One significant factor in the decrease in US oil production has been the restrictions put in place on exploration in the coastal waters of the US due to environmental conditions. There may be oil in those places or not. But failure to explore in certain areas has to be considered as a factor in the decline in US oil production.
Another factor has been the price level of crude oil; as the chart below shows the real price of oil was essentially flat between 1975 and 2004 with an anomaly due to the first Gulf War. There has been no economic incentive to seek new sources of oil or alternative sources of energy.
The USGS estimated US oil consumption at about 7 billion barrels per year in 1998; given that rate the North American reserves shown in the chart below would be exhausted in about 28 years.