Complex sentence: Because oil was priced in dollars, this meant that oil producers real income decreased and in September 1971 OPEC issued a joint communiqué stating that, from then on, they would price a barrel of oil against gold.
Split into simpler sentences: Because oil was priced in dollars, this meant that oil producers received less real income. The OPEC issued a joint communiqué stating that, from then on, they would price a barrel of oil against gold.