I'm an engineering (not a petroleum) geologist, so this is not my areas of expertise. I thought we had been doing horizontal drilling prior to the 1990s. I thought that horizontal drilling with fracking, and the discovery of the oil shale in the eastern US all came together to make it possible for the US to become indpendent with regards to oil?
Shale formations were discovered a long time ago but never exploited. I am pretty sure Mitchell Energy in Houston was the first oil company to successfully develop tools and fraccing techniques to extract hydrocarbons from shale around the early 1990s. Mitchell is now Devon Energy in Oklahoma.
Horizontal drilling has been around since before the 1990s and I have seen fields that were in production (not many) but not in shale. The term was more commonly "directional" drilling and was very precise and could extract hydrocarbons with conventional fraccing in conventional formations.
With respect to shales, there are several big plays and a lot of smaller ones, all of which got developed around the same time (a decade perhaps). Many of these shale formations were primarily natural gas plays: (from Wiki)
2.0 Natural gas shale deposits in the United States
2.1 Antrim Shale, Michigan
2.2 Barnett Shale, Texas
2.3 Caney Shale, Oklahoma
2.4 Conesauga Shale, Alabama
2.5 Fayetteville Shale, Arkansas
2.6 Floyd Shale, Alabama
2.7 Gothic Shale, Colorado
2.8 Haynesville Shale, Louisiana
2.9 Mancos Shale, New Mexico and Coloardo
2.10 Monterey Shale, California
2.11 New Albany Shale, Illinois Basin
2.12 Niobrara Shale, Colorado, Kansas, Nebraska and Wyoming
2.13 Pearsall Shale, Texas
2.14 Eagle Ford Shale, Texas
2.15 Devonian shales, Appalachian Basin
2.15.1 Chattanooga and Ohio Shales
2.15.2 Marcellus Shale
2.16 Utica Shale, New York
2.17 Woodford Shale, Oklahoma
3.0 Oil shale deposits in the United States
3.1 Bakken formation, North Dakota
Although the gas plays are listed as "gas" above, several of them provide huge quantities of crude oil like the Eagle Ford in south Texas.
Interestingly, about 60% of the Bakken formation is really in Canada, but the field is huge.
And yes, the concurrent timing of the shale extraction techniques, the advances in horizontal drilling (tools) and fraccing technology has made the U.S. "somewhat" energy independent.