UPDATE 1-Centrica doubles stake in Statfjord Field

* British Gas owner pays 144 mln stg to up stake to 19 pct

* Secures extra 172 bln cubic feet of oil and gas reserves

* To spend about 100 mln stg over 3 yrs to maximise output

(Adds details)

LONDON, Sept 17 (Reuters) - Centrica (CNA.L) doubled its stake in the North Sea's Statfjord Field in a 144 million pound ($224.6 million) deal that provides the British Gas owner with an additional 172 billion cubic feet of oil and gas reserves.

Centrica is buying a further 9.44 percent interest from A/S Norske Shell and Enterprise Oil Norge AS (Shell) (RDSa.L), raising its interest to 19.13 percent in the field, which is directly linked to the UK market.

Centrica has been expanding its upstream business by investing in the North Sea as some oil majors start to reduce activity in the area.

"(The deal) represents a further step in our strategy to grow and deliver value from our upstream business," said Centrica Energy Managing Director Mark Hanafin on Friday.

"It also increases our level of gas reserves to supply British Gas customers and underlines our continued commitment to invest in North Sea production."

The company said it would spend about 100 million pounds over the next three years to maximise production from the field, which is in Norwegian sector of the North Sea.

(Reporting by Paul Sandle; Editing by Julie Crust)

($1=.6412 Pound)