Demystifying the Fjerritslev Formation

The Fjerritslev Fm. is a geological formation, that when drilling Danish onshore wells is related with great concern, as it is known to be the origin of instability during and after its penetration. As green energy is becoming increasingly important, it is natural to shed light on this apparent problem with respect to the development of geothermal wells in Denmark.

The Fjerritslev Formation

The Fjerritslev Fm. consists of deep marine clay of Early Jurassic origin (c. 201-191 Ma). The lithology is generally related to instability due to its plastic nature upon reaction with water based drilling mud, and globally, clayey formations constitutes more than 75% of drilled formations, and more than 90% of drilling problems associated with instability of the borehole. As a rule of thumb, problems with clayey formations are related to the time of exposure to the drilling mud. That is, the longer time a borehole is open without casing (see Figure 1), the more likely the borehole is to become unstable and undergo plastic deformation.

The problem with clay

The term clay relates to particles with a size <3.9 µm, but also refers to a broad family of minerals. The minerals are of secondary origin, and are formed through chemical weathering and transformation of existing rocks. By example, a granitic rock exposed on the surface of Earth will react with rainwater by substitution of atoms in the mineral feldspar with ions in the water. The new mineral is called kaolinite and will easily be eroded as microscopic particles. In general, the hydrated clay mineral consists of thin sheets, weakly bound to other sheets. Because of substitution of ions with different valences – e.g. Mg+2 instead of Al+3 and Al+3 instead of Si+4 – clay minerals are negatively charged, and therefore attract the water molecules which are dipolar. All clay minerals will, to different degrees, adsorb water on the surface and between the individual sheets when such a contact occurs, and as a direct consequence the clay expands. This reaction forms the basis of instability and plastic movement, which can result in collapse of clay-rich material into an open borehole. The problem with plastic, sticky, and expanding clay may manifest itself as collapsing borehole walls, stuck equipment in hole, and broken drill string which in the end results in prolonged and more costly drilling operations.

The problem with the Fjerritslev Formation

In Denmark, the subsurface is rather uniform, with several geological formations of specific lithologies, which laterally varies in thickness and depths as a consequent of the geological development of the Danish Basin. The general sequence is given in Figure 1, but may be described as a thick package of limestone, followed by alternating packages of clay and sand. Knowledge of the Danish subsurface is mainly based on deep wells, of which some 175 has been drilled onshore, since the 1930’ies. To investigate the actual influence that the clay of the Fjerritslev Fm. has had on drilling, 47 final well reports have been analysed to form a general picture. Many variables may impact a drilling operation – e.g. drilling equipment, mud composition, mud weight, hole diameter, rate of penetration, and geology – but a simple and quantitative approach to the problem, could be to merely investigate the time elapsed from the beginning to end of drilling the well. By selecting a time-limit, and assuming that wells above this have had problems during the drilling operation, it is possible to narrow the number of wells and examine their relation to a problematic Fjerritslev Fm.

Figure 2 illustrates this relationship, i.e. time elapsed compared with the total depths of the respective onshore wells. The apparent pattern is a Fjerritslev Fm. that in most cases has been drilled within the expected time period, and it is therefore easy to jump to the conclusion that the assigned problems to the clayey formation have been exaggerated. However, it should be pointed out that almost every well in Denmark have had problems with the Fjerritslev Fm, however, only to an expected extent – e.g. decreased rate of penetration resulting from sticky clay – which in general have not caused significant delays or issues. Figure 2 shows 17 wells plotting above the chosen time limits, however, only three wells – Aars-1, Karlebo-1, and Stenlille-7 – cannot be claimed unproblematic through the Fjerritslev Fm. as a consequence of other circumstances. The problems in Aars-1, Karlebo-1, and Stenlille-7 may be due to planned deviated drilling and mechanical failures, thus enforcing additional instability to the borehole and a prolonged exposure of the formation to the drilling mud. The remaining wells have had prolonged drilling time which may be explained by large total depths, a large Fjerritslev Fm. thickness, choice of casing point within the Fjerritslev Fm., or planned mechanical reparations, and are therefore not correlated with a problematic Fjerritslev Fm.

We hereby hope, with this short essay, to have shown that even though large economic risk is associated with onshore drilling in Denmark, the overwhelming majority of projects have had successful drilling operations, finished within an acceptable and planned time schedule. It is not easy to identify exactly why certain wells have had great struggles drilling through the Fjerritslev Fm., however, that is perhaps exactly due to an insufficient basis for comparison. In the end, minimization of problems can only be strived for, but may be helped by thorough preparation, precise prognoses, and an experienced and competent personnel.

Figure 1: link
Figure 2: link

Written by
Rasmus B. Engler

For any elaboration, questions, or comments please contact Dorthe Juul Kann at

For the article in Danish: link

For inquiry of Danish onshore final well reports you are referred to the Frisbee database by GEUS: