Final answer:
The EROEI of oil and natural gas has decreased over time due to the progression to more challenging and unconventional sources of these energy resources.
Step-by-step explanation:
The Energy Return on Energy Invested (EROEI) of oil and natural gas has decreased over time due to the progression to more challenging and unconventional sources of these energy resources. In the 1950s, the EROEI of oil and natural gas was around 30:1, meaning that for every unit of energy invested, 30 units of energy were returned.
Currently, the EROEI has decreased to approximately 11:1 or even as low as 3:1 for tar sands. This decrease is primarily attributed to the increased difficulty and energy required to extract these unconventional resources.