Scotland, which has formed part of the United Kingdom since 1707, will vote Sept. 18 in a referendum on whether it wants to become an independent nation. The debate has captured imaginations far beyond U.K. borders, whether due to the draw of the Scots' notoriously fiery and romantic spirit, or due to concerns over the precedent it sets for other separatist nations.

But oil companies have little time for romance, and they look wary. The oil and gas the U.K. has been exploiting since 1976 off Scotland's North Sea coast, and on which an independent Scotland hopes to depend, is firmly at the center of this debate.

The choice has been characterized as a "head versus heart" issue, but highly politicized estimates of remaining oil reserves (ranging from 10 to 24 billion barrels) and future annual tax revenues from the industry (from £5 billion to £7 billion) make it difficult for voters to make an informed decision. With little time left, the outcome among Scottish voters is hard to call, but for potential investors the head will certainly be leading the heart.

Oil companies do not like change. However romantic the times, they do not like protests, nor do they like unexpected changes of government. They certainly do not like calls for secession from a power they have been dealing with for years.

An investment in drilling an oil rig is a multiyear (often multidecade) and multimillion-dollar decision. Given the inherent risk of turning up an expensive dry hole, investors at least hope for some political and fiscal stability. Shell CEO Ben Van Beurden, when commenting on Scottish independence, has said, "we want to know as accurately as possible what investment conditions will look like 10 or 20 years from now."

This priority is the reason the North Sea is still being exploited in the first place. Geologically speaking, Iraqi, Nigerian and Turkmen oil and gas plays may be more impressive than the North Sea's mature fields, but the developed world offers priceless political stability to more risk-averse investors or larger majors looking to diversify their risk portfolio.

The language of certainty and stability is driving the debate among the investor community. BP chief Bob Dudley has spoken out and said that a vote for Scottish independence would mean "greater uncertainty" for the energy industry. Van Beurden soon joined the chorus of business leaders when he said outright that "we'd like to see Scotland remain part of the United Kingdom."

We should not get carried away. Alex Salmond, the leader of the Scottish National Party (SNP) is no Hugo Chavez; do not expect any appropriations of oil fields or punishing of foreign investors.

The Scottish also hope to woo international oil companies to explore and produce in their waters, but a new government would need to craft its own tax and regulatory structure in response to competing demands. But they would be faced with regaining the trust of oil companies after a number of years of instability in the tax regime.

Back in 2003, BP's sale of its stake in the Forties field to Houston-based Apache Energy was typical of a seeming exit of the North Sea by oil majors. But in 2003 the price of a barrel of oil on global markets was still hovering around $30. Once this figure started to rise — ultimately almost fivefold to US $147 in July 2008 — the maturing fields of the U.K. Continental Shelf (UKCS) once again made more economic sense.

To keep energy companies in U.K. waters, in 2009 the government in Westminster dangled the incentive of extra tax allowances. However, the reversal of this policy in 2011, which raised taxes for operators from 62 percent to 81 percent for older fields and from 50 percent to 62 percent for newer ones, caused outrage and caused a dip in exploration again.

A new raft of allowances introduced as a consequence led to today's higher level of investment. After this fiscal roller coaster, what operators will be pushing for from policymakers, whether in London or Edinburgh, is stability and predictability in the tax regime.

There are already signs that investors are getting twitchy. A report published by Deloitte's Petroleum Services Group revealed that the number of wells drilled on the UKCS dropped from 17 in the second quarter of 2013 to just seven over the same period this year. The 2011 tax hike and perceived unpredictability of the tax regime is likely to have influenced the drop-off in activity, in combination with historically high operating costs and the added costs of decommissioning old facilities.

Some oil companies have voted with their feet. Despite the assurances of Scottish politicians, in late 2012 Ireland's Tullow Oil pulled out of the North Sea to focus on its African assets. More worryingly, they struggled to find a buyer for the Schooner and Ketch fields, with investors scared away by rising costs and lower returns. Faroe Petroleum, which specializes in Norway, the U.K. and Iceland, eventually stepped up and paid $75.6 million.

We should not only be listening to the investors, of course. Many Scots are understandably angry that during their own North Sea oil boom of the 1980s no sovereign wealth fund was set up by the U.K. government to save their natural resource wealth for future generations, as Norway did. There is a strong argument that the U.K. has not been a careful steward of Scotland's resources, and many want to see that reversed.

But the reality is that the 1980s are behind us, and Scotland will need these investors — especially the 200,000 Scots who are employed by the industry. Oil companies will never be satisfied with the tax relief offered, but Tullow's experience shows that there are increasingly international alternatives that may offer a better risk-reward calculation and where oil majors can refocus their assets.

If Scotland votes for independence this month, the new sovereign government will face high expectations. The expectations will come firstly from freshly minted Scottish citizens, who have been promised their country can become another Norway; and secondly from oil companies who have been holding back on investment, but whose lobbyists would soon fill the corridors in Edinburgh's parliament.

Striking a fair balance may be a more difficult job than they imagine.