DUBAI, July 27 (Reuters) - Saudi International Petrochemical Co (Sipchem) reported a 76.4 percent slump in second-quarter net profit on Wednesday, as lower sales prices for its products weighed on its earnings for a fifth straight quarter.

Sipchem made a profit of 26 million riyals ($6.9 million) in the three months to June 30, down from 110.1 million riyals in the prior-year period, according to a bourse filing.

The average estimate of four analysts polled by Reuters had been for a quarterly profit of 50.2 million riyals.

The company said its profit margins fell as sales prices for all its products dropped, while production costs increased because of higher feedstock and energy prices. This offset the benefits of higher production and sales.

Petrochemical product prices are closely linked to oil, so lower crude prices have reduced margins and the competitive advantage over rival manufacturers from non-oil producing regions which Saudi firms used to enjoy because of subsidised energy and feedstock.

Saudi Arabia is reforming its energy subsidies to help bring down a huge budget deficit. Sipchem had previously announced it expected 120 million riyals of extra costs due to the reforms, without specifying the time period.

Sipchem said in May that a 15-day maintenance period for three plants at subsidiaries in which it holds majority control would cost the company around 22.5 million riyals.

(Reporting by David French; Editing by Andrew Torchia) ((davidj.french@thomsonreuters.com; +971 4 362 5864; Reuters Messaging: davidj.french.thomsonreuters.com@reuters.net))