Carbonate nodules/concretions are common in North Sea mudstone/shales - which form the overburden rocks for many reservoirs. As they are generally localised, they are not of significance for hydrocarbon exploration/recovery. But geological observations around the nodules can provide valuable insights into the geological history. I’ve also observed core jam-offs at the level of zones of concretions. So, it’s very useful to know something about their genesis and likelihood of occurrence.
The dolomite concretions in the shale core in Fig. 1a have been sheared along the bedding planes i.e., bedding parallel shear. Note the haloes of cement around each concretion showing that there has been some recrystallisation around the original concretion.
Fig. 1b is a close up of the biggest concretion in Figure 1a. There is a lot of detail here ! - recrystallisation (note the mosaic of crystals) at the top right of the concretion before it tails off into 2 arms - which appear to follow the concretion-related fractures.
The pencil (in Fig. 1b) points to a large healed fracture. Thin-sections would be useful here to check the type of mineral fill and the chronology of events. Note also the compaction of shale layers at the base of the concretion indicating that the substrate was relatively soft at the time.
Figure 1 (a) Bedding parallel dolomite concretions in Cretaceous shale in the N Sea. (b) Zoom into largest concretion showing a lot of detail which provides insights into the geological history.
The concretion in Fig. 2 is found in greyish brown (Tertiary-age) mudstones in the Central North Sea. No bedding parallel shear is observed but chemical zonation indicates a rapidly evolving flow. The associated curved fractures (in Fig. 2) look almost like they are induced but they are natural given the evidence of shear on the concretion rim. This rounded concretion indicates slow deposition, whereas a smaller, more irregular (stunted) concretions would indicate more rapid deposition (Fig. 3).
Figure 2. Carbonate concretion with related fractures - in a Central North Sea (Tertiary age) Mudstone
Figure 3. Concretion shape and rate of deposition. Density log signature on right hand side.