In 2007 oil prices were rising, and the Chávez government sought more revenue as the investments made by the international oil companies began to pay off. Venezuela demanded changes to the agreements made by the international oil companies that would give PDVSA majority control of the projects.
ExxonMobil and ConocoPhillips refused, and as a result, their assets were expropriated. (These expropriations were later ruled to be illegal, and compensation was granted to both companies).
So there are two related causes that have resulted in the steep decline of Venezuela's oil production, despite the the country's top rank in proved reserves.
The first is the removal of expertise required to develop the country's heavy oil. This started with the firing of PDVSA employees in 2003 and continued with pushing international expertise out of the country in 2007.
Second, the Chávez government failed to appreciate the level of capital expenditures required to continue developing the country's oil. This was in no small part due to inexperience among the Chávez loyalists that were now running PDVSA.
When oil prices were high, Chávez funneled billions from the oil industry into the country's social programs. But he failed to reinvest adequately in this capital-intensive industry.
Following the 2007 expropriations, Venezuela's oil production went on a steep decline. In 2018, Venezuela's oil production fell to 1.5 million BPD, a decrease of more than 50% below 2006 levels.