China has invested an estimated $40 billion in Iran's oil and gas sector Deputy Oil Minister Hossein Noqrehkar Shirazi said in an AFP report Saturday.Speaking to the Mehr news agency, Shirazi stated that oil exports from Iran to China have decreased in the first six months of 2010 by 30 percent to 66.12 million barrels, compared to the same time period in 2009.RELATED:US: Iran sanctions starting to bite"Although Iran is still among top 10 oil exporters to China, it is the only country which in the first six months of 2010 has seen its exports to China falling," Shirazi stated in the report."The volume (of Chinese investment) in upstream projects is 29 billion dollars," the oil minister explained. China signed contracts worth an additional $10 billion in petrochemicals, refineries and oil and gas pipeline projects, he added.China has proposed participating in the construction of seven new refineries in Iran.Iran has been pushed to import gasoline and other petroleum products to meet internal needs as a result of its deteriorating refining sector. Iran is currently OPEC's second biggest exporter of oil.China has compensated for downfalls in the Iranian energy sector which occurred as a result of western businesses leaving due to international sanctions. The country became Iran's main trading partner, with bilateral trade worth $21.2 billion compared to three years' earlier sum of $14.4 billion. Fifteen years ago, commercial ties between Iran and China amounted to only 400 million dollars.Western sanctions paved the way for Chinese companies doing business with Iran, directly supplying Iran with $7.9 billion (13%) of its imports, according to official data.China has urged the international community solve the Iranian nuclear crisis diplomatically but has supported the fourth set of UN sanctions against Iran.China on Friday, opposed the European Union's latest unilateral sanctions against Iran.