Queen turned the tap to open Forties Field

Secrecy Part of North Sea Exploration Life

American Association of Petroleum Geologists (AAPG)

I have always counted myself to be one of those lucky geologists who became involved in a major petroleum province before it was discovered and stayed with it until it reached maturity.

The United Kingdom awarded its first round of offshore licences in the southern North Sea in 1964. BP was quickly off the mark and made the first gas discovery at West Sole the following year.

It was while this was drilling that I arrived on the scene – I had been appointed review geologist based in BP’s small East Midlands oilfields office, from where the North Sea operation was being run.

It was not an onerous job, as I had only three traded wells to review. (UK allows licensees six years before their data is released into the public domain and therefore companies acquire it by well trades.)

So secret was BP’s drilling operation that I was not allowed access to the well data. Nor was I allowed to see any maps; only the senior geologist had that privilege.

Data security was paramount but lacked a budget. I locked away my three sets of well logs with a hasp and padlock bought from Woolworths, while the senior geologist hid his West Sole material amongst the thousands of Coal Measure maps stored in eight large cabinets.

Retrieval presented problems – and some may still be there today!

Risking It All

Following the West Sole discovery I moved to London to undertake my review work amidst a small geophysical team that was struggling to interpret the analog seismic then being acquired.

Upper Permian salt diapirism masked reflections associated with the underlying Lower Permian reservoir, while the existence of multiples and problems of migration and velocity made depth conversion exceedingly tricky. Had the geophysicists not worked wonders, the West Sole well would have missed its target – as did a number of other exploration wells.

By 1966 exploration in the south was in full swing and others were soon making bigger gas discoveries than the 1.5 tcf West Sole field. Meanwhile BP took delivery of “Sea Quest,” the first North Sea semi-submersible rig, which would allow exploration to move into deeper waters to the north.

Early mapping of the 1962 seismic northward had identified an elongated basin running north-south up the center of the North Sea. The lines were widely spaced and the only decent reflector was thought to be the base of the Tertiary. It reached a depth of nearly 10,000 feet in the center of the basin (see map left), and from a structural point of view was completely featureless.

In 1965 the UK embarked on a second round of licensing, including some blocks in northern waters. BP was offered five of these, amongs them blocks 21/9 and 21/10, some 110 miles east of Peterhead. Of all that featureless seismic just one north-south line had shown a very slight turnover. Contouring indicated a nose plunging southeastwardly into the basin running through 21/9 and 21/10.

Throwing caution to the wind we accepted these blocks and the drilling obligation that went with them.

This was in October 1965, and before any gas had been discovered in the southern North Sea.

The water depth was around 400 feet, the sea was extremely hostile and there were no rigs available that could drill in such conditions. This was exploration by the seat of one’s pants.

We would never have got it through the board today now that risk assessment has been invented!

A Major Discovery

As things progressed in the south a tentative drilling program was taking place up north, both in British and Norwegian waters. Things were not encouraging. Over 50 wells had been drilled before Phillips made their major discovery at Ekofisk in Norwegian waters in December 1969. The race for North Sea Oil was on!

Almost simultaneously Amoco discovered oil in Palaeocene sands in 22/18 (Montrose/Arbroath field). Having contracted Sea Quest to drill this well it was very hard for them to keep the news secret from BP, much as they would have preferred.

By then I was regional geologist and was fully aware of what was going on at Arbroath. Fortunately we soon managed a well data exchange so that I no longer had to keep up the pretense of knowing nothing.

By now we had carried out new-fangled digital seismic across blocks 21/9 and 21/10 and the surrounding areas, and the results were encouraging because we had mapped a large low-relief closure. The Amoco well was of particular interest because Montrose was the first closure down the nose from 21/10 and, importantly, it was full to spill point.

Things were suddenly looking up but disaster nearly struck. One of our senior managers popped his head round my door after having had a convivial lunch with his Shell counterpart and said, “I don’t suppose there is any reason why we shouldn’t farm out 21/9 and 21/10, is there?”

I guess the expression on my face was answer enough and, fortunately, nothing more was heard of the idea.

Sea Quest quickly moved to drill 21/10-1 and, in October 1970 at a depth of just under 7,000 feet, it encountered oil bearing Palaeocene sands (just three feet from prediction; well done, geophysicists). Subsequent work showed we had 35 square miles of closure, an oil column of just over 500 feet and 4.4 billion barrels oil in place.

A field of major proportions had been discovered.

Pin-Point Accuracy

The decision to develop was taken immediately, despite the fact that no one knew quite how it could be done in what was then such deep water.

I remember a discussion about laying a pipeline to shore. Some thought it would collapse under the weight of water while others thought it would float to the surface. Such was the knowledge of offshore technology at that time. How things have changed in 40 years!

The field was given the name “Forties.” “Forties” is a fishing/meteorological area so called because it occupies a large part of the North Sea that is almost universally 40 fathoms (240 feet) deep. It was ironic that our Forties Field was discovered in the only part of that area that was around 400 feet deep!

Five years later the first of four production platforms was in place and drilling had commenced. The pipeline to shore had been laid (without collapsing or floating) and an operations HQ had been set up in Aberdeen, Scotland.

All was ready for Her Majesty the Queen to visit Aberdeen on Nov. 3, 1975, for a grand celebration – during which she pressed the button that allowed first oil to come ashore.

Full marks must go to the geophysicists whose painstaking work led to the discovery of the Forties structure.

For my part I felt much gratification at being the guy that stuck the pin in the right place on one of their maps.

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