The town of Whitby on the coast of north-east Yorkshire in the UK is known more for its associations with Dracula and the famous explorer Captain James Cook than as a centre of industry. Whitby has a long history, dating back to the year 656, when Whitby Abbey was founded. The ruined 13th-century abbey building, destroyed during the time of Henry VIII, still looms imposingly above the harbour where James Cook first learnt his seafaring skills. Later, author Bram Stoker was inspired by the Gothic ruins to write his novel about vampires.
Whitby does also have an industrial past, albeit not on a large scale. Much of that industry was a result of the local geology, such as the extraction of alum, dating from the 16th century, and the discovery of jet, from fossilised Jurassic conifers, which was used in jewellery. Iron-rich springs led to the development of a spa and hint at the ironstone mining and smelting that occurred in the locality and further inland in the 19th century.
Deposits Linked to Hydrocarbon Geology
Schematic stratigraphy of Zechstein sequence in NE Yorkshire. Ca. 400m column (not to scale)This relatively small scale industrial past is poised on the brink of a larger scale future thanks to the discovery of minerals that can be used to manufacture fertilisers, now a product much in demand in the face of a growing global population and the need to produce more and more crops in an efficient and cost-effective way.
These minerals are not visible in surface exposures but are located at depths of between 1,000m and about 1,600m below ground level. They were first discovered during petroleum exploration in the 1930s and are inextricably linked to the familiar oil and gas geology of the Southern North Sea, as they are found in the evaporite sequence of the mid and lower Zechstein Formations. Two main types of minerals have been found in quantity here, sylvite, or potassium chloride (KCl), and polyhalite, a complex hydrated sulphate (K2SO4.MgSO4.2CaSO4.2H2O), either a product of evaporation or the alteration of minerals such as gypsum.
The deposits were not exploited until the 1960s, when three companies started investigations in the Whitby area. Only one of these resulted in a commercial project when, in 1973, Cleveland Potash Ltd. opened the Boulby mine, about 15 km north-west of Whitby. This mine has been in continuous production since opening, mining mainly sylvite from a seam in a halite sequence above the Billingham Anhydrite. A by-product has also been salt, which is used in the grit spread on the roads, much appreciated by UK motorists in the recent harsh winters!
Polyhalite, however, is considered more economically attractive, as it is more effective as a fertiliser and can be processed into a wide variety of other high value potassium and magnesium minerals. Cleveland Potash started mining polyhalite, found deeper in the Zechstein, in 2011. These deposits are found offshore in the area of Boulby mine, but the zone of deposition moves inland further south, following the edge of the North Sea Zechstein Basin.
Using Legacy Data
Seismic line within area of interest: (top) line from UKOGL data base; (bottom) pre-stack processing by Spectrum. Source: UKOGL and Sirius Minerals
Map of north-east Yorkshire from UKOGL database, showing legacy seismic lines (green) and boreholes (blue and red dots). Orange outline shows general area of interest where York Potash holds mineral rights. Inset map shows distribution of seismic available online in the UKOGL data base. Source: UK Onshore Geophysical LibraryIn 2010, a new company, York Potash, was established to explore in this area, which comprises an onshore part following the coastal zone between Whitby and Scarborough and also further offshore. Mineral mining rights are not the same in the UK as those for hydrocarbons, so the company defined an area of interest in which mineral rights would be acquired. York Potash engaged regional mining geological specialists, FWS Consultants of Spennymoor near Durham, to provide an initial evaluation of the expected resource based on the rights so far agreed. Based on a reasonably extensive suite of boreholes, some from the potash exploration of the 1960s and some from hydrocarbon exploration ranging from the 1930s to the 1990s, the evaluated resource proved more than sufficient for a commercial project and an exploration programme was embarked upon. Early in 2011, York Potash was acquired by the AIM-listed, potash exploration and development company, Sirius Minerals, giving the project further impetus.
Coincidentally, FWS’ investigations were in progress at the time the UK Onshore Geophysical Library (UKOGL) was launched by Lynx Information Systems (see also Geo ExPro, Vol. 8, No. 1). Rick Smith, lead consultant at FWS, says, “We realised that now there was a major resource available online, showing maps and cross sections of almost all of the legacy seismic data onshore UK. Because of past hydrocarbon exploration, north-east Yorkshire is particularly rich in such seismic. By browsing the data on the Internet, we could see that much of it, in particular Vibroseis lines from the 1980s, were of a good enough quality to use as mapping, connecting up the boreholes.”
In order to make maps, however, the data needed to be obtained digitally and conditioned so that the various vintages and acquisition types matched up. York Potash engaged Spectrum ASA, well known in the hydrocarbon business as a provider of multi-client seismic data, to process the data. Spectrum also specialises in seismic imaging. Andy Billings of Hornet Geoconsulting was also contracted for the interpretation. Various data purchase options are available through UKOGL, either temporary or permanent licences, so it was possible to acquire the data in a cost-effective manner, with post-stack conditioning of the data providing a data set to be interpreted in parallel with the initial borehole programme. Processing tests indicated that some vintages were considerably enhanced by pre-stack time migration, so a second phase was directed towards processing the more ‘modern’ lines in order to connect the boreholes.
Essentially an Oilfield Operation
Tie between surface seismic and VSP. Source: Sirius MineralsThe similarities between this mining project and the oil and gas business does not stop with the geology and the seismic. The drilling programme is essentially an oilfield operation, with the difference that York Potash are trying not to find gas in the Zechstein! Oilfield geologists might be surprised (and perhaps jealous) of the amount of core that is cut – necessary in the mining business in order to get an accurate assay of the mineable seams. In the first well, over 500m of core were recovered, including the entire Zechstein sequence that was penetrated.
Another familiar hydrocarbon player, Schlumberger, also comes onto the scene to log the wells. The high grade potash and polyhalite seams are characterised by high and specific gamma ray readings, so gamma and density logs are also important. In order to tie to the seismic, as well as sonic and density for the synthetic seismic, VSP surveys are conducted.
Successful Results
York Potash drilling site near Whitby. Source: Paul WoodSo far, the results have been very successful. The initial assessment of resource volumes assumed a 5m thickness of ‘ore grade’ polyhalite. The first well found a 23m thick deposit, split across three seams and the second and third wells also found thick deposits. Chris Fraser, Managing Director and CEO of Sirius Minerals, said: “These are fantastic results that are a validation of the world-class status of the project. The first hole we have drilled has delivered one of the world’s single thickest potash intersections ever reported.”
The project looks set to progress further and to develop into a mining operation that will provide a much needed boost to this region of the UK during a time of economic uncertainty. One thing York Potash and Sirius Minerals are very conscious of is that the mine site will most likely be within the North York Moors National Park (as indeed is Boulby Mine). But mining technology has moved on in 40 years and they aim to have as little impact on the environment as possible. The mine head will be designed for low visibility and it is the intention that the product be transported by buried pipeline – another link to the oil industry.
The industrial heritage of Whitby should now be revived, albeit in a high-tech way. What would the Victorian jet, alum and ironstone miners have thought of our ability to find and develop minerals at a depth of 1,500 metres? Even Dracula would want a stake in it!
