Always regarded as a miner first, and an oil and gas producer second, BHP Billiton has long been Australia's biggest producer of petroleum products.

Petroleum was so far in second place in the pecking order that speculation would surface from time to time that BHPP (BHP Petroleum) would be spun out as a separate ASX-listed business or sold to one of the oil majors.

History shows that nothing happened about a spin-off, though planning in that direction did cost a few senior BHP Billiton executives their careers when the parent company board thought it had gone too far.

It is also history that BHP Billiton had a habit of ignoring opportunities to get bigger in oil and gas, despite invitations to acquire more of Woodside or its LNG projects.

None of that matters a jot now that Andrew Mackenzie is getting ready to slip his brogues under the chief executive's desk (as soon as Marius Kloppers exits). A new man at a big company is invariably the equivalent of a new broom.

In Mackenzie's case, and to a lesser extent Kloppers, there is a deep interest in oil and gas that could become even deeper if the evidence is being interpreted the right way, with tell-tales blowing in the wind, such as:

• hints that the sometimes criticised US shale oil and gas investment could become BHP Billiton's equivalent of holding a tiger by the tail
• the temptation to snap up the remaining shares Royal Dutch Shell has in Woodside, potentially triggering a full-scale bid
• the search by BHP Billiton for growth options once the fad for cost-cutting runs its course in the next 12-18 months.

Over the top of those three points is the undoubted first area of interest for a man who started training as a geologist before moving into a 22-year career with British Petroleum, rising through the senior ranks to the posts of chief technology officer and then vice-president petrochemicals.

There is no doubt that with such training, Mackenzie understands the shale challenge and potential better than anyone else in the modern mining world. He "gets it" when the question of how big will the shale revolution become - in non-technical terms, bloody big.

Kloppers, who ultimately had to sign off on BHP Billiton's plunge into US shale, also gets it. This caused him to tell one of the many analysts to interview him in the company's equivalent of the departure lounge that US shale could become so big that it has the potential to "threaten the balance of the company".

Precisely what "threaten the balance" means is open to discussion, but The Slug's interpretation is that the board of BHP Billiton is concerned oil and gas production from unconventional sources could upset the core operational and financial structure of the company, which is governed by a single word: "diversified".

How, it might be argued, could BHP Billiton continue to claim to be diversified with a balanced portfolio of mineral and petroleum assets if one dominates the others.

For the past few years, that "dominance" role has been played by iron ore, but the start of a change can be seen in profit flow, with oil and gas in 2012 generating pre-tax earnings equivalent to 63% of iron ore ($US9.56 billion for oil v $US15 billion for iron ore) - but with a clear rising trend for oil and gas in the future.

The investment bank Goldman Sachs reckons that in the current year, the oil to iron ratio will be 71%, largely a result of iron ore profits falling, and then up to 88% in 2015.

The point about that exercise is to demonstrate the rising importance of oil and gas to BHP Billiton's profits. That and the fact the board has just chosen an oil and gas man as its chief executive - over the head of the existing oil and gas chief, J Michael Yeager.

Mackenzie's first 100 days are unlikely to yield many clues as to the man's intentions, except perhaps the abandonment of some of the silly rules enforced by Kloppers (shoe colour, clean desks and no food in the office).

However, after the obligatory 100 days touring BHP Billiton's outposts, it will be game on for the Scot with oil in his blood. That is when the Shell stake in Woodside could become a fresh talking point, along with other petroleum matters that should soon have iron ore playing second fiddle.