Complex sentence: The industry shrank in the mid- 20th century following the discovery of large reserves of conventional oil, but high petroleum prices at the beginning of the 21st century have led to renewed interest, accompanied by the development and testing of newer technologies.
Split into simpler sentences: The industry shrank in the mid- 20th century following the discovery of large reserves of conventional oil, but gained new attention at the beginning of the 21st century due to high petroleum prices. The renewed interest was accompanied by the development and testing of newer technologies.