Had to happen eventually. Shale was placing a cap on what OPEC could charge, then there were the Russians playing games. Eventually, it became clear that the Russians wouldn't go along with OPEC holding back productions, so it's now a price war with the Saudis trying to put the US and Russians out of business (with N. Sea oil as collateral damage). We'll see how long they can keep it up, but at prices below $40 a lot of shale is losing money now, and the Saudis can probably push it down to $25 under present low-demand circumstances and keep it there for a while.

As for diesel generators with cheap fuel, "cheap" depends on where you are and what your smog regs look like. Though SCR emission control really doesn't care much about the quality of the fuel. Low sulfur content is important for particulate control (with or without a filter), but practically all diesel sold in the US now is low sulfur. It'll still be expensive in California due to its "island" refining system (little connection with the rest of the US) and extra regs/taxes, but other places in the US might use a little more diesel. It'll still be expensive compared to solar and wind if that's available (no fuel cost; if the up-front is paid for the energy is nearly free), and coal and natural gas in big plants, so it'll make sense only for peak production needs.

Total non-sequitur: do the casinos in Vegas still use F-M diesel powered gensets for emergency and peak power?