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ON THE face of it the world's big and publicly quoted oil companies should be celebrating some pleasing results this week. Royal Dutch Shell unveiled its results on Thursday February 4th, reporting that it had made $9.8 billion in 2009. Two days earlier BP boasted profits of $14 billion for the same year. Yet these billions are a disappointment compared with the bonanza of previous years (Shell, for example, raked in $31.4 billion in 2008 alone) when soaring oil prices pulled profits ever higher.

In the long term, however, the firms' success depends on sustaining reserves. The big western oil companies are trying to expand through acquisitions and investment, but the opportunities do so are becoming scarcer. The firms are spending where they can. Exxon Mobil, the biggest listed oil company, says that exploration and capital spending hit $27.1 billion in 2009, 4% higher than in 2008. The company expects to spend $25 billion to $30 billion annually to the same end over the next five years. BP intends to spend some $20 billion this year on investment in new projects and drilling, roughly the same level as last year.

But there are limits to what money can buy. State-controlled rivals—in the Middle East, Russia and beyond—jealously guard oil reserves on their home patches. Few new big fields of oil, at least those that are easy to reach and cheap to exploit, have been discovered in recent years. And where new opportunities emerge, such as in Iraq, Western oil giants are scrambling to pay big sums at auctions for drilling rights in territory where the local government tightly limits their returns. Even then, competition from Chinese, Russian and other state-run oil firms can be severe. National oil companies will often pay prices that would alarm shareholders in the big listed oil companies.

Thus Western firms are increasingly looking for different sorts of growth. One option is to deploy their expertise in the hunt for oil that is harder to reach, for example deep offshore, or to go for reserves such as tar sands that are trickier, and so much pricier, to refine.

Another route is to speed up the quest for other energy reserves. France's Total has branched out into nuclear-power generation. This week Shell announced a $12 billion joint-venture with Cosan, a Brazilian producer of ethanol from sugar cane. This is something of a change of tack. Exxon and Shell are both spending money on “second generation” biofuels made from algae or waste materials, but these could take years to develop. Now Shell can sell Cosan's “first generation” wares through it global distribution network.

By far the biggest bet laid, however, has been on natural gas. Around 40% of Shell's daily production is now in the form of gas. Total and BP are not far behind. Gas is increasingly important for power generation and heating and the global market is expected to grow by half by 2030. Big oil companies are keen to expand, calculating that their skills at managing huge capital projects will be useful when building gas-liquefaction plants that make the stuff readily transportable. Late last year Chevron, Shell and Exxon agreed to spend $37 billion to develop the Gorgon field off Australia, another potentially huge source of gas.

Nonetheless investors remain cautious because prices are volatile. Exxon's shares fell in December when the firm bid $30 billion for XTO Energy, which gets its gas from “unconventional” shale beds. Gas prices have tumbled in the past couple of years as new projects, especially shale, came on stream just as the global recession lessened demand. And technological improvements have made the huge reserves of gas in America, from shale beds and the like, a commercial prospect. BP believes that these factors will keep gas prices weak for three or four years, but it could be longer. Though national oil companies have forced the Western oil giants to look farther afield for new reserves, the oil prices, unlike gas prices, are supported by the state-owned firms that make up OPEC.