Based in Mahone Bay, nova scotia, agile libre is an independent publisher of technical books about the subsurface.  

Geochemical alchemy

Alchemy was a medieval chemical philosophy aimed at turning base metals into gold. Long discredited, but ever hopeful, we humans continue with the fervently wished for in preference to the strictly logical. Explorers for oil and gas are always being approached with magical devices to help them. Many big, reputable companies have been conned this way. These days we apply geochemistry in much the same spirit, often pushing the conclusions beyond the bounds of logic.

In my career of over 50 years, many of the worst mistakes I’ve witnessed have come from too strict a consideration of geochemical principles. In the early stages of my career there was no clear accepted view of how oil and gas originates. I remember a talk in Colombia in 1966 that postulated that oil was of primary origin from the earth’s crust and many years later in Sweden a deep well was drilled in Lower Palaeozoic rocks of the Siljan Ring to prove this idea.

One of the pioneers of oil industry geochemistry was Jim Momper who was based at Amoco’s Research Laboratory in Tulsa. He was a great scientist and a real pioneer. When I joined Amoco in 1969, new hires were quickly indoctrinated. But like all ‘holy writ,’ eventually we started to realise that his groundbreaking work needed to be modified and expanded. For instance, he thought that carbonates could not be source rocks because they were not compressible. Saudi Arabia seemed to us to disprove this rather conclusively.

In 1975 Mesa Petroleum offered Amoco a farm-in to an Inner Moray Firth block they acquired in the fourth licensing round. We thought this was a no-hoper based on the shallow depth of burial of the source rocks. The Upper Jurassic crest of the structure is about 1800 m (6000 ft) deep, with the potential source rocks of the Middle and Lower Jurassic at about 2740 m (9000 ft). In 1976 the discovery well was drilled in block 11/30 and eventually the Beatrice Field with oil in place of 73 MSm3 (460 million barrels) was found. There is still controversy about the source of all this oil, with the possibility that the Devonian may have contributed. At our own post-mortem the conclusion was that we had underestimated the geothermal gradient. The structure was formed as a result of movement along the Great Glen Fault, and mineral veins onshore — had we known about them at the time — would have given us the clue to hotter rocks. So strong was our adherence to geochemical principles I doubt whether we could have persuaded the company to take a risk on what seemed a very risky venture. In exploration one should not worry about dry holes, they are an accepted hazard of the game. What one should really worry about are the prospects which were evaluated and rejected, but which turned out to be big fields.

On a more positive note, EnCore Oil led a partnership to the unlikely discovery of the Catcher Field in the central North Sea. This field and a cluster of others perhaps amounts to more than 24 MSm3 (150 million barrels) of recoverable oil. In my days as head of exploration for Amerada Hess, we discovered the filled-to-spill Bittern Field in the Palaeocene, following which we and others searched up-dip for more prospects. Despite advanced seismic acquisition and processing, none were found. In 2010 EnCore decided to ignore conventional geochemical wisdom which said that any oil found even further up-dip in block 28/9 would be heavy and drilled 28/9-1. Oil was found at a depth of 1400 m (4600 ft) and, somewhat to the surprise of everyone, turned out to be crude of medium gravity. Thus ignoring geochemical advice led to a sound decision.

It seems geochemical calculations often are pushed beyond the limits of knowledge — either because a company wishes to do something bold or more likely because it does not. The important point is to be honest and avoid the presumption of understanding when none exists. 

A walk through time, part 1

Geology comes first

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