BHP Billiton Group has made the decision to exit its U.S. shale business, but while recent shareholder criticisms may have had some influence over the move, CEO Andrew Mackenzie told reporters that the mining heavyweight's exit from the business has been years in the making.

"What we're announcing today is the culmination of five years of effort and also five years of maturing how we want to run this business," he said Aug. 22.

"Of course we listen to shareholders and all shareholders' inputs count very high in the decisions that we make in how to run this business, but this has been a long time in its creation."

BHP believes its best option is a trade sale, but Mackenzie admitted the company does have other options up its sleeve that may provide a quicker exit, but could potentially be less "value-adding."

"We will be patient to make sure we restore value for shareholders," the executive said.

BHP has faced opposition to its petroleum business from several shareholder groups that believe the best way forward for the company is to ditch the US$22 billion worth of U.S. oil and gas assets.

However, the dual-listed mining giant is instead dropping its pursuit of unconventional sources and returning to the conventional.

"Really the path to this decision started quite a few years back when we realized that [shale] wasn't going to be a business that we could expand globally because we just didn't think the geology and the commercial conditions existed anywhere seriously outside the U.S. and maybe Canada," Mackenzie said.

Over the past five years, BHP has halved its shale acreage and put half of the remaining acreage on the sale block, and is now moving to offload the entire landholding.

Mackenzie said the company has also cut the number of its rigs to four from 40 and substantially reduced CapEx spend on shale assets to US$600 million from US$6 billion.

"Our pivot back to conventional petroleum is working," he said. "We realized that we needed to find more oil that we could develop conventionally as we steadily over the last few years backed away from shale."

BHP expects to increase its CapEx spend on the shale assets slightly prior to any sale to increase the profitability and marketability of the business.

Mackenzie said there have been "plenty of people interested in taking a look" at the unit.

"We do know what our assets are worth and we do believe the application of our technical and commercial skills and some investment in them will continue to increase their value and therefore, if you like, our target selling price," he noted.

Mackenzie argued that BHP's motivation for divesting its shale business was driven by the limited availability of good projects and not the sliding oil price.

"Actually before the oil price fell, we did conclude through a global geological study that the opportunities to expand into shale elsewhere in the world were very limited and it was more a geological reason that took us back towards conventional," he said.

Mackenzie believes the limited opportunities in shale globally will see its contribution start to dwindle over the next decade, which is expected to put a floor under the oil price.

This bodes well for BHP's development of the Mad Dog phase two project in the Gulf of Mexico, which the company approved in the 2017 financial year.

BHP believes it can quickly convert its conventional oil and gas discoveries in the U.S., Mexico and Trinidad to production.