. Additional ichnofacies have since been defined for marine, non-marine and continental environments, producing a refined and comprehensive system.
In the 1970s sequence stratigraphy concepts, first developed on seismic data calibrated by biostratigraphy and core facies analysis, were spreading in both industry and academia. These ideas subsequently promoted the integration of data arising from all techniques, including trace fossil analysis complementary to sedimentology.
The use of ichnofabrics has increased steadily since the concept was first developed in 1984 by Bromley and Ekdale, who defined it as: “all aspects of the texture and internal structure of a sediment that result from bioturbation at all scales.”
In 1991, the first International Ichnofabric Workshop was hosted by Norsk Hydro in Bergen in Norway. Since then, meetings have been organised every second year to demonstrate applications of ichnofabrics and present case studies to ichnologists, sedimentologists and geologists from both academia and industry.
Using Ichnofabrics in Hydrocarbon Reservoir Exploration
Combining the broader ichnofacies system with detailed ichnofabric analysis has been found to yield a better result than employing the systems separately. Ichnofabrics provide a finer resolution than ichnofacies but neglect the broader picture; they allow not only large third-order cycle recognition but are imperative for the identification of higher frequency cycles (parasequences) by comparing ichnofabric cycles and their vertical stacking.
Ichnofabric techniques involve the identification of trace fossils, the analysis of their cross-cut relationships and their grouping in recurring cycles which classically represent either shallowing-upward or deepening-upward trends. This information, which is complementary to classical sedimentary attributes – grain size, sorting, texture, bedding – supply reliable constraints to aid the clearer identification of significant geological surfaces and sequences when correlating wells.
The level of resolution supplied by ichnofabrics is therefore compatible with and comparable to that needed for reservoir characterisation. From a reservoir viewpoint, the impact of bioturbation on early diagenetic fluid flow is being increasingly emphasised, as it may ultimately control micro-scale heterogeneity by enhancing (leaching) or deteriorating (mixing mud and sand or cementing) the reservoir quality of the sediment during burial history.
Trace Fossil Identification
Each trace fossil has a series of characteristics which have been accepted by the International Commission on Zoological Nomenclature. Most of this work, including description, identification and classification (ichnotaxonomy) is based upon analysis of specimens coming from outcrops, although sediment boxes and cores provide additional data.
In fully bioturbated rocks, trace fossil identification is more difficult and even impossible, as the burrows cross-cut each other giving rise to all sorts of comments in core rooms when geologists try to describe bioturbated cores: “the sediment is churned, the funny stuff again, the rock is a mess…”.
Published diagnostic criteria (see ‘About the Author’) can help sedimentologists in identifying many important burrows common in the rock record. The diagnosis must be established and supported by description (shape and size), substrate and behaviour, the most common depositional environments, ichnofabrics occurrence, ichnofacies and the age for each trace fossil. Forms and structures of the burrows vary in response to the type of substrate, which induces changes in the behaviour of the trace makers. Each type of trace fossil shows a few classical sections – although uncommon views of the burrows might just reflect an increasing complexity, such as branching, meandering or twisting.
Ichnofabrics for Depositional Environment Interpretation
Identification of trace fossils and their grouping into different trace fossil associations or ichnofabrics can be distinguished on the basis of crosscutting relationships between the ichnotaxa within each trace fossil association, which reveals the tiering pattern and thereby the tiered structure of the endobenthic community. Ichnofabrics from similar depositional settings generally show recurring patterns, which can be related to specific depositional environments, spanning from the continental realm to deep seas.
A series of classic ichnofabrics ranging from the terrestrial zone to deep water has been identified, although complications arise when directly comparing Palaeozoic with Cenozoic ichnofabrics, as tracemakers have evolved through geological times. Nevertheless, the animal and vegetal communities adapt to physical processes such as currents, tides, waves and storms. Chemical, oxygen and redox conditions, for instance, affect the distribution of animals and algae in the different realms of the earth. Species occurrence grades from absent, where conditions are hostile, to present and even abundant if conditions allow blooming of the community. Community diversity may be very low and eventually limited to a monospecific colonisation if stressed and hostile conditions favour the development of only one type of animal and prevent other species from occupying the biotope. On the other hand, diverse ichnofabrics indicate steady environmental conditions which provide equilibrium to the community.
Distribution and Depositional Profile Interpretation
A case study from the Late Jurassic is an example of interpretation of the depositional profile based upon the relationship between ichnotaxa, bioturbation index and lithofacies. The summary diagram clearly indicates that ichnofabric examination is a building block in the sedimentary analysis. Lithofacies must be analysed first: grain size, sorting, texture and sedimentary structures. Intensity of bioturbation must be quantified, followed by the identification of burrows and their grouping into recurring ichnofabrics.
Compilation of all the analytical data supports the reconstruction of the depositional profile as it passes from non-bioturbated continental deposits to intertidal domain to marine open shelf. Transgressive surfaces are highlighted by colonisation of the back-stepping seafloor by crustaceans during transgressive pulses at the base of each parasequence (flooding surface).
Identification of a Genetic Depositional Sequence
In a large correlation project with abundant core data recurring ichnofabrics were observed and compared between twenty wells. Sequences show stacked decimetre to metre scale fining-upward beds bounded by a sharp base, although bioturbation sometimes make the identification of the contact difficult. The facies succession within a bed shows a cleaning-upward pattern associated with an upward dilution of the glauconite grains, coarse quartz grains and wood debris. At the base of the beds, shell debris may also be found.
Common coarsening-upward beds with mud drapes are interpreted as sigmoidal cross-bedded sediments from sub-tidal bars. Coarseningupward cycles, based on the analysis of both facies succession and ichnofabrics, are interpreted as the genetic depositional sequences. The ichnofabrics evolution from base to top shows which can be interpreted as shallowing-upward cycles. Abundant deep burrows, Diplocraterion habichi, commonly occur at the top of these cycles and are interpreted to be indicative of firm ground colonisation. The cycles are frequently bounded by early-diagenetic siderite-cemented horizons or ‘rubified levels’ that are indicative of early precipitation of iron in the upper part of the sediment, recording a depositional break.
Ichnofabrics and Reservoir Characteristics
Core description within a broader reservoir project is a two-fold process:
- To describe and interpret depositional facies and propose a sedimentological model mainly from facies mapping, correlations and prediction of facies distribution;
- To tie sedimentological facies with porosity-permeability measurements provided by standard core analysis.
Once the sedimentologist has produced the ichnofabrics core log, results must be tied to standard core analysis. It is a good opportunity to check the validity of the ichnofabrics scheme as demonstrated in a case study from the Lower Cretaceous (see image below).
The best reservoirs, shown in yellow, red and purple, are clearly associated with the upper part of prograding cycles, i.e. coarser grained and better-sorted, whilst poorer reservoirs, in blue, correspond to the more argillaceous and finer grained sandstones from the distal part of the depositional systems.
Another Useful Tool
Sedimentological core studies dedicated to reservoir description, characterisation and correlation are better achieved when combined with other techniques, including ichnofabric techniques, particularly when sedimentary rocks lack diagnostic sedimentary structures as a result of intense bioturbation.
Nevertheless, ichnology should not be used as a standalone technique, but as another tool available to sedimentologists who need to use it to describe complicated bioturbated sections rather than avoiding them.
No need for “the funny stuff again, the rock is a mess” anymore; there is information in there!