Extraction cost of unconventional, shale oil

I was watching a You Tube documentary, published about 6 years ago, on Alaska's North Slope oil reserves, particularly shale oil. The focus was on the extraction cost, estimated to be about USD 50 per barrel, as well as the significant advancements in technology to extract shale oil. The advancements were in terms of identifying commercial shale deposits and reduction in the time to assemble and dis-assemble necessary production infrastructure.

Another documentary popped up, published about 2 years ago, in a similar theme, where the estimated extraction cost was below USD 40 and even further advancements in the identification of commercial reserves of shale deposits, and at least a further 30% reduction in the time to assemble production and dis-assemble infrastructure.

Both documentaries were positive about shale, or its by-products oil and gas, as the long-term energy source for USA . What occurred to me what has changed in the last 2 years in terms of extraction cost, identification of shale reserves, and reduction in the time to assemble production and dis-assemble infrastructure.

Bos