Although oil prices rose strongly in 2021, they have been on the decline in recent days. Ken Cedeno/Getty ImagesOil prices dropped as high as 4.5% Monday, as investors worried about the spread Delta.Many countries around the world are experiencing an increase in COVID-19 cases, which is threatening their economies.Investors will be keeping an eye on the key oil reports this week to see if there are any clues as to demand.Subscribe to our daily newsletter 10 Things Before The Opening Bell.Oil prices fell Monday due to investors' concerns that the continuing rise in cases Delta-variant coronavirus would hamper the global economic recovery as well as reduce energy demand.Brent crude oil fell 4.17% to $67.75 per barrel in European trading. This was its lowest level in three week. WTI crude oil was 6.25 dollars per barrel lower at $65.25, down 4.44% from its previous low of 4.5%.The Delta coronavirus variant spreads rapidly throughout the globe, driving up COVID cases in the US, Europe, and even China.According to New York Times data, the average number of cases in the US was 110,360 for Sunday. This is up from 56,987 just two weeks ago.Steen Jakobsen is chief investment officer at Saxo Bank. "The COVID-19 return led by the rapidly-spreading Delta variation continues to raise concerns about short-term outlook," he said.In 2021, oil prices rebounded strongly as economies have reopened and the OPEC plus group of oil-producing nations has limited supply. Brent crude oil rose from $50 at the beginning of the year, to $77 in July.Investors are starting to worry about the impact of the Delta variant on the global economy, as countries continue to restrict international travel and reintroduce coronavirus controls.Goldman Sachs lowered its 2021 forecast for Chinese GDP growth from 8.6% to 8.3% Monday, citing Delta variant restrictions.Brent crude oil fell by around 7% in the week before, making it its worst week since October. However, the selling picked up on Monday.Continue reading: Goldman Sachs recommends buying these 21 energy stocks that have unique catalysts. This will allow them to rise through the year as the sector surpasses the wider market.Jakobsen, from Saxo Bank, said that a stronger dollar and higher yields on US bonds after strong economic data last Wednesday could also be impacting oil. Oil is denominated as dollars.He said that the market will still be able absorb the gradual production increase announced by OPEC+. "And if it doesn't, the group will likely step in to support prices."Traders will keep an eye on the International Energy Agency's monthly oil market reports and OPEC's weekly report this week for clues about the strength of the demand.