The US is a mature oil province.  To understand what this means, watch this temporal sequence originally prepared by the US Geological Service in 1996, showing decade by decade how the very few 19th century plays turn into the modern world. Literally millions of wells have been dug and many hundreds of thousands remain active.  Note that grey depicts dry holes.

If you're interested in the historical resource development of any of the major fields, Pete highly recommends the 1996 release 2 version of 1995 National Assessment of United States Oil and Gas Resources--Results, Methodology, and Supporting Data.  The USGS used to give it away for free on a CD.

[Please allow enough time for the images to be downloaded to your computer]

Here is USGS' most recent: