> Does anyone know how the extraction cost of shale compare to the declining cost of solar?
The solar market is so tiny it doesn't matter in the energy discussion. Despite what you read from the cheerleaders ( paid or unpaid ) on social media and the news.
The two important changes in energy markets the last decade are fracking/shale oil and natural gas. That's it.
> Are we at the point already where some oil fields become non-profitable?
Solar doesn't determine profitability of oil field. Oil prices determine profitability of oil fields. And it's all variable depending on location/quality/etc of the oil fields and the oil. Yes, as oil prices drop, there are oil fields that became unprofitable and many oil fields were shuttered. As oil prices rise, they will become profitable and the activity will increase on those fields.
Low oil prices and the rise of natural gas along with regulation has absolutely crushed coal markets in the last 10 years. It has also crushed the solar markets as well ( see all the solar companies going bankrupt ). Whether oil and gas can keep this trend going, your guess is good as mine.
I think Trump is very pro-energy and he will support low oil prices, low LNG prices and even push for more coal investments. That won't bode well for solar. But what do I know.