The answer lies in the price of Santos shares, and what has happened in the days after Ellice-Flint's sudden departure was announced last Tuesday.

Between then and now the Santos share price has risen from $13.10 to $14.86, meaning the value of Santos has increased by exactly $1 billion in less than a week.

It is probably unrealistic to say that one event caused the other, but there is unmistakably something happening inside Santos which has little to do with the oil price, or exploration news, and everything to do with what stock market players call "corporate events", such as a major change in strategic direction.

To underline the fact that the market smells change ahead, Santos on Monday morning charged ahead by another 48c, or up 3.3%, while its arch-rival, Woodside fell 49c, or down 1%.

One oil stock going up while another comes down is a phenomena that greatly interests The Slug and deserves a little investigation.

Let's start with Santos itself, a company which has promised much during the eight years under the management of Ellice-Flint, but which never seemed to quite deliver.

To be fair, the man inherited a near-basket case which had been wallowing for years under the control of the South Australian Government, and a lack of desire to explore far beyond its backyard.

The result was a company trapped in the Cooper Basin, a declining legacy asset, and with a management team which seemed more interested in supporting the Adelaide arts community than enriching shareholders.

Ellice-Flint swept in like the new broom he was, promising to break the shackles, and free Santos from the shareholder restrictions placed on it by the Government.

He achieved some of his goals. Santos did spread its wings. It did diversify into liquefied natural gas and Asian exploration, and it has won its freedom from the shareholder cap.

So, having done these things, why is Ellice-Flint leaving?

Or, more to the point, why is he leaving now, and why is he leaving with so little notice?

There are four possible explanations for the departure:

1. He has achieved what he set out to do.

2. He has failed to achieve what he set out to do.

3. There's a disagreement at Santos board level about explanations 1 or 2.

4. There's a disagreement about the future direction of the company.

On the first point, Ellice-Flint has achieved much and perhaps now is a good time to move on.

It is even possible that he has received a better job offer. If nothing else that would explain the sudden departure - his new role of "consultant" certainly takes him out of the day-to-day management loop.

On the second point, Ellice-Flint has not impressed everyone. Stockbrokers remain unimpressed with Santos.

Goldman Sachs even renewed a "sell" notice on the stock after the management change was announced, adding somewhat sourly that the company's exploration-led growth strategy "has not been successful in ameliorating the declines in the Cooper Basin and driving material production growth".

Both points - perhaps the chief executive has done all he could, and perhaps he hasn't been sufficiently successful - will have caused much boardroom angst, especially as so much more was expected.

To use the Santos v Woodside example again, it is worth pointing out that in the eight years under Ellice Flint, the Santos share price has risen 250%, which looks fabulous, until you see that Woodside has risen by 511% - twice as fabulous.

Perhaps what most rattled the board is a realisation that the year ahead for Santos is likely to be the most traumatic since the company was listed on the stock exchange 54 years ago.

Lifting the shareholder cap in November will undoubtedly put the stock "into play", a perfect issue on which a chief executive and board can disagree.

Finding a technology partner for its Gladstone LNG plant will be a second challenge - and an issue which ought to have been resolved already.

Fixing the disastrous Indonesia mud flow, or suffering more bad publicity, will weigh heavily on public sentiment towards the company, which must also find a way to boost overall oil output.

Issues such as these might explain the decision of Ellice-Flint to leave. But what explains the suddenness of the announcement?