* Global oil price slump hits earnings at BASF unit
* Say will expand production, invest, to offset this
* Focus on Norway, Russia, Argentina (Adds details, background from statement)
FRANKFURT, March 19 (Reuters) - BASF oil and gas subsidiary Wintershall expects lower oil prices to considerably reduce operating profit in 2015, but will widen output to try to offset the problem.
Earnings before interest and tax (EBIT) adjusted for special items were 3 percent down at 1.795 billion euros ($1.92 billion) in 2014, the Kassel-based firm said on Thursday.
Wintershall also said it anticipated a slight decrease in sales this year. In 2014, sales rose 2 percent to 15.145 billion euros.
"Our planning for the year 2015 is based on an average oil price between $60 and $70 per barrel and a U.S. dollar exchange rate of $1.20 per euro," chief executive Rainer Seele said.
It based last year's figures on an average 2014 Brent oil price for North Sea crude oil at $99/barrel.
Wintershall is a big earnings contributor to parent company BASF.
Its EBIT represented 22 percent of that at the BASF group, which reported results on Feb 27.
Wintershall said it will aim for higher production in Norway, where it has reached oil output of 60,000 barrels per day (BPD), and in Russia, where it exploits cost-competitive oil and gas fields jointly with its partner Gazprom.
It said it planned to raise its long-term production to 190 million barrels of oil equivalent (BOE) by 2018.
In 2014, it had raised production to 136 million BOE from 132 million BOE a year earlier.
Wintershall plans to invest at least 4 billion euros over the next five years in its key oil and gas activities.
Oil production in Libya, a big revenue factor before conflict took hold there, is currently down at 30,000 barrels per day (bpd). In early 2013, Libyan output had been 85,000 bpd and in pre-war times 100,000 bpd.
Wintershall was also stung on the Russian side of its business when a planned gas assets swap fell through last year due to geopolitical tensions over Ukraine.
The deal would have given Russia greater access to gas trading and storage in Germany, in return for Wintershall getting more stakes in Siberian gas fields.
But joint gas production in Siberia and gas trading in Europe at its joint venture firm with Gazprom, Wingas, will continue on their current basis, it said. ($1 = 0.9357 euros) (Reporting by Vera Eckert; Editing by Kirsti Knolle/Ruth Pitchford)