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It is acquiring up to 25% in the South Weslaco field, in Hidalgo County, which is predominantly a gas property.
Northcote will pay US$450,000 for the assets, with US$150,000 paid in cash and the rest being paid in shares.
The deal is, in part, being facilitated by Northcote's part owned associate Northcote Drilling partners which is taking just over 8% and in return is providing the US$150,000 in cash.
South Weslaco spans 6,000 acres and in six months, to June 30, it provided Aminex with net production of 29 barrels of oil equivalent per day, generating monthly revenues of US$16,000.
Chief executive Randy Connally said: "This acquisition does not alter our focus on growing our production from our Mississippi Lime assets in Oklahoma; but the opportunity to acquire an accretive interest in a set of established assets, located in an area of South Texas that we like and which holds significant acreage for several decades to come, was too good to pass up."
Northcote joined the AIM in January and has moved quickly since. It been progressing a value-adding fracking programme and it continues to show an appetite for bolt on acquisitions.
Earlier this month the firm hit its full year production schedule and lifted the bar on a new target.
Northland Capital analyst Andrew McGeary describes the deal as an 'interesting bolt-on' which moves the group into gas properties.
He also points out that at US$300 per acre the total consideration is modest, from one perspective modest. Although on the basis of existing production multiples he says it is neither expensive nor cheap.
McGeary does, however, indicate that Northcote may be looking at the bigger picture, beyond the four existing wells on the property.
"The company has clearly bought into the wider potential of the lease position that is held by production for over 10 years and the use of Northcote Drilling Partners third-party funding option also increases scope," the analyst said in a note.
"It is difficult to gauge the attractiveness of the assets at this time, but assuming they are prospective the per acre multiple should allow plenty of upside potential.
"The statement suggests this was an opportunistic move and does not deter from the company’s main focus in the Mississippi Lime."
McGeary also says that while US gas prices remain low they have firmed up from their lowest and the emergence of an export option could see prices rise in the medium to longer term.
For Aminex the deal represents just one step in its plan to divest its American assets as it pursues a separate deal to bring in a new partner for its project in Tanzania.
South Weslaco is the smallest of Aminex's American properties.
"The sale of this gas producing interest is the second in a divestment programme previously announced and follows our declared strategy for our US assets," said Brian Hall, Aminex chairman.