Today, in a note released by Goldman titled "OPEC: To cut or not to cut, that is the question", the firm presents a chart which shows just as graphically how exactly OPEC lost the war against US shale: in one word: the cost curve has massively flattened and extended as a result of "shale productivity" driving oil breakeven in the US from $80 to $50-$55, in the process sweeping Saudi Arabia away from the post of global oil price setter to merely inventory manager. 

This is how Goldman explains it:

Shale's short time to market and ongoing productivity improvements have provided an efficient answer to the industry's decade-long search for incremental hydrocarbon resources in technically challenging, high cost areas and has kicked off a competition amongst oil producing countries to offer attractive enough contracts and tax terms to attract incremental capital. This is instigating a structural deflationary change in the oil cost curve, as shown in Exhibit 2. This shift has driven low cost OPEC producers to respond by focusing on market share, ramping up production where possible, using their own domestic resources or incentivizing higher activity from the international oil companies through more attractive contract structures and tax regimes. In the rest of the world, projects and countries have to compete for capital, trying to drive costs down to become competitive through deflation, FX and potentially lower tax rates.