Horse Hill, one of the most talked about discoveries in the UK for decades – not just by the industry – lies in the Weald Basin in southeast England, an area considered potentially rich in hydrocarbons, but which has been ignored for decades.
With the help of experienced UK geologist, Chris Pullan, we look at the geology, the newly identified plays and the prospectivity behind the hype.
“More than 70% of the wells in the Weald Basin, most of which were drilled between the 1960s and early ’80s, have encountered hydrocarbon shows,” says Chris Pullan, a consultant with Magellan Petroleum.
Esso actually drilled not one but two wells at Horse Hill in the 1960’s and didn’t realise the potential of the Kimmeridge limestones either. They never even tested the limestones!
Back in 1987 BP, one of the great global oil majors, found oil at the base of the Surrey Hills just outside the M25. The nodding donkeys at the Brockham X1 oil well have been quietly pumping Portland oil for the UK economy
But UK Oil & Gas was about to change all that, the area really hit the headlines late in 2014 when preliminary results from Horse Hill-1, drilled on PEDL (Petroleum Exploration and Development Licence) 137 on the northern flank of the Weald Basin, suggested the total in-place resource for the area could be considerably larger than previously estimated.
Further positive analysis and testing of the well led to news reports suggesting the discovery – nicknamed by the press the ‘Gatwick Gusher’ – maybe ‘a world-class resource with millions of barrels of oil’, which could play a ‘key role in transforming the UK’s energy balance’.
Less positive press exposure to the hydrocarbon potential of the area had occurred in 2013 and 2014, when Cuadrilla Resources proposed a test well near the village of Balcombe in the centre of the Weald Basin. There was considerable public concern and active anti-fracking protests over the proposed well, although no application to use hydraulic fracturing had been made.
The first exploration wells were drilled in the Weald Basin in the 1930s. A major phase of exploration from 1930 to the 1960s focused on drilling surface features, but with little success.
The 1973 discovery of oil in Triassic sandstone at Wytch Farm – at the time the largest onshore oil field in Europe (see Wytch Farm Ploughs Ahead by Will Thornton) – in the adjacent Wessex Basin to the west of the Weald Basin, led to a resurgence of interest in the region.
A number of new wells in the first half of the 1980s resulted in the first commercial discoveries in the Weald Basin. These included the Humbly Grove, Herriard, Horndean and Singleton oil fields in the western parts of the basin, reservoired in the Middle Jurassic Great Oolite Formation. Fields found in the Upper Jurassic sandstones and limestones in the northern and central part of the basin at this time included the Palmers Wood and Brockham oil fields and Albury and Godley Bridge gas fields.
After oil prices crashed in the late 1980s there was little interest in the region for nearly two decades. At the moment, there are 13 producing sites in the Weald Basin, but some are almost 30 years old and many reservoirs are declining. However, the last decade has seen a considerable resurgence of interest in the area, with some very interesting well results.
Meanwhile, some of the questions as to whether Upper Jurassic Kimmerigian Clays could be a source were answered by geochemical analysis of oil from a test of the Kimmeridgian Limestones in the Balcombe-1 well, originally drilled by Conoco, in the centre of the basin, which suggested the presence of a second, less mature, source.
As Chris says, “The unconventional industry in the US had breathed new life into the UK onshore. We realised that the Kimmeridgian Clays might not just be sourcing the overlying Portland Sandstone, but also a new, unexplored play: the interbedded and stacked Kimmeridgian Limestones. We needed to map out the extent of the mature Kimmeridgian Clays as well as the interbedded limestones, known as the Micrites.”
Horse Hill Rears Up
Magellan Petroleum invested in more blocks in subsequent licensing rounds and by 2010 had a large acreage position in the Weald Basin.
One prospect the company started looking at closely was Horse Hill, a tilted horst block prospect in PEDL137, which lies on the northern edge of the basin, close to Gatwick Airport.
A previous well on the block, Collingdean Farm, drilled in 1967, encountered ‘good oil shows’ in the Jurassic reservoirs and was even tested but did not flow. However, seismic reprocessing suggested that the well was off the structure, on the downthrown side of the major bounding fault.
“By 2013, Magellan was ready to drill a new well on the upthrown side of the fault. The prospect was farmed-out to Angus Energy Ltd, who later created Horse Hill Developments Ltd, with multiple consortium partners in order to fund the exploration drilling, and who became the operator,” continues Chris.
“The well was completed in November 2014 and encountered several hydrocarbon-bearing horizons, including the Portland Sandstone and the Kimmeridge Limestone.
Along with the new vitrinite reflectance data, the test results have redefined the limit of the maturity of the Kimmeridge source, suggesting that it is mature over a greater area than previously expected, possibly up to the northern bounding fault, thus considerably expanding the overall potential of the Basin.
As time has passed we have watched the growth of interest from the media, investors as the story has unfolded.
Today UK Oil & Gas are the main operators of HorseHill and are testing the Kimmeridge Limestone with future planning permission to compleat a further two drills in 2019.
South–north composite interpreted seismic through the Weald Basin, modified from M. Butler and R. Jamieson, 2013. For line of section see map at top of page. (Source: UK Onshore Geophysical Library)
A Large Resource
There was additional interest when the results of technical studies of the well carried out by Nutech and Schlumberger were released, which suggested large estimates of oil in place. This led to a media frenzy about the somewhat inaccurately named ‘Gatwick Gusher’ and the millions of barrels of oil underlying the protected ‘Green Belt’ areas of the Weald.
“There is a very large potential resource here, but it is far too soon to make any realistic estimate of its size and how much of it can be recovered,” Chris says. “We need more data and a lot more fundamental analysis needs to be done, particularly so we can gain a much greater understanding of the hydrocarbon trapping; all wells so far have been drilled on structural highs, for example, so we need to prove that a stratigraphic trapping mechanism is effective. We also need to test long term and also look at several other horizons, before we can have a reliable idea of the size of recoverable reserves.
“The potential of the Kimmeridge is particularly exciting; we don’t have any good core from the section, as previously we just drilled through it to look at the deeper horizons. More work needs to be done on the geochemistry of the clays to analyse their oil potential and maturity.
The centre of the basin is underexplored and we have yet to fully understand the level of uplift it has undergone during the Tertiary and the amount of resulting deformation. Also, the conventional Triassic gas play could be very significant, but needs a lot more data, work and analysis.”
A Large Resource – Unlocking the Potential
The Weald Basin is home to millions of people and environmentally very sensitive, with large swathes being protected from development, which adds an extra layer to the issues surrounding the exploitation of this potential resource.
“For me, the most thrilling thing was realising that there are really interesting plays literally in my backyard and, with recent technological developments, developing these opportunities is possible. There are lots of undrilled structures and plays out there and I cannot wait to help unlock the potential of this area.”
We have used images under the fair use act and claim no copyrights
All information is provided on an as-is basis. Where we allow Bloggers to publish articles on our platform please note these are not our opinions or views and we have no affiliation with the companies mentioned