Iberian prompt power undercut by wind forecasts

* Spain day-ahead 36.17 euros ($52.70)/MWh, down 1.57

* Portugal spot falls 2.05 to 36.68 euros/MWh

MADRID, Oct 1 (Reuters) - Iberian prompt power prices on Thursday gave back some gains made over the previous two sessions due to predictions that becalmed wind parks were about to step up supplies to the grid.

Spain is the world's third-biggest producer of wind power and generation was expected to rise to 4,000 megawatts on Friday from about 1,500 MW at midday on Thursday, or enough to supply some 4 percent of demand.

Gas-fired plants' share of the generation mix had consequently dropped to 36.1 percent from 40.5 on Wednesday.

Wind power moves the spot market because it provides 11 percent of Spain's electricity, on average, and utilities can sell it to the pool cheaper than electricity from gas- or coal-burning generators.

Output can vary from virtually nil to more than 11,000 MW, however, which makes spot prices -- the market benchmark -- volatile.

Looking ahead, spot prices often fall on Fridays due to a predictable drop in demand over the weekend ahead.

A pull-back by crude pared benchmark forward contracts in early over-the-counter dealing, although mid-market quotes were flat after Brent crude LCOc1 recovered slightly from early lows.

Baseload calendar-year 2010 was heard trading at 39.25 euros/MWh, down 0.15, with quotes at 39.20/39.60.

Data from national grid operator REE (REE.MC) and watchdog CSN showed six of Spain's eight nuclear power stations were producing 5,176 MW in all, or 14.2 percent of demand.

The 1,000 MW Vandellos II plant is undergoing repairs after an unscheduled halt last week and may be back this weekend. Meanwhile, the 1,000 MW Cofrentes reactor is refuelling and due to be reconnected in about three weeks.

Greenhouse gas emissions in Spain were a hefty 12,207 tonnes per hour, REE said.

(Reporting by Martin Roberts; Editing by Keiron Henderson)