How a recently acquired mining core became a new reference section for the Triassic of the North Sea

In north Yorkshire, United King­dom, a huge mine is being con­structed by AngloAmerican to produce polyhalite from an Up­per Permian evaporite succession. In order to design the shafts, a borehole was drilled to better understand the overburden, which includes Triassic and Lower Jurassic rocks. The sec­tion was entirely cored along its full 900 m length. This provides a unique dataset to understand the Triassic depositional system of the North Sea, even though it was not intended for that purpose.

Although the mine facilities are obviously onshore, the Jurassic, Tri­assic and Zechstein successions in the area are very much the equivalents of what is commonly found offshore. For that reason, it was recognised by geologists from CASP that the core of the so-called SM14 borehole offered a great opportunity to better character­ize not only the Bunter reservoir, but also its sealing units. It is especially the seals that have hardly been cored offshore, for obvious reasons, but even cores from Triassic reservoirs are rare, because the interval has never been a major target in the UK North Sea.

“We’re extremely grateful to An­gloAmerican for granting permission to access the SM14 core,” says Steve Vincent, CASP‘s Chief Geologist, “it really was a unique opportunity.”

It resulted in the collection of about 600 samples, a detailed logging exercise of the core, and a hand-held XRF scan­ning project to have a better grip on the bulk chemistry of the rocks.

CASP

The organisation now known as CASP originated in 1948, when Brian Harland, also known for his work on the Geological Timescale and the Snowball Earth Concept, began the Cambridge Spitsbergen Expeditions (CSE). Many senior figures in academia and industry gained early field experience on these expeditions. CSE gave rise to the Cambridge Arctic Shelf Programme in 1975. Subsequently, CASP greatly increased its regional scope, diversifying its research to areas outside the Arctic for conducting field-based geological research in frontier and under-explored basins. This has led to the accumulation of about 46,000 samples from all over the world and a large analytical database. In more recent times, the organisation has pivoted towards geological carbon storage research.

A better grip on time

“The Triassic succession in the North Sea, especially the continental Bunter Sandstone and its more dis­tal playa lake equivalents, have been mostly considered barren in terms of the microfossil content,” says Niall Paterson, who has been working on a new palyno-zonation of the Trias­sic based on the data from the SM14 core. Another advantage of the core is that we don’t suffer from the effects of caving, which happens a lot when relying on cutting materi­al for doing palynological research. The new zonation we have been able to define is therefore a great addition to our knowledge of the Triassic in the Southern North Sea, and rather than conventional wisdom that the Triassic is all barren, actually some of these formations are full of palyno­morphs; not only the mudstones but also the evaporitic intervals.”

Cement

Where Niall was mainly focused on the finer-grained lithologies in the SM14 core because of their microfossil recov­ery, sedimentologist Michelle Shiers mainly looked at the sandstone itself. Using whatever material there was available from offshore cores as well, she focused particularly on the distribution of cement in the sandstones, as this is a main driver for the ultimate storage and injection potential. “Some people may think that the top of the Bunter is always cemented by halite because of the overlying Röt evaporites,” says Mi­chelle. “We saw that the opposite is true in some cases, revealing a more com­plex diagenetic history than expected.”

The seal

Going back to SM14, and it may sound counterintuitive, but it is the sealing units overlying the Bunter sandstone that formed the real boon in this pro­ject. “Combined with the wireline logs that were available from the borehole and the strength testing results, we have a unique dataset covering the equivalent of the sealing units of some of the future carbon stores in the UK Southern North Sea,” says Simon Sch­neider, who has been involved with this part of the project.

A secondary seal

The sealing unit that sits on top of the Bunter reservoir is generally consid­ered as just Triassic in age, but there is a Lower Jurassic fine-grained succes­sion that also forms part of the storage complex, as well as being a primary seal elsewhere. This Lower Jurassic interval is not only present in the SM14 core, but also in coastal outcrops not too far away from where the core was cut. That presented the CASP team with the opportunity to both study the core as well as the cliffs, such that a region­al element could be introduced to the overall seal assessment. “It allowed us to provide more tangible data when it comes to the heterogeneities that char­acterise the Lower Jurassic succession,” says Colm Pierce, who worked on the outcrops and core, “which are ultimate­ly helpful to upscale into storage com­plex performance models.”

Should you be interested in knowing more about the study, please get in touch with the team at CASP.