Shares in China Oilfield Services surged 160 percent on their Shanghai debut on Friday, far exceeding analysts' expectations, as the firm's growth is anchored by a virtual monopoly in offshore oil services in the world's second-largest energy market.

The yuan-denominated A shares of the offshore drilling and equipment arm of China's dominant offshore oil producer, CNOOC Group, opened at 35 yuan (US$4.66) compared with its IPO price of 13.48 yuan.

The opening price represented a 115-percent premium to the firm's Hong Kong-listed H shares, which last closed at HK$16.84. This was much higher than the average premium of 54 percent for A shares over H shares among more than 40 firms listed on both bourses.

China Oilfield, which competes with Schlumberger, Halliburton and Baker Hughes, dominates the country's offshore oilfield service industry as it derives most of its revenue from sister company CNOOC Ltd.

The firm raised 6.74 billion yuan through its Shanghai IPO, which was 320 times oversubscribed and drew a total of 2.17 trillion yuan in subscriptions - making it the country's third most sought-after domestic share offer.